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[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index] [Xen-devel] [PATCH v1 1/5] xsplice: Design document.
A mechanism is required to binarily patch the running hypervisor with new opcodes that have come about due to primarily security updates. This document describes the design of the API that would allow us to upload to the hypervisor binary patches. This document has been shapped by the input from: Martin Pohlack <mpohlack@xxxxxxxxx> Jan Beulich <jbeulich@xxxxxxxx> Thank you! Input-from: Martin Pohlack <mpohlack@xxxxxxxxx> Input-from: Jan Beulich <jbeulich@xxxxxxxx> Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@xxxxxxxxxx> --- docs/misc/xsplice.markdown | 1370 ++++++++++++++++++++++++++++++++++++++++++++ 1 file changed, 1370 insertions(+) create mode 100644 docs/misc/xsplice.markdown diff --git a/docs/misc/xsplice.markdown b/docs/misc/xsplice.markdown new file mode 100644 index 0000000..18a264e --- /dev/null +++ b/docs/misc/xsplice.markdown @@ -0,0 +1,1370 @@ +# xSplice Design v1 + +## Rationale + +A mechanism is required to binarily patch the running hypervisor with new +opcodes that have come about due to primarily security updates. + +This document describes the design of the API that would allow us to +upload to the hypervisor binary patches. + +The document is split in four sections: + - Detailed descriptions of the problem statement. + - Design of the data structures. + - Design of the hypercalls. + - Implementation notes that should be taken into consideration. + + +## Glossary + + * splice - patch in the binary code with new opcodes + * trampoline - a jump to a new instruction. + * payload - telemetries of the old code along with binary blob of the new + function (if needed). + * reloc - telemetries contained in the payload to construct proper trampoline. + +## Multiple ways to patch + +The mechanism needs to be flexible to patch the hypervisor in multiple ways +and be as simple as possible. The compiled code is contiguous in memory with +no gaps - so we have no luxury of 'moving' existing code and must either +insert a trampoline to the new code to be executed - or only modify in-place +the code if there is sufficient space. The placement of new code has to be done +by hypervisor and the virtual address for the new code is allocated dynamically. + +This implies that the hypervisor must compute the new offsets when splicing +in the new trampoline code. Where the trampoline is added (inside +the function we are patching or just the callers?) is also important. + +To lessen the amount of code in hypervisor, the consumer of the API +is responsible for identifying which mechanism to employ and how many locations +to patch. Combinations of modifying in-place code, adding trampoline, etc +has to be supported. The API should allow read/write any memory within +the hypervisor virtual address space. + +We must also have a mechanism to query what has been applied and a mechanism +to revert it if needed. + +We must also have a mechanism to: (optional) provide an copy of the old code - so +that the hypervisor can verify it against the code in memory; the new code; +the symbol name of the function to be patched; or offset from the symbol; +or virtual address. + +The complications that this design will encounter are explained later +in this document. + +## Workflow + + +The expected workflows of higher-level tools that manage multiple patches +on production machines would be: + + * The first obvious task is loading all available / suggested + hotpatches around system start. + * Whenever new hotpatches are installed, they should be loaded too. + * One wants to query which modules have been loaded at runtime. + * If unloading is deemed safe (see unloading below), one may want to + support a workflow where a specific hotpatch is marked as bad and + unloaded. + * If we do no restrict module activation order and want to report tboot + state on sequences, we might have a complexity explosion problem, in + what system hashes should be considered acceptable. + +## Patching code + +The first mechanism to patch that comes in mind is in-place replacement. +That is replace the affected code with new code. Unfortunately the x86 +ISA is variable size which places limits on how much space we have available +to replace the instructions. That is not a problem if the change is smaller +than the original opcode and we can fill it with nops. Problems will +appear if the replacement code is longer. + +The second mechanism is by replacing the call or jump to the +old function with the address of the new function. + +A third mechanism is to add a jump to the new function at the +start of the old function. + +### Example of trampoline and in-place splicing + +As example we will assume the hypervisor does not have XSA-132 (see +*domctl/sysctl: don't leak hypervisor stack to toolstacks* +4ff3449f0e9d175ceb9551d3f2aecb59273f639d) and we would like to binary patch +the hypervisor with it. The original code looks as so: + +<pre> + 48 89 e0 mov %rsp,%rax + 48 25 00 80 ff ff and $0xffffffffffff8000,%rax +</pre> + +while the new patched hypervisor would be: + +<pre> + 48 c7 45 b8 00 00 00 00 movq $0x0,-0x48(%rbp) + 48 c7 45 c0 00 00 00 00 movq $0x0,-0x40(%rbp) + 48 c7 45 c8 00 00 00 00 movq $0x0,-0x38(%rbp) + 48 89 e0 mov %rsp,%rax + 48 25 00 80 ff ff and $0xffffffffffff8000,%rax +</pre> + +This is inside the arch_do_domctl. This new change adds 21 extra +bytes of code which alters all the offsets inside the function. To alter +these offsets and add the extra 21 bytes of code we might not have enough +space in .text to squeeze this in. + +As such we could simplify this problem by only patching the site +which calls arch_do_domctl: + +<pre> +<do_domctl>: + e8 4b b1 05 00 callq ffff82d08015fbb9 <arch_do_domctl> +</pre> + +with a new address for where the new `arch_do_domctl` would be (this +area would be allocated dynamically). + +Astute readers will wonder what we need to do if we were to patch `do_domctl` +- which is not called directly by hypervisor but on behalf of the guests via +the `compat_hypercall_table` and `hypercall_table`. +Patching the offset in `hypercall_table` for `do_domctl: +(ffff82d080103079 <do_domctl>:) +<pre> + + ffff82d08024d490: 79 30 + ffff82d08024d492: 10 80 d0 82 ff ff + +</pre> +with the new address where the new `do_domctl` is possible. The other +place where it is used is in `hvm_hypercall64_table` which would need +to be patched in a similar way. This would require an in-place splicing +of the new virtual address of `arch_do_domctl`. + +In summary this example