[Xen-devel] [PATCH v5 06/16] livepatch: ARM/x86: Check displacement of old_addr and new_addr

[Konrad Rzeszutek Wilk <konrad.wilk@xxxxxxxxxx>]

If the distance is too big we are in trouble - as our relocation
distance can surely be clipped, or still have a valid width - but
cause an overflow of distance.

On various architectures the maximum displacement for a unconditional
branch/jump varies. ARM32 is +/- 32MB, ARM64 is +/- 128MB while x86
for 32-bit relocations is +/- 2G.

Note: On x86 we could use the 64-bit jmpq instruction which
would provide much bigger displacement to do a jump, but we would
still have issues with the new function not being able to reach
any of the old functions (as all the relocations would assume 32-bit
displacement). And "furthermore would require an register or
memory location to load/store the address to." (From Jan).

On ARM the conditional branch supports even a smaller displacement
but fortunately we are not using that.

Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@xxxxxxxxxx>

---
Cc: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
Cc: Jan Beulich <jbeulich@xxxxxxxx>
Cc: Stefano Stabellini <sstabellini@xxxxxxxxxx>
Cc: Julien Grall <julien.grall@xxxxxxx>

v3: New submission.
v4: s/arch_livepatch_verify_distance/livepatch_verify_distance/
    s/LIVEPATCH_ARCH_RANGE/ARCH_LIVEPATCH_RANGE/
v5: Updated commit description with Jan's comment
    Ditch the casting of long on calculating offset.
    Move most of the docs in "livepatch: Initial ARM64 support."
    Drop the cast on assigning ARCH_LIVEPATCH_RANGE to an variable.
---
 docs/misc/livepatch.markdown    |  3 +++
 xen/arch/arm/arm64/livepatch.c  |  1 +
 xen/common/livepatch.c          |  4 ++++
 xen/include/asm-arm/livepatch.h | 11 +++++++++++
 xen/include/asm-x86/livepatch.h |  3 +++
 xen/include/xen/livepatch.h     | 17 ++++++++++++++++-
 6 files changed, 38 insertions(+), 1 deletion(-)

diff --git a/docs/misc/livepatch.markdown b/docs/misc/livepatch.markdown
index ff2cfb8..16f45f3 100644
--- a/docs/misc/livepatch.markdown
+++ b/docs/misc/livepatch.markdown
@@ -1151,6 +1151,9 @@ with proper offset is used for an unconditional branch to 
the new code.
 This means that that `old_size` **MUST** be at least four bytes if patching
 in trampoline.
 
+The instruction offset is limited on ARM32 to +/- 32MB to displacement
+and on ARM64 to +/- 128MB displacement.
+
 The new code is placed in the 8M - 10M virtual address space while the
 Xen code is in 2M - 4M. That gives us enough space.
 
diff --git a/xen/arch/arm/arm64/livepatch.c b/xen/arch/arm/arm64/livepatch.c
index 7cb1812..d3fc9ac 100644
--- a/xen/arch/arm/arm64/livepatch.c
+++ b/xen/arch/arm/arm64/livepatch.c
@@ -40,6 +40,7 @@ void arch_livepatch_apply(struct livepatch_func *func)
     else
         insn = aarch64_insn_gen_nop();
 
+    /* Verified in livepatch_verify_distance. */
     ASSERT(insn != AARCH64_BREAK_FAULT);
 
     new_ptr = func->old_addr - (void *)_start + vmap_of_xen_text;
diff --git a/xen/common/livepatch.c b/xen/common/livepatch.c
index 66f23e0..f2f866c 100644
--- a/xen/common/livepatch.c
+++ b/xen/common/livepatch.c
@@ -545,6 +545,10 @@ static int prepare_payload(struct payload *payload,
         rc = resolve_old_address(f, elf);
         if ( rc )
             return rc;
+
+        rc = livepatch_verify_distance(f);
+        if ( rc )
+            return rc;
     }
 
     sec = livepatch_elf_sec_by_name(elf, ".livepatch.hooks.load");
diff --git a/xen/include/asm-arm/livepatch.h b/xen/include/asm-arm/livepatch.h
index 929c7d9..482d74f 100644
--- a/xen/include/asm-arm/livepatch.h
+++ b/xen/include/asm-arm/livepatch.h
@@ -6,6 +6,8 @@
 #ifndef __XEN_ARM_LIVEPATCH_H__
 #define __XEN_ARM_LIVEPATCH_H__
 
+#include <xen/sizes.h> /* For SZ_* macros. */
+
 /* On ARM32,64 instructions are always 4 bytes long. */
 #define ARCH_PATCH_INSN_SIZE 4
 
@@ -15,6 +17,15 @@
  */
 extern void *vmap_of_xen_text;
 
+/* These ranges are only for unconditional branches. */
+#ifdef CONFIG_ARM_32
+/* ARM32: A4.3 IN ARM DDI 0406C.j -  we are using only ARM instructions in 
Xen.*/
+#define ARCH_LIVEPATCH_RANGE SZ_32M
+#else
+/* ARM64: C1.3.2 in ARM DDI 0487A.j */
+#define ARCH_LIVEPATCH_RANGE SZ_128M
+#endif
+
 #endif /* __XEN_ARM_LIVEPATCH_H__ */
 
 /*
diff --git a/xen/include/asm-x86/livepatch.h b/xen/include/asm-x86/livepatch.h
index 5e04aa1..7dfc2e7 100644
--- a/xen/include/asm-x86/livepatch.h
+++ b/xen/include/asm-x86/livepatch.h
@@ -6,7 +6,10 @@
 #ifndef __XEN_X86_LIVEPATCH_H__
 #define __XEN_X86_LIVEPATCH_H__
 
+#include <xen/sizes.h> /* For SZ_* macros. */
+
 #define ARCH_PATCH_INSN_SIZE 5
+#define ARCH_LIVEPATCH_RANGE SZ_2G
 
 #endif /* __XEN_X86_LIVEPATCH_H__ */
 
diff --git a/xen/include/xen/livepatch.h b/xen/include/xen/livepatch.h
index b7f66d4..b7b84e7 100644
--- a/xen/include/xen/livepatch.h
+++ b/xen/include/xen/livepatch.h
@@ -12,6 +12,7 @@ struct livepatch_elf_sym;
 struct xen_sysctl_livepatch_op;
 
 #include <xen/elfstructs.h>
+#include <xen/errno.h> /* For -ENOSYS or -EOVERFLOW */
 #ifdef CONFIG_LIVEPATCH
 
 /*
@@ -79,6 +80,21 @@ unsigned int livepatch_insn_len(const struct livepatch_func 
*func)
 
     return ARCH_PATCH_INSN_SIZE;
 }
+
+static inline int livepatch_verify_distance(const struct livepatch_func *func)
+{
+    long offset;
+    long range = ARCH_LIVEPATCH_RANGE;
+
+    if ( !func->new_addr ) /* Ignore NOPs. */
+        return 0;
+
+    offset = func->old_addr - func->new_addr;
+    if ( offset < -range || offset >= range )
+        return -EOVERFLOW;
+
+    return 0;
+}
 /*
  * These functions are called around the critical region patching live code,
  * for an architecture to take make appropratie global state adjustments.
@@ -103,7 +119,6 @@ void arch_livepatch_unmask(void);
 #define init_or_livepatch_data        __initdata
 #define init_or_livepatch             __init
 
-#include <xen/errno.h> /* For -ENOSYS */
 static inline int livepatch_op(struct xen_sysctl_livepatch_op *op)
 {
     return -ENOSYS;
-- 
2.4.11


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