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| author | Alexandre Chartre <alexandre.chartre@oracle.com> | 2024-06-20 16:47:47 +0200 |
|---|---|---|
| committer | Josh Poimboeuf <jpoimboe@kernel.org> | 2024-07-02 23:40:54 -0700 |
| commit | 8e366d83edce3065ff3372bedc281c5e217c0550 (patch) | |
| tree | e2d7ea29fa5624773cbebf9e106701ede727afe8 | |
| parent | b13e9f6da4cc34240dae05418b9876b2758ebe35 (diff) | |
objtool/x86: objtool can confuse memory and stack access
The encoding of an x86 instruction can include a ModR/M and a SIB
(Scale-Index-Base) byte to describe the addressing mode of the
instruction.
objtool processes all addressing mode with a SIB base of 5 as having
%rbp as the base register. However, a SIB base of 5 means that the
effective address has either no base (if ModR/M mod is zero) or %rbp
as the base (if ModR/M mod is 1 or 2). This can cause objtool to confuse
an absolute address access with a stack operation.
For example, objtool will see the following instruction:
4c 8b 24 25 e0 ff ff mov 0xffffffffffffffe0,%r12
as a stack operation (i.e. similar to: mov -0x20(%rbp), %r12).
[Note that this kind of weird absolute address access is added by the
compiler when using KASAN.]
If this perceived stack operation happens to reference the location
where %r12 was pushed on the stack then the objtool validation will
think that %r12 is being restored and this can cause a stack state
mismatch.
This kind behavior was seen on xfs code, after a minor change (convert
kmem_alloc() to kmalloc()):
>> fs/xfs/xfs.o: warning: objtool: xfs_da_grow_inode_int+0x6c1: stack state mismatch: reg1[12]=-2-48 reg2[12]=-1+0
Reported-by: kernel test robot <lkp@intel.com>
Closes: https://lore.kernel.org/oe-kbuild-all/202402220435.MGN0EV6l-lkp@intel.com/
Signed-off-by: Alexandre Chartre <alexandre.chartre@oracle.com>
Link: https://lore.kernel.org/r/20240620144747.2524805-1-alexandre.chartre@oracle.com
Signed-off-by: Josh Poimboeuf <jpoimboe@kernel.org>
| -rw-r--r-- | tools/objtool/arch/x86/decode.c | 8 |
1 files changed, 7 insertions, 1 deletions
diff --git a/tools/objtool/arch/x86/decode.c b/tools/objtool/arch/x86/decode.c index 3a1d80a7878d..ed6bff0e01dc 100644 --- a/tools/objtool/arch/x86/decode.c +++ b/tools/objtool/arch/x86/decode.c @@ -125,8 +125,14 @@ bool arch_pc_relative_reloc(struct reloc *reloc) #define is_RIP() ((modrm_rm & 7) == CFI_BP && modrm_mod == 0) #define have_SIB() ((modrm_rm & 7) == CFI_SP && mod_is_mem()) +/* + * Check the ModRM register. If there is a SIB byte then check with + * the SIB base register. But if the SIB base is 5 (i.e. CFI_BP) and + * ModRM mod is 0 then there is no base register. + */ #define rm_is(reg) (have_SIB() ? \ - sib_base == (reg) && sib_index == CFI_SP : \ + sib_base == (reg) && sib_index == CFI_SP && \ + (sib_base != CFI_BP || modrm_mod != 0) : \ modrm_rm == (reg)) #define rm_is_mem(reg) (mod_is_mem() && !is_RIP() && rm_is(reg)) |