patched the callee of the affected function by + * allocating memory for the new code to live in, + * changing the virtual address in all the functions which called the old + code (computing the new offset, patching the callq with a new callq). + * changing the function pointer tables with the new virtual address of + the function (splicing in the new virtual address). Since this table + resides in the .rodata section we would need to temporarily change the + page table permissions during this part. + + +However it has severe drawbacks - the safety checks which have to make sure +the function is not on the stack - must also check every caller. For some +patches this could mean - if there were an sufficient large amount of +callers - that we would never be able to apply the update. + +### Example of different trampoline patching. + +An alternative mechanism exists where we can insert a trampoline in the +existing function to be patched to jump directly to the new code. This +lessens the locations to be patched to one but it puts pressure on the +CPU branching logic (I-cache, but it is just one unconditional jump). + +For this example we will assume that the hypervisor has not been compiled +with fe2e079f642effb3d24a6e1a7096ef26e691d93e (XSA-125: *pre-fill structures +for certain HYPERVISOR_xen_version sub-ops*) which mem-sets an structure +in `xen_version` hypercall. This function is not called **anywhere** in +the hypervisor (it is called by the guest) but referenced in the +`compat_hypercall_table` and `hypercall_table` (and indirectly called +from that). Patching the offset in `hypercall_table` for the old +`do_xen_version` (ffff82d080112f9e <do_xen_version>) + +</pre> + ffff82d08024b270 <hypercall_table> + ... + ffff82d08024b2f8: 9e 2f 11 80 d0 82 ff ff + +</pre> +with the new address where the new `do_xen_version` is possible. The other +place where it is used is in `hvm_hypercall64_table` which would need +to be patched in a similar way. This would require an in-place splicing +of the new virtual address of `do_xen_version`. + +An alternative solution would be to patch insert a trampoline in the +old `do_xen_version' function to directly jump to the new `do_xen_version`. + +<pre> + ffff82d080112f9e <do_xen_version>: + ffff82d080112f9e: 48 c7 c0 da ff ff ff mov $0xffffffffffffffda,%rax + ffff82d080112fa5: 83 ff 09 cmp $0x9,%edi + ffff82d080112fa8: 0f 87 24 05 00 00 ja ffff82d0801134d2 <do_xen_version+0x534> +</pre> + +with: + +<pre> + ffff82d080112f9e <do_xen_version>: + ffff82d080112f9e: e9 XX YY ZZ QQ jmpq [new do_xen_version] +</pre> + +which would lessen the amount of patching to just one location. + +In summary this example patched the affected function to jump to the +new replacement function which required: + * allocating memory for the new code to live in, + * inserting trampoline with new offset in the old function to point to the + new function. + * Optionally we can insert in the old function a trampoline jump to an function + providing an BUG_ON to catch errant code. + +The disadvantage of this are that the unconditional jump will consume a small +I-cache penalty. However the simplicity of the patching and higher chance +of passing safety checks make this a worthwhile option. + +### Security + +With this method we can re-write the hypervisor - and as such we **MUST** be +diligent in only allowing certain guests to perform this operation. + +Furthermore with SecureBoot or tboot, we **MUST** also verify the signature +of the payload to be certain it came from a trusted source and integrity +was intact. + +As such the hypercall **MUST** support an XSM policy to limit what the guest +is allowed to invoke. If the system is booted with signature checking the +signature checking will be enforced. + +## Design of payload format + +The payload **MUST** contain enough data to allow us to apply the update +and also safely reverse it. As such we **MUST** know: + + * (optional) What the old code is expected to be. We **MUST** be able verify it + against the runtime code if old code is included in the payload. + * Verify the build-id of hypervisor against the payload build-id. + * The locations in memory to be patched. This can be determined dynamically + via symbols or via virtual addresses. + * The new code (or data) that will be patched in. + * Signature to verify the payload. + +This binary format can be constructed using an custom binary format but +there are severe disadvantages of it: + + * The format might need to be changed and we need an mechanism to accommodate + that. + * It has to be platform agnostic. + * Easily constructed using existing tools. + +As such having the payload in an ELF file is the sensible way. We would be +carrying the various sets of structures (and data) in the ELF sections under +different names and with definitions. The prefix for the ELF section name +would always be: *.xsplice* to match up to the names of the structures. + +Note that every structure has padding. This is added so that the hypervisor +can re-use those fields as it sees fit. + +Earlier design attempted to ineptly explain the relations of the ELF sections +to each other without using proper ELF mechanism (sh_info, sh_link, data +structures using Elf_* types, etc). This design will explain in detail +the structures and how they are used together and not dig in the ELF +format - except mention that the section names should match the +structure names. + +### ASCII art of structures. + +The diagram below is omitting some entries to easy the relationship explanation. + +<pre> + /---------------------\ + +->| xsplice_reloc_howto | + / \---------------------/ + /---------------\ 1:1/ + +->| xsplice_reloc | / + / | - howto +--/ 1:1 /----------------\ + / | - symbol +-------->| xsplice_symbol | + 1:N / \---------------/ / \----------------/ +/----------\ /--------------\ / / +| xsplice | 1:1 | xsplice_code | / 1:1/ +| - new +------->| - relocs +---/ 1:N /-----------------\ / +| - old +------->| - sections +----------->| xsplice_section | / +\----------/ | - patches +--\ | - symbol +/ 1:1 /----------------\ + \--------------/ \ | - addr +------->| .text or .data | + \ \----------------/ \----------------/ + \ + 1:N \ + \ /----------------\ + +-->| xsplice_patch | 1:1 /----------------\ + | - content +------>| binary code or | + \----------------/ | data | + \----------------/ + +</pre> + +### xsplice structures + +From the top (or left in the above diagram) the structures are: + + * `xsplice`. The top most structure - contains the the name of the update, + the id to match against the hypervisor, the pointer to the metadata for + the