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[Xen-devel] [PATCH v5 05/28] 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 shaped 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> Signed-off-by: Ross Lagerwall <ross.lagerwall@xxxxxxxxxx> --- Cc: Ian Jackson <ian.jackson@xxxxxxxxxxxxx> Cc: Jan Beulich <jbeulich@xxxxxxxx> Cc: Keir Fraser <keir@xxxxxxx> Cc: Tim Deegan <tim@xxxxxxx> v1-2: review v3: Split document in v1 and v2 (todo) to simplify implementation goals. - Add const on some structures. Truncate size to uint16_t where it makes sense. - Convert 'id' to 'name', Add Ross's comments about what is implemented. - Wei's and Ross's reviews. - Jan's review comments. - Jan's review comments. s/int32_t state/uint32_t state/ now that return code is in seperate field (rc). Add various other types, such as R_X86_64_PC64 in the list. Mention the need for compiler check. v4: - Drop the LOADED->CHECKED state and go directly to CHECKED state. Drop LOADED. v5: Julien mentioned ARM 32-bit would not use ELF64, so make the .xsplice.func use uintXX_t types instead of ELF ones. Remove the OUT on idx subfield. Mention that 'nr' being zero can be used for probing the number of payloads. Update what 'idx' means. --- docs/misc/xsplice.markdown | 1039 ++++++++++++++++++++++++++++++++++++++++++++ 1 file changed, 1039 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..a5303f0 --- /dev/null +++ b/docs/misc/xsplice.markdown @@ -0,0 +1,1039 @@ +# 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. + +## History + +The document has gone under various reviews and only covers v1 design. + +The end of the document has a section titled `Not Yet Done` which +outlines ideas and design for the future version of this work. + +## 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. + +## 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 when they are available. + * 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. + +## 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 ti replace 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. N.B. The Xen hypervisor implements the third +mechanism. See `Trampoline (e9 opcode)` section for more details. + +### 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 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. + +Having the patching done at predetermined instances where the stacks +are not deep mostly solves this problem. + +### 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. + +This patching has a similar drawback as inline patching - the safety +checks have to make sure the function is not on the stack. However +since we are replacing at a higher level (a full function as opposed +to various offsets within functions) the checks are simpler. + +Having the patching done at predetermined instances where the stacks +are not deep mostly solves this problem as well. + +### 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: + + * The locations in memory to be patched. This can be determined dynamically + via symbols or via virtual addresses. + * The new code that will be patched in. + +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. + +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 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. + +The xSplice payload is a relocatable ELF binary. A typical binary would have: + + * One or more .text sections. + * Zero or more read-only data sections. + * Zero or more data sections. + * Relocations for each of these sections. + +It may also have some architecture-specific sections. For example: + + * Alternatives instructions. + * Bug frames. + * Exception tables. + * Relocations for each of these sections. + +The xSplice core code loads the payload as a standard ELF binary, relocates it +and handles the architecture-specifc sections as needed. This process is much +like what the Linux kernel module loader does. + +The payload contains a section (xsplice_patch_func) with an array of structures +describing the functions to be patched: + +<pre> +struct xsplice_patch_func { + const char *name; + uint64_t new_addr; + uint64_t old_addr; + uint32_t new_size; + uint32_t old_size; + uint8_t pad[32]; +}; +</pre> + +The size of the structure is 64 bytes. + +* `name` is the symbol name of the old function. Only used if `old_addr` is + zero, otherwise will be used during dynamic linking (when hypervisor loads + the payload). + +* `old_addr` is the address of the function to be patched and is filled in at + payload generation time if hypervisor function address is known. If unknown, + the value *MUST* be zero and the hypervisor will attempt to resolve the address. + +* `new_addr` is the address of the function that is replacing the old + function. The address is filled in during relocation. The value **MUST** be + the address of the new function in the file. + +* `old_size` and `new_size` contain the sizes of the respective functions in bytes. + The value of `old_size` **MUST** not be zero. + +* `pad` **MUST** be zero. + +The size of the `xsplice_patch_func` array is determined from the ELF section +size. + +When applying the patch the hypervisor iterates over each `xsplice_patch_func` +structure and the core code inserts a trampoline at `old_addr` to `new_addr`. + +When reverting a patch, the hypervisor iterates over each `xsplice_patch_func` +and the core code copies the data from the undo buffer (private internal copy) +to `old_addr`. + +## 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. + +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 name 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 and one padding field which +*MUST* always be zero. + +<pre> +struct xen_sysctl_xsplice_op { + uint32_t cmd; /* IN: XEN_SYSCTL_XSPLICE_*. */ + uint32_t pad; /* IN: Always zero. */ + union { + ... see below ... + } u; +}; + +</pre> +while the rest of hypercall specific structures are part of the this structure. + +### Basic type: struct xen_xsplice_name + +Most of the hypercalls employ an shared structure called `struct xen_xsplice_name` +which contains: + + * `name` - pointer where the string for the name is located. + * `size` - the size of the string + * `pad` - padding - to be zero. + +The structure is as follow: + +<pre> +/* + * Uniquely identifies the payload. Should be human readable. + * Includes the NUL terminator + */ +#define XEN_XSPLICE_NAME_SIZE 128 +struct xen_xsplice_name { + XEN_GUEST_HANDLE_64(char) name; /* IN, pointer to name. */ + uint16_t size; /* IN, size of name. May be upto + XEN_XSPLICE_NAME_SIZE. */ + uint16_t pad[3]; /* IN: MUST be zero. */ +}; +</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_name` called `name` which has the unique name. + * `size` the size of the ELF payload (in bytes). + * `payload` the virtual address of where the ELF payload is. + +The `name` could be an UUID that stays fixed forever for a given +payload. 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 -XEN_EXX return value is provided. Duplicate `name` are not supported. + +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_name_t name; /* 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_name` called `name` which has the unique name. + * A `struct xen_xsplice_status` structure. The member values will + be over-written upon completion. + +Upon completion the `struct xen_xsplice_status` is updated. + + * `status` - indicates the current status of the payload: + * *XSPLICE_STATUS_CHECKED* (1) loaded and the ELF payload safety checks passed. + * *XSPLICE_STATUS_APPLIED* (2) loaded, checked, and applied. + * No other value is possible. + * `rc` - -XEN_EXX type errors encountered while performing the last + XSPLICE_ACTION_* operation. The normal values can be zero or -XEN_EAGAIN which + respectively mean: success or operation in progress. Other values + imply an error occurred. If there is an error in `rc`, `status` will **NOT** + have changed. + +The return value of the hypercall is zero on success and -XEN_EXX on failure. +(Note that the `rc`` value can be different from the return value, as in +rc=-XEN_EAGAIN and return value can be 0). + +For example, supposing there is an payload: + +<pre> + status: XSPLICE_STATUS_CHECKED + rc: 0 +</pre> + +We apply an action - XSPLICE_ACTION_REVERT - to revert it (which won't work +as we have not even applied it. Afterwards we will have: + +<pre> + status: XSPLICE_STATUS_CHECKED + rc: -XEN_EINVAL +</pre> + +It has failed but it remains loaded. + +This operation is synchronous and does not require preemption. + +The structure is as follow: + +<pre> +struct xen_xsplice_status { +#define XSPLICE_STATUS_CHECKED 1 +#define XSPLICE_STATUS_APPLIED 2 + uint32_t state; /* OUT: XSPLICE_STATE_*. */ + int32_t rc; /* OUT: 0 if no error, otherwise -XEN_EXX. */ +}; + +struct xen_sysctl_xsplice_get { + xen_xsplice_name_t name; /* 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`. Version of the payload. Caller should re-use the field provided by + the hypervisor. If the value differs the data is stale. + * `idx` index iterator. The index into the hypervisor's payload count. It is + recommended that on first invocation zero be used so that `nr` (which the + hypervisor will update with the remaining payload count) be provided. + Also the hypervisor will provide `version` with the most current value. + * `nr` the max number of entries to populate. Can be zero which will result + in the hypercall being a probing one and return the number of payloads + (and update the `version`). + * `pad` - *MUST* be zero. + * `status` virtual address of where to write `struct xen_xsplice_status` + structures. Caller *MUST* allocate up to `nr` of them. + * `name` - virtual address of where to write the unique name of the payload. + Caller *MUST* allocate up to `nr` of them. Each *MUST* be of + **XEN_XSPLICE_NAME_SIZE** size. Note that **XEN_XSPLICE_NAME_SIZE** includes + the NUL terminator. + * `len` - virtual address of where to write the length of each unique name + of the payload. Caller *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 (upto `nr` +provided to the hypercall) 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`, `name`, and `len`' are updated at their designed index +value (`idx`) with the returned value of data. + +If the hypercall returns -XEN_E2BIG the `nr` is too big and should be +lowered. + +If the hypercall returns an zero value there are no more payloads. + +Note that due to the asynchronous nature of hypercalls the control domain might +have added or removed a 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` - indicates the current status of the payload: + * *XSPLICE_STATUS_CHECKED* (1) loaded and the ELF payload safety checks passed. + * *XSPLICE_STATUS_APPLIED* (2) loaded, checked, and applied. + * No other value is possible. + * `rc` - -XEN_EXX type errors encountered while performing the last + XSPLICE_ACTION_* operation. The normal values can be zero or -XEN_EAGAIN which + respectively mean: success or operation in progress. Other values + imply an error occurred. If there is an error in `rc`, `status` will **NOT** + have changed. + +The structure is as follow: + +<pre> +struct xen_sysctl_xsplice_list { + uint32_t version; /* OUT: Hypervisor stamps value. + If varies between calls, we are + getting stale data. */ + uint32_t idx; /* IN: Index into hypervisor array. + Should be between [0, nr). */ + uint32_t nr; /* IN: How many status, names, and len + should be filled out. Can be zero to get + amount of payloads and version. + 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 nr of them. */ + XEN_GUEST_HANDLE_64(char) id; /* OUT: Array of names. Each member + MUST XEN_XSPLICE_NAME_SIZE in size. + Must have nr of them. */ + XEN_GUEST_HANDLE_64(uint32) len; /* OUT: Array of lengths of name's. + Must have nr of them. */ +}; +</pre> + +### XEN_SYSCTL_XSPLICE_ACTION (3) + +Perform an operation on the payload structure referenced by the `name` 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. + +The caller provides: + + * A 'struct xen_xsplice_name` `name` containing the unique name. + * `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. This is the initial state an payload is in. + * *XSPLICE_ACTION_UNLOAD* (2) unload the payload. + Any further hypercalls against the `name` will result in failure unless + **XEN_SYSCTL_XSPLICE_UPLOAD** hypercall is perfomed with same `name`. + * *XSPLICE_ACTION_REVERT* (3) revert the payload. If the operation takes + more time than the upper bound of time the `rc` in `xen_xsplice_status' + retrieved via **XEN_SYSCTL_XSPLICE_GET** will be -XEN_EBUSY. + * *XSPLICE_ACTION_APPLY* (4) apply the payload. If the operation takes + more time than the upper bound of time the `rc` in `xen_xsplice_status' + retrieved via **XEN_SYSCTL_XSPLICE_GET** will be -XEN_EBUSY. + * *XSPLICE_ACTION_REPLACE* (5) revert all applied payloads and apply this + payload. If the operation takes more time than the upper bound of time + the `rc` in `xen_xsplice_status' retrieved via **XEN_SYSCTL_XSPLICE_GET** + will be -XEN_EBUSY. + * `time` the upper bound of time (ms) 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 +#define XSPLICE_ACTION_REPLACE 5 +struct xen_sysctl_xsplice_action { + xen_xsplice_name_t name; /* IN, name of the patch. */ + uint32_t cmd; /* IN: XSPLICE_ACTION_* */ + uint32_t time; /* IN: Zero if no timeout. */ + /* Or upper bound of time (ms) */ + /* for operation to take. */ +}; + +</pre> + +## State diagrams of XSPLICE_ACTION commands. + +There is a strict ordering state of what the commands can be. +The XSPLICE_ACTION prefix has been dropped to easy reading and +does not include the XSPLICE_STATES: + +<pre> + /->\ + \ / + UNLOAD <--- CHECK ---> REPLACE|APPLY --> REVERT --\ + \ | + \-------------------<-------------/ + +</pre> +## State transition table of XSPLICE_ACTION commands and XSPLICE_STATUS. + +Note that: + + - The CHECKED state is the starting one achieved with *XEN_SYSCTL_XSPLICE_UPLOAD* hypercall. + - The REVERT operation on success will automatically move to the CHECKED state. + - There are two STATES: CHECKED and APPLIED. + - There are five actions (aka commands): CHECK, APPLY, REPLACE, REVERT, and UNLOAD. + +The state transition table of valid states and action states: + +<pre> + ++---------+---------+--------------------------------+-------+--------+ +| ACTION | Current | Result | Next STATE: | +| ACTION | STATE | |CHECKED|APPLIED | ++---------+----------+-------------------------------+-------+--------+ +| CHECK | CHECKED | Check payload (once more, no) | x | | +| | | errors) | | | ++---------+---------+--------------------------------+-------+--------+ +| CHECK | CHECKED | Check payload (once more, with | | | +| | | errors) | | | ++---------+---------+--------------------------------+-------+--------+ +| UNLOAD | CHECKED | Unload payload. Always works. | | | +| | | No next states. | | | ++---------+---------+--------------------------------+-------+--------+ +| APPLY | CHECKED | Apply payload (success). | | x | ++---------+---------+--------------------------------+-------+--------+ +| APPLY | CHECKED | Apply payload (error|timeout) | x | | ++---------+---------+--------------------------------+-------+--------+ +| REPLACE | CHECKED | Revert payloads and apply new | | x | +| | | payload with success. | | | ++---------+---------+--------------------------------+-------+--------+ +| REPLACE | CHECKED | Revert payloads and apply new | x | | +| | | payload with error. | | | ++---------+---------+--------------------------------+-------+--------+ +| REVERT | APPLIED | Revert payload (success). | x | | ++---------+---------+--------------------------------+-------+--------+ +| REVERT | APPLIED | Revert payload (error|timeout) | | x | ++---------+---------+--------------------------------+-------+--------+ +</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 `->rc`. If *-XEN_EAGAIN* spin. If zero go to next step. + 3. *XEN_SYSCTL_XSPLICE_ACTION* with *XSPLICE_ACTION_APPLY* to apply the patch. + 4. *XEN_SYSCTL_XSPLICE_GET* to check the `->rc`. If in *-XEN_EAGAIN* spin. If zero 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. + +Note that patching functions that copy to or from guest memory requires +to support alternative support. For example this can be due to SMAP +(specifically *stac* and *clac* operations) which is enabled on Broadwell +and later architectures. It may be related to other alternative instructions. + +### 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. Safety checks such as not patching code which +is on the stack - which can lead to corruption. + +#### 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 execute lockstep instructions +on all CPUs. + +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. + +This is implemented in the Xen Project hypervisor. + +### 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. + +This is implemented in a seperate tool which lives in a seperate +GIT repo. + +Currently it resides at https://github.com/rosslagerwall/xsplice-build + +### 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. + +As found almost every patch (XSA) to a non-trivial function requires +additional entries in the exception table and/or the bug frames. + +This is implemented in the Xen Project hypervisor. + +### .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. + +The trampoline patching is implemented in the Xen Project hypervisor. + +### .bss and .data sections. + +In place 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. + +However, functions which are being patched can bring in changes to strings +(.data or .rodata section changes), or even to .bss sections. + +As such the ELF payload can introduce new .rodata, .bss, and .data sections. +Patching in the new function will end up also patching in the new .rodata +section and the new function will reference the new string in the new +.rodata section. + +This is implemented in the Xen Project hypervisor. + +### Security + +Only the privileged domain should be allowed to do this operation. + + +# Not Yet Done + +This is for further development of xSplice. + +## Goals + +The implementation must also have a mechanism for: + + * An dependency mechanism for the payloads. To use that information to load: + - The appropiate payload. To verify that payload is built against the + hypervisor. This can be done via the `build-id` + or via providing an copy of the old code - so that the hypervisor can + verify it against the code in memory. + - To construct an appropiate order of payloads to load in case they + depend on each other. + * Be able to lookup in the Xen hypervisor the symbol names of functions from the ELF payload. + * Be able to patch .rodata, .bss, and .data sections. + * Further safety checks (blacklist of which functions cannot be patched, check + the stack, make sure the payload is built with same compiler as hypervisor). + * NOP out the code sequence if `new_size` is zero. + * Deal with other relocation types: R_X86_64_[8,16,32,32S], R_X86_64_PC[8,16,64] in payload file. + +### 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. + +### 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. + +For BUG(), WARN(), etc., the line number is embedded into the bug frame, not +the function itself. + +Similar considerations are true to a lesser extent for __FILE__, but it +could be argued that file renaming should be done outside of hotpatches. + +## 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.). + + +### .bss and .data sections. + +In place 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. + +That said we should provide hook functions so that the existing data +can be changed during payload application. + + +### 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. + +The tool for generating payloads currently does perform a compile-time +check to ensure that the function to be replaced is large enough. + -- 2.5.0 _______________________________________________ Xen-devel mailing list Xen-devel@xxxxxxxxxxxxx http://lists.xen.org/xen-devel

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