new code and optionally the metadata for the old code. + + * `xsplice_code`. The structure that ties all of this together and defines + the payload. Contains arrays of `xsplice_reloc`, `xsplice_section`, and + `xsplice_patch`. + + * `xsplice_reloc` contains telemetry used for patching - which describes the + targets to be patched and how to do it. + + * `xsplice_section` - the safety data for the code. Contains pointer to the + symbol (`xsplice_symbols`) and pointer to the code (`.text`) or data (`.data`), + which are to be used during safety and dependency checking. + + * `xsplice_patch`: the description of the new function to be patched in + along with the binary code or data. + + * ` xsplice_reloc_howto`: the howto properly construct trampolines for an patch. + We may have multiple locations for which we need to insert a trampoline for a + payload and each location might require a different way of handling it. + + * `xsplice_symbols `. The symbol that will be patched. + +In short the *.xsplice* sections (with `xsplice` being the top) represent +various structures to define the new code and safety checks for the old +code (optional). The ELF provides the mechanism to glue it all together when +loaded in memory. + + +Note that a lot of these ideas are borrowed from kSplice which is +available at: https://github.com/jirislaby/ksplice + +### struct xsplice + +The top most structure is quite simple. It defines the name, the id +of the hypervisor, pointer to the new code & data and an pointer to +the old code & data (optional). + +The `new` code uses all of the `xsplice_*` structures while the +`old` does not use the `xsplice_reloc` structures. + +The sections defining the structures will explicitly state +when they are not used. + +<pre> +struct xsplice { + const char *name; /* A sensible name for the patch. Up to 40 characters. */ + const char *build_id; /* ID of the hypervisor this binary was built against. */ + uint32_t version; /* Version of payload. */ + uint32_t id_size; /* Size of the ID. */ + struct xsplice_code *new; /* Pointer to the new code & data to be patched. */ + struct xsplice_code *old; /* Pointer to the old code & data to be checked against. */ + uint8_t pad[24]; /* Must be zero. */ +}; +</pre> + +The size of this structure should be 64 bytes. + +### xsplice_code + +The structure embedded within this section ties the other +structures together. It has the pointers with an start and end +address for each set of structures. This means that an update +can be split in multiple changes - for example to accomodate +an update that contains both code and data and will need patching +in both .text and .data sections. + +<pre> +struct xsplice_code { + struct xsplice_reloc *relocs; /* How to patch it. */ + struct xsplice_section *sections; /* Safety data. */ + struct xsplice_patch *patches; /* Patch code and data */ + uint32_t n_relocs; + uint32_t n_sections; + uint32_t n_patches; + uint8_t pad[28]; /* Must be zero. */ +}; +</pre> + +The size of this structure is 64 bytes. + +There can be at most two of those structures in the payload. +One for the `new` and another for the `old` (optional). + +If it is for the old code the relocs, and relocs_end values will be ignored. + + +### xsplice_reloc + +The `xsplice_code` defines an array of these structures. As such +an singular structure defines an singular point where to patch the +hypervisor. + +The structure contains the address of the hypervisor (if known), +the symbol associated with this address, how the patching is to +be done, and platform specific details. + +The `isns_added` is an value to be used to compute the new offset +due to the quirks of the operands of the instruction. For example +to patch in an jump operation to the new code - the offset is relative +to the program counter of the next instruction - hence the offset +value has to be subtracted by four bytes - hence this would contain -4 . + +The `isns_target` is the offset against the symbol. + +The relation of this structure with `xsplice_patch` is 1:1, even +for inline patches. See the section detailing the structure +`xsplice_reloc_howto`. + +The relation of this structure with `xsplice_section` is 1:1. + +This structure is as follow: + +<pre> +struct xsplice_reloc { + uint64_t addr; /* The address of the relocation (if known). */ + struct xsplice_symbol *symbol; /* Symbol for this relocation. */ + int64_t isns_target; /* rest of the ELF addend. This is equal to the offset against the symbol that the relocation refers to. */ + struct xsplice_reloc_howto *howto; /* Pointer to the above structure. */ + int64_t isns_added; /* ELF addend resulting from quirks of instruction one of whose operands is the relocation. For example, this is -4 on x86 pc-relative jumps. */ + uint8_t pad[24]; /* Must be zero. */ +}; + +</pre> + +The size of this structure is 64 bytes. + +### xsplice_section + +The structure defined in this section is used during pre-patching and +during patching. Pre-patching it is used to verify that it is safe +to update with the new changes - and contains safety data on the old code +and what kind of matching we are to expect. + +That is whether the address (either provided or resolved when payload is +loaded by referencing the symbols) is: + + * in memory, + * correct size, + * in it's proper ELF section, + * has been already patched (or not), + * is expected not to be on any CPU stack - (or if it is OK for it be on the CPU stack). + +with what we expect it to be. + +Some of the checks can be relaxed, as such the `flag` values +can be or-ed together. + +Depending on the time when patching is done, stack checking might not +be required. +<pre> + +#define XSPLICE_SECTION_TEXT 0x00000001 /* Section is in .text */ +#define XSPLICE_SECTION_RODATA 0x00000002 /* Section is in .rodata */ +#define XSPLICE_SECTION_DATA 0x00000004 /* Section is in .data */ +#define XSPLICE_SECTION_STRING 0x00000008 /* Section is in .str */ + +#define XSPLICE_SECTION_TEXT_INLINE 0x00000200 /* Change is to be inline. */ +#define XSPLICE_SECTION_MATCH_EXACT 0x00000400 /* Must match exactly. */ +#define XSPLICE_SECTION_NO_STACKCHECK 0x00000800 /* Do not check the stack. */ + + +struct xsplice_section { + struct xsplice_symbol *symbol; /* The symbol associated with this change. */ + uint64_t address; /* The address of the section (if known). */ + uint32_t size; /* The size of the section. */ + uint32_t flags; /* Various XSPLICE_SECTION_* flags. */ + uint8_t pad[2]; /* To be zero. */ +}; + +</pre> + +The size of this structure is 32 bytes. + +### xsplice_patch + +This structure has the binary code (or data) to be patched. Depending on the +type it can either an inline patch (data or text) or require an relocation +change (which requires a trampoline). Naturally it also points to a blob +of the binary data to patch in, and the size of the patch. + +The `addr` is used when the patch is for inline change. It can be +an virtual address or an offset from the symbol start. + +If it is an relocation (requiring a trampoline), the `addr` should be zero. + +There must be an corresponding ` struct xsplice_reloc` and +`struct xsplice_section` describing this patch. + +<pre> +#define XSPLICE_PATCH_INLINE_TEXT 0x1 +#define XSPLICE_PATCH_INLINE_DATA 0x2 +#define XSPLICE_PATCH_RELOC_TEXT 0x3 + +struct xsplice_patch { + uint32_t type; /* XSPLICE_PATCH_* .*/ + uint32_t size; /* Size of patch. */ + uint64_t addr; /* The address (or offset from symbol) of the inline new code (or data). */ + void *content; /* The bytes to be installed. */ + uint8_t pad[40]; /* Must be zero. */ +}; + +</pre> + +The size of this structure is 64 bytes. + +### xsplice_symbols + +The structure contains an pointer to the name of the ELF symbol +to be patched and as well an unique name for the symbol. + +The `label` is used for diagnostic purposes - such as including the +name and the offset. + +The structure is as follow: + +<pre> +struct xsplice_symbol { + const char *name; /* The ELF name of the symbol. */ + const char *label; /* A unique xSplice name for the symbol. */ + uint8_t pad[16]; /* Must be zero. */ +}; +</pre> + +The size of this structure is 32 bytes. + + +### xsplice_reloc_howto + +The howto defines in the detail the change. It contains the type, +whether the relocation is relative, the size of the relocation, +bitmask for which parts of the instruction or data are to be replaced, +amount the final relocation is shifted by (to drop unwanted data), and +whether the replacement should be interpreted as signed value. + +The structure is as follow: + +<pre> +#define XSPLICE_HOWTO_INLINE 0x1 /* It is an inline replacement. */ +#define XSPLICE_HOWTO_RELOC_PATCH 0x2 /* Add a trampoline. */ + +#define XSPLICE_HOWTO_FLAG_PC_REL 0x1 /* Is PC relative. */ +#define XSPLICE_HOWOT_FLAG_SIGN 0x2 /* Should the new value be treated as signed value. */ + +struct xsplice_reloc_howto { + uint32_t howto; /* XSPLICE_HOWTO_* */ + uint32_t flag; /* XSPLICE_HOWTO_FLAG_* */ + uint32_t size; /* Size, in bytes, of the item to be relocated. */ + uint32_t r_shift; /* The value the final relocation is shifted right by; used to drop unwanted data from the relocation. */ + uint64_t mask; /* Bitmask for which parts of the instruction or data are replaced with the relocated value. */ + uint8_t pad[8]; /* Must be zero. */ +}; + +</pre> + +The size of this structure is 32 bytes. + +### Example + +There is a wealth of information that the payload must have to define a simple +patch. For this example we will assume that the hypervisor has not been compiled +with fe2e079f642effb3d24a6e1a7096ef26e691d93e (XSA-125: *pre-fill structures +for certain HYPERVISOR_xen_version sub-ops*) which mem-sets an structure +in `xen_version` hypercall. This function is not called **anywhere** in +the hypervisor (it is called by the guest) but referenced in the +`compat_hypercall_table` and `hypercall_table` (and indirectly called +from that). There are two ways to patch this: +inline patch `hvm_hypercall64_table` and `hvm_hypercall` with a new +address for the new `do_xen_version` , or insert +trampoline in `do_xen_version` code. The example will focus on the later. + +The `do_xen_version` code is located at virtual address ffff82d080112f9e. + +<pre> +struct xsplice_code xsplice_xsa125; +struct xsplice_reloc relocs[1]; +struct xsplice_section sections[1]; +struct xsplice_patch patches[1]; +struct xsplice_symbol do_xen_version_symbol; +struct xsplice_reloc_howto do_xen_version_howto; +char do_xen_version_new_code[1728]; + +#ifndef BUILD_ID +#define BUILD_ID "92dd05a61556c554155b1508c9cf67d993336d28" +#endif + +struct xsplice xsa125 = { + .name = "xsa125", + .build_id = BUILD_ID, + .old = NULL, + .new = &xsplice_xsa125, +}; + +struct xsplice_code xsplice_xsa125 = { + .relocs = &relocs[0], + .n_relocs = 1, + .sections = §ions[0], + .n_sections = 1, + .patches = &patches[0], + .n_patches = 1, +}; + +struct xsplice_reloc relocs[1] = { + { + .addr = 0xffff82d080112f9e, + .symbol = &do_xen_version_symbol, + .isns_target = 0, + .howto = &do_xen_version_howto, + .isns_added = -4, + }, +}; + +struct xsplice_symbol do_xen_version_symbol = { + .name = "do_xen_version", + .label = "do_xen_version+<0x0>", +}; + +struct xsplice_reloc_howto do_xen_version_howto = { + .type = XSPLICE_HOWTO_RELOC_PATCH, + .flag = XSPLICE_HOWTO_FLAG_PC_REL, + .r_shift = 0, + .mask = (-1ULL), +}; + + +struct xsplice_section sections[1] = { + { + .symbol = &do_xen_version_symbol, + .address = 0xffff82d080112f9e, + .size = 1728, + .flags = XSPLICE_SECTION_TEXT, + }, +}; + +struct xsplice_patch patches[1] = { + { + .type = XSPLICE_PATCH_RELOC_TEXT, + .size = 1728, + .addr = 0, + .content = &do_xen_version_new_code, + }, +}; + +char do_xen_version_new_code[1728] = { 0x83, 0xff, 0x09, /* And more code. */}; +</pre> + + +## Signature checking requirements. + +The signature checking requires that the layout of the data in memory +**MUST** be same for signature to be verified. This means that the payload +data layout in ELF format **MUST** match what the hypervisor would be +expecting such that it can properly do signature verification. + +The signature is based on the all of the payloads continuously laid out +in memory. The signature is to be appended at the end of the ELF payload +prefixed with the string '~Module signature appended~\n', followed by +an signature header then followed by the signature, key identifier, and signers +name. + +Specifically the signature header would be: + +<pre> +#define PKEY_ALGO_DSA 0 +#define PKEY_ALGO_RSA 1 + +#define PKEY_ID_PGP 0 /* OpenPGP generated key ID */ +#define PKEY_ID_X509 1 /* X.509 arbitrary subjectKeyIdentifier */ + +#define HASH_ALGO_MD4 0 +#define HASH_ALGO_MD5 1 +#define HASH_ALGO_SHA1 2 +#define HASH_ALGO_RIPE_MD_160 3 +#define HASH_ALGO_SHA256 4 +#define HASH_ALGO_SHA384 5 +#define HASH_ALGO_SHA512 6 +#define HASH_ALGO_SHA224 7 +#define HASH_ALGO_RIPE_MD_128 8 +#define HASH_ALGO_RIPE_MD_256 9 +#define HASH_ALGO_RIPE_MD_320 10 +#define HASH_ALGO_WP_256 11 +#define HASH_ALGO_WP_384 12 +#define HASH_ALGO_WP_512 13 +#define HASH_ALGO_TGR_128 14 +#define HASH_ALGO_TGR_160 15 +#define HASH_ALGO_TGR_192 16 + + +struct elf_payload_signature { + u8 algo; /* Public-key crypto algorithm PKEY_ALGO_*. */ + u8 hash; /* Digest algorithm: HASH_ALGO_*. */ + u8 id_type; /* Key identifier type PKEY_ID*. */ + u8 signer_len; /* Length of signer's name */ + u8 key_id_len; /* Length of key identifier */ + u8 __pad[3]; + __be32 sig_len; /* Length of signature data */ +}; + +</pre> +(Note that this has been borrowed from Linux module signature code.). + + +## Hypercalls + +We will employ the sub operations of the system management hypercall (sysctl). +There are to be four sub-operations: + + * upload the payloads. + * listing of payloads summary uploaded and their state. + * getting an particular payload summary and its state. + * command to apply, delete, or revert the payload. + * querying of the hypervisor build ID. + +Most of the actions are asynchronous therefore the caller is responsible +to verify that it has been applied properly by retrieving the summary of it +and verifying that there are no error codes associated with the payload. + +We **MUST** make some of them asynchronous due to the nature of patching +it requires every physical CPU to be lock-step with each other. +The patching mechanism while an implementation detail, is not an short +operation and as such the design **MUST** assume it will be an long-running +operation. + +The sub-operations will spell out how preemption is to be handled (if at all). + +Furthermore it is possible to have multiple different payloads for the same +function. As such an unique id per payload has to be visible to allow proper manipulation. + +The hypercall is part of the `xen_sysctl`. The top level structure contains +one uint32_t to determine the sub-operations: + +<pre> +struct xen_sysctl_xsplice_op { + uint32_t cmd; + union { + ... see below ... + } u; +}; + +</pre> +while the rest of hypercall specific structures are part of the this structure. + +### Basic type: struct xen_xsplice_id + +Most of the hypercalls employ an shared structure called `struct xen_xsplice_id` +which contains: + + * `name` - pointer where the string for the id is located. + * `size` - the size of the string + * `_pad` - padding - to be zero. + +The structure is as follow: + +<pre> +#define XEN_XSPLICE_ID_SIZE 128 +struct xen_xsplice_id { + XEN_GUEST_HANDLE_64(char) name; /* IN, pointer to name. */ + uint32_t size; /* IN, size of name. May be upto + XEN_XSPLICE_ID_SIZE. */ + uint32_t _pad; +}; +</pre> +### XEN_SYSCTL_XSPLICE_UPLOAD (0) + +Upload a payload to the hypervisor. The payload is verified +against basic checks and if there are any issues the proper return code +will be returned. The payload is not applied at this time - that is +controlled by *XEN_SYSCTL_XSPLICE_ACTION*. + +The caller provides: + + * A `struct xen_xsplice_id` called `id` which has the unique id. + * `size` the size of the ELF payload (in bytes). + * `payload` the virtual address of where the ELF payload is. + +The `id` could be an UUID in mind that stays fixed forever for a given +hotpatch. It can be embedded into the Elf payload at creation time +and extracted by tools. + +The return value is zero if the payload was succesfully uploaded. +Otherwise an EXX return value is provided. Duplicate `id` are not supported. +The payload at this point is verified against the basic checks. + +The `payload` is the ELF payload as mentioned in the `Payload format` section. + +The structure is as follow: + +<pre> +struct xen_sysctl_xsplice_upload { + xen_xsplice_id_t id; /* IN, name of the patch. */ + uint64_t size; /* IN, size of the ELF file. */ + XEN_GUEST_HANDLE_64(uint8) payload; /* IN: ELF file. */ +}; +</pre> + +### XEN_SYSCTL_XSPLICE_GET (1) + +Retrieve an status of an specific payload. This caller provides: + + * A `struct xen_xsplice_id` called `id` which has the unique id. + * A `struct xen_xsplice_status` structure which has all members + set to zero: That is: + * `status` *MUST* be set to zero. + * `_pad` *MUST* be set to zero. + +Upon completion the `struct xen_xsplice_status` is updated. + + * `_pad` - reserved for further usage. + * `status` - whether it has been: + * *XSPLICE_STATUS_LOADED* (0x1) has been loaded. + * *XSPLICE_STATUS_PROGRESS* (0x2) acting on the **XEN_SYSCTL_XSPLICE_ACTION** command. + * *XSPLICE_STATUS_CHECKED* (0x4) the ELF payload safety checks passed. + * *XSPLICE_STATUS_APPLIED* (0x8) loaded, checked, and applied. + * *XSPLICE_STATUS_REVERTED* (0x10) loaded, checked, applied and then also reverted. + * Negative values is an error. The error would be of EXX format. + +The return value is zero on success and EXX on failure. This operation +is synchronous and does not require preemption. + +If the status has a negative value more details on the payload failure +can be retrieved via **XEN_SYSCTL_XSPLICE_INFO** hypercall. + +The structure is as follow: + +<pre> +struct xen_xsplice_status { +#define XSPLICE_STATUS_LOADED 0x01 +#define XSPLICE_STATUS_PROGRESS 0x02 +#define XSPLICE_STATUS_CHECKED 0x04 +#define XSPLICE_STATUS_APPLIED 0x08 +#define XSPLICE_STATUS_REVERTED 0x10 + /* Any negative value is an error. The error would be in -EXX format. */ + int32_t status; /* OUT, On IN has to be zero. */ + uint32_t _pad; /* IN: Must be zero. */ +}; + +struct xen_sysctl_xsplice_summary { + xen_xsplice_id_t id; /* IN, the name of the payload. */ + xen_xsplice_status_t status; /* IN/OUT: status of the payload. */ +}; +</pre> + +### XEN_SYSCTL_XSPLICE_LIST (2) + +Retrieve an array of abbreviated status and names of payloads that are loaded in the +hypervisor. + +The caller provides: + + * `version`. Initially (on first hypercall) *MUST* be zero. + * `idx` index iterator. On first call *MUST* be zero, subsequent calls varies. + * `nr` the max number of entries to populate. + * `_pad` - *MUST* be zero. + * `status` virtual address of where to write `struct xen_xsplice_status` + structures. *MUST* allocate up to `nr` of them. + * `id` - virtual address of where to write the unique id of the payload. + *MUST* allocate up to `nr` of them. Each *MUST* be of + **XEN_XSPLICE_ID_SIZE** size. + * `len` - virtual address of where to write the length of each unique id + of the payload. *MUST* allocate up to `nr` of them. Each *MUST* be + of sizeof(uint32_t) (4 bytes). + +If the hypercall returns an positive number, it is the number (up to `nr`) +of the payloads returned, along with `nr` updated with the number of remaining +payloads, `version` updated (it may be the same across hypercalls. If it +varies the data is stale and further calls could fail). The `status`, +`id`, and `len`' are updated at their designed index value (`idx`) with +the returned value of data. + +If the hypercall returns E2BIG the `count` is too big and should be +lowered. + +This operation can be preempted by the hypercall returning EAGAIN. +Retry. + +Note that due to the asynchronous nature of hypercalls the domain might have +added or removed the number of payloads making this information stale. It is +the responsibility of the toolstack to use the `version` field to check +between each invocation. if the version differs it should discard the stale +data and start from scratch. It is OK for the toolstack to use the new +`version` field. + +The `struct xen_xsplice_status` structure contains an status of payload which includes: + + * `status` - whether it has been: + * *XSPLICE_STATUS_LOADED* (0x1) has been loaded. + * *XSPLICE_STATUS_PROGRESS* (0x2) acting on the **XEN_SYSCTL_XSPLICE_ACTION** command. + * *XSPLICE_STATUS_CHECKED* (0x4) the ELF payload safety checks passed. + * *XSPLICE_STATUS_APPLIED* (0x8) loaded, checked, and applied. + * *XSPLICE_STATUS_REVERTED* (0x10) loaded, checked, applied and then also reverted. + * Any negative values means there has been error. The value is in EXX format. + +If the status has a negative value more details on the payload failure +can be retrieved via **XEN_SYSCTL_XSPLICE_INFO** hypercall. + +The structure is as follow: + +<pre> +struct xen_sysctl_xsplice_list { + uint32_t version; /* IN/OUT: Initially *MUST* be zero. + On subsequent calls reuse value. + If varies between calls, we are + * getting stale data. */ + uint32_t idx; /* IN/OUT: Index into array. */ + uint32_t nr; /* IN: How many status, id, and len + should populate. + OUT: How many payloads left. */ + uint32_t _pad; /* IN: Must be zero. */ + XEN_GUEST_HANDLE_64(xen_xsplice_status_t) status; /* OUT. Must have enough + space allocate for n of them. */ + XEN_GUEST_HANDLE_64(char) id; /* OUT: Array of ids. Each member + MUST XEN_XSPLICE_ID_SIZE in size. + Must have n of them. */ + XEN_GUEST_HANDLE_64(uint32) len; /* OUT: Array of lengths of ids. + Must have n of them. */ +}; +</pre> + +### XEN_SYSCTL_XSPLICE_ACTION (3) + +Perform an operation on the payload structure referenced by the `id` field. +The operation request is asynchronous and the status should be retrieved +by using either **XEN_SYSCTL_XSPLICE_GET** or **XEN_SYSCTL_XSPLICE_LIST** hypercall. +If the operation fails more details on the operation can be retrieved via +**XEN_SYSCTL_XSPLICE_INFO** hypercall. + +The caller provides: + + * A 'struct xen_xsplice_id` `id` containing the unique id. + * `cmd` the command requested: + * *XSPLICE_ACTION_CHECK* (1) check that the payload will apply properly. + This also verfies the payload - which may require SecureBoot firmware + calls. + * *XSPLICE_ACTION_UNLOAD* (2) unload the payload. + Any further hypercalls against the `id` will result in failure unless + **XEN_SYSCTL_XSPLICE_UPLOAD** hypercall is perfomed with same `id`. + * *XSPLICE_ACTION_REVERT* (3) revert the payload. If the operation takes + more time than the upper bound of time the `status` will EBUSY. + * *XSPLICE_ACTION_APPLY* (4) apply the payload. If the operation takes + more time than the upper bound of time the `status` will be EBUSY. + * *XSPLICE_ACTION_LOADED* is an initial state and cannot be requested. + * `time` the upper bound of time the cmd should take. Zero means infinite. + If within the time the operation does not succeed the operation would go in + error state. + * `_pad` - *MUST* be zero. + +The return value will be zero unless the provided fields are incorrect. + +The structure is as follow: + +<pre> +#define XSPLICE_ACTION_CHECK 1 +#define XSPLICE_ACTION_UNLOAD 2 +#define XSPLICE_ACTION_REVERT 3 +#define XSPLICE_ACTION_APPLY 4 + +struct xen_sysctl_xsplice_action { + xen_xsplice_id_t id; /* IN, name of the patch. */ + uint32_t cmd; /* IN: XSPLICE_ACTION_* */ + uint32_t _pad; /* IN: MUST be zero. */ + uint64_t time; /* IN, upper bound of time (ms) for the operation to take. */ +}; + +</pre> + +### XEN_SYSCTL_XSPLICE_INFO (4) + +Retrieve information useful for the patching tools. + +The calleer provides: + + * `cmd` The command of the sub-command requested, which can be: + * **XEN_SYSCTL_XSPLICE_INFO_BUILD_ID** (0) - which request the build-id + of the hypervisor. + * **XEN_SYSCTL_XSPLICE_INFO_TRACE_CLEAR** (1) - clear the hypervisor + patching trace. + * **XEN_SYSCTL_XSPLICE_INFO_TRACE_GET** (2) - retrieve the trace. The + return value will be the number of bytes retrieved. Zero means end of trace. + * `size` - The size of the `info` char array. *MUST* not be zero. + * `info` - virtual address where to write requested information. + *MUST* not be zero. + +On completion if the return value contains an positive value it signifies +that this many bytes were written into `info`. + +The return value can also be EXX format. EINVAL if incorrect values have been provided, +ENOENT the requested information cannot be supplied, or any other error occurred. + +The structure is as a follow: +<pre> +#define XEN_SYSCTL_XSPLICE_INFO_BUILD_ID 0 +struct xen_sysctl_xsplice_info { + uint32_t cmd; /* IN: XEN_SYSCTL_XSPLICE_INFO_* */ + uint32_t size; /* IN: Size of info: OUT: Amount of + bytes filed out in info. */ + union { + XEN_GUEST_HANDLE_64(char) info; /* OUT: Requested information. */ + } u; +}; +</pre> + +## State diagrams of XSPLICE_ACTION values. + +There is a strict ordering state of what the commands can be. +The XSPLICE_ACTION prefix has been dropped to easy reading: + +<pre> + /->\ + \ / + /-------< CHECK + | | + | + + | UNLOAD<--\ + | \ + | \ + /-> APPLY -----------> REVERT --\ + | | + \-------------------------------/ + +</pre> +Or an state transition table of valid states: +<pre> ++-------+-------+--------+--------+---------+-------+------------------+ +| CHECK | APPLY | REVERT | UNLOAD | Current | Next | Result | ++-------+-------+--------+--------+---------+-------+------------------+ +| x | | | | LOADED | CHECK | Check payload. | ++-------+-------+--------+--------+---------+-------+------------------+ +| | | | x | LOADED | UNLOAD| unload payload. | ++-------+-------+--------+--------+---------+-------+------------------+ +| x | | | | CHECK | CHECK | Check payload. | ++-------+-------+--------+--------+---------+-------+------------------+ +| | x | | | CHECK | APPLY | Apply payload. | ++-------+-------+--------+--------+---------+-------+------------------+ +| | | | x | CHECK | UNLOAD| Unload payload. | ++-------+-------+--------+--------+---------+-------+------------------+ +| | | x | | APPLY | REVERT| Revert payload. | ++-------+-------+--------+--------+---------+-------+------------------+ +| | x | | | REVERT | APPLY | Apply payload. | ++-------+-------+--------+--------+---------+-------+------------------+ +| | | | x | REVERT | UNLOAD| Unload payload. | ++-------+-------+--------+--------+---------+-------+------------------+ +</pre> +All the other state transitions are invalid. + +## Sequence of events. + +The normal sequence of events is to: + + 1. *XEN_SYSCTL_XSPLICE_UPLOAD* to upload the payload. If there are errors *STOP* here. + 2. *XEN_SYSCTL_XSPLICE_GET* to check the `->status`. If in *XSPLICE_STATUS_PROGRESS* spin. If in *XSPLICE_STATUS_LOADED* go to next step. + 3. *XEN_SYSCTL_XSPLICE_ACTION* with *XSPLICE_ACTION_CHECK* command to verify that the payload can be succesfully applied. + 4. *XEN_SYSCTL_XSPLICE_GET* to check the `->status`. If in *XSPLICE_STATUS_PROGRESS* spin. If in *XSPLICE_STATUS_CHECKED* go to next step. + 5. *XEN_SYSCTL_XSPLICE_ACTION* with *XSPLICE_ACTION_APPLY* to apply the patch. + 6. *XEN_SYSCTL_XSPLICE_GET* to check the `->status`. If in *XSPLICE_STATUS_PROGRESS* spin. If in *XSPLICE_STATUS_APPLIED* exit with success. + + +## Addendum + +Implementation quirks should not be discussed in a design document. + +However these observations can provide aid when developing against this +document. + + +### Alternative assembler + +Alternative assembler is a mechanism to use different instructions depending +on what the CPU supports. This is done by providing multiple streams of code +that can be patched in - or if the CPU does not support it - padded with +`nop` operations. The alternative assembler macros cause the compiler to +expand the code to place a most generic code in place - emit a special +ELF .section header to tag this location. During run-time the hypervisor +can leave the areas alone or patch them with an better suited opcodes. + +However these sections are part of .init. and as such can't reasonably be +subject to patching. + +### .rodata sections + +The patching might require strings to be updated as well. As such we must be +also able to patch the strings as needed. This sounds simple - but the compiler +has a habit of coalescing strings that are the same - which means if we in-place +alter the strings - other users will be inadvertently affected as well. + +This is also where pointers to functions live - and we may need to patch this +as well. And switch-style jump tables. + +To guard against that we must be prepared to do patching similar to +trampoline patching or in-line depending on the flavour. If we can +do in-line patching we would need to: + + * alter `.rodata` to be writeable. + * inline patch. + * alter `.rodata` to be read-only. + +If are doing trampoline patching we would need to: + + * allocate a new memory location for the string. + * all locations which use this string will have to be updated to use the + offset to the string. + * mark the region RO when we are done. + +### .bss and .data sections. + +Patching writable data is not suitable as it is unclear what should be done +depending on the current state of data. As such it should not be attempted. + + +### Patching code which is in the stack. + +We should not patch the code which is on the stack. That can lead +to corruption. + +### Inline patching + +The hypervisor should verify that the in-place patching would fit within +the code or data. + +### Trampoline (e9 opcode) + +The e9 opcode used for jmpq uses a 32-bit signed displacement. That means +we are limited to up to 2GB of virtual address to place the new code +from the old code. That should not be a problem since Xen hypervisor has +a very small footprint. + +However if we need - we can always add two trampolines. One at the 2GB +limit that calls the next trampoline. + +Please note there is a small limitation for trampolines in +function entries: The target function (+ trailing padding) must be able +to accomodate the trampoline. On x86 with +-2 GB relative jumps, +this means 5 bytes are required. + +Depending on compiler settings, there are several functions in Xen that +are smaller (without inter-function padding). + +<pre> +readelf -sW xen-syms | grep " FUNC " | \ + awk '{ if ($3 < 5) print $3, $4, $5, $8 }' + +... +3 FUNC LOCAL wbinvd_ipi +3 FUNC LOCAL shadow_l1_index +... +</pre> +A compile-time check for, e.g., a minimum alignment of functions or a +runtime check that verifies symbol size (+ padding to next symbols) for +that in the hypervisor is advised. + +### When to patch + +During the discussion on the design two candidates bubbled where +the call stack for each CPU would be deterministic. This would +minimize the chance of the patch not being applied due to safety +checks failing. + +#### Rendezvous code instead of stop_machine for patching + +The hypervisor's time rendezvous code runs synchronously across all CPUs +every second. Using the stop_machine to patch can stall the time rendezvous +code and result in NMI. As such having the patching be done at the tail +of rendezvous code should avoid this problem. + +However the entrance point for that code is +do_softirq->timer_softirq_action->time_calibration +which ends up calling on_selected_cpus on remote CPUs. + +The remote CPUs receive CALL_FUNCTION_VECTOR IPI and execute the +desired function. + + +#### Before entering the guest code. + +Before we call VMXResume we check whether any soft IRQs need to be executed. +This is a good spot because all Xen stacks are effectively empty at +that point. + +To randezvous all the CPUs an barrier with an maximum timeout (which +could be adjusted), combined with forcing all other CPUs through the +hypervisor with IPIs, can be utilized to have all the CPUs be lockstep. + +The approach is similar in concept to stop_machine and the time rendezvous +but is time-bound. However the local CPU stack is much shorter and +a lot more deterministic. + +### Compiling the hypervisor code + +Hotpatch generation often requires support for compiling the target +with -ffunction-sections / -fdata-sections. Changes would have to +be done to the linker scripts to support this. + + +### Generation of xSplice ELF payloads + +The design of that is not discussed in this design. + +The author of this design envisions objdump and objcopy along +with special GCC parameters (see above) to create .o.xsplice files +which can be used to splice an ELF with the new payload. + +The ksplice code can provide inspiration. + +### Exception tables and symbol tables growth + +We may need support for adapting or augmenting exception tables if +patching such code. Hotpatches may need to bring their own small +exception tables (similar to how Linux modules support this). + +If supporting hotpatches that introduce additional exception-locations +is not important, one could also change the exception table in-place +and reorder it afterwards. + + +### xSplice interdependencies + +xSplice patches interdependencies are tricky. + +There are the ways this can be addressed: + * A single large patch that subsumes and replaces all previous ones. + Over the life-time of patching the hypervisor this large patch + grows to accumulate all the code changes. + * Hotpatch stack - where an mechanism exists that loads the hotpatches + in the same order they were built in. We would need an build-id + of the hypevisor to make sure the hot-patches are build against the + correct build. + * Payload containing the old code to check against that. That allows + the hotpatches to be loaded indepedently (if they don't overlap) - or + if the old code also containst previously patched code - even if they + overlap. + +The disadvantage of the first large patch is that it can grow over +time and not provide an bisection mechanism to identify faulty patches. + +The hot-patch stack puts stricts requirements on the order of the patches +being loaded and requires an hypervisor build-id to match against. + +The old code allows much more flexibility and an additional guard, +but is more complex to implement. + +### Hypervisor ID (buid-id) + +The build-id can help with: + + * Prevent loading of wrong hotpatches (intended for other builds) + + * Allow to identify suitable hotpatches on disk and help with runtime + tooling (if laid out using build ID) + +The build-id (aka hypervisor id) can be easily obtained by utilizing +the ld --build-id operatin which (copied from ld): + +<pre> +--build-id + --build-id=style + Request creation of ".note.gnu.build-id" ELF note section. The contents of the note are unique bits identifying this + linked file. style can be "uuid" to use 128 random bits, "sha1" to use a 160-bit SHA1 hash on the normative parts of the + output contents, "md5" to use a 128-bit MD5 hash on the normative parts of the output contents, or "0xhexstring" to use a + chosen bit string specified as an even number of hexadecimal digits ("-" and ":" characters between digit pairs are + ignored). If style is omitted, "sha1" is used. + + The "md5" and "sha1" styles produces an identifier that is always the same in an identical output file, but will be + unique among all nonidentical output files. It is not intended to be compared as a checksum for the file's contents. A + linked file may be changed later by other tools, but the build ID bit string identifying the original linked file does + not change. + + Passing "none" for style disables the setting from any "--build-id" options earlier on the command line. + +</pre> + +### Symbol names + + +Xen as it is now, has a couple of non-unique symbol names which will +make runtime symbol identification hard. Sometimes, static symbols +simply have the same name in C files, sometimes such symbols get +included via header files, and some C files are also compiled +multiple times and linked under different names (guest_walk.c). + +As such we need to modify the linker to make sure that the symbol +table qualifies also symbols by their source file name. + +For the awkward situations in which C-files are compiled multiple +times patches we would need to some modification in the Xen code. + + +The convention for file-type symbols (that would allow to map many +symbols to their compilation unit) says that only the basename (i.e., +without directories) is embedded. This creates another layer of +confusion for duplicate file names in the build tree. + +That would have to be resolved. + +<pre> +> find . -name \*.c -print0 | xargs -0 -n1 basename | sort | uniq -c | sort -n | tail -n10 + 3 shutdown.c + 3 sysctl.c + 3 time.c + 3 xenoprof.c + 4 gdbstub.c + 4 irq.c + 5 domain.c + 5 mm.c + 5 pci.c + 5 traps.c +</pre> + +### Security + +Only the privileged domain should be allowed to do this operation. + + +### Handle inlined __LINE__ + + +This problem is related to hotpatch construction +and potentially has influence on the design of the hotpatching +infrastructure in Xen. + +For example: + +We have file1.c with functions f1 and f2 (in that order). f2 contains a +BUG() (or WARN()) macro and at that point embeds the source line number +into the generated code for f2. + +Now we want to hotpatch f1 and the hotpatch source-code patch adds 2 +lines to f1 and as a consequence shifts out f2 by two lines. The newly +constructed file1.o will now contain differences in both binary +functions f1 (because we actually changed it with the applied patch) and +f2 (because the contained BUG macro embeds the new line number). + +Without additional information, an algorithm comparing file1.o before +and after hotpatch application will determine both functions to be +changed and will have to include both into the binary hotpatch. + +Options: + +1. Transform source code patches for hotpatches to be line-neutral for + each chunk. This can be done in almost all cases with either + reformatting of the source code or by introducing artificial + preprocessor "#line n" directives to adjust for the introduced + differences. + + This approach is low-tech and simple. Potentially generated + backtraces and existing debug information refers to the original + build and does not reflect hotpatching state except for actually + hotpatched functions but should be mostly correct. + +2. Ignoring the problem and living with artificially large hotpatches + that unnecessarily patch many functions. + + This approach might lead to some very large hotpatches depending on + content of specific source file. It may also trigger pulling in + functions into the hotpatch that cannot reasonable be hotpatched due + to limitations of a hotpatching framework (init-sections, parts of + the hotpatching framework itself, ...) and may thereby prevent us + from patching a specific problem. + + The decision between 1. and 2. can be made on a patch--by-patch + basis. + +3. Introducing an indirection table for storing line numbers and + treating that specially for binary diffing. Linux may follow + this approach. + + We might either use this indirection table for runtime use and patch + that with each hotpatch (similarly to exception tables) or we might + purely use it when building hotpatches to ignore functions that only + differ at exactly the location where a line-number is embedded. + +Similar considerations are true to a lesser extent for __FILE__, but it +could be argued that file renaming should be done outside of hotpatches. + -- 2.1.0 _______________________________________________ Xen-devel mailing list Xen-devel@xxxxxxxxxxxxx http://lists.xen.org/xen-devel
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