Messages in this thread Patch in this message | | | From | Alexandre Chartre <> | Subject | [PATCH] objtool/x86: objtool can confuse memory and stack access | Date | Thu, 28 Mar 2024 14:46:34 +0100 |
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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> --- tools/objtool/arch/x86/decode.c | 8 ++++++++ 1 file changed, 8 insertions(+)
diff --git a/tools/objtool/arch/x86/decode.c b/tools/objtool/arch/x86/decode.c index 3a1d80a7878d3..18a9140173326 100644 --- a/tools/objtool/arch/x86/decode.c +++ b/tools/objtool/arch/x86/decode.c @@ -412,6 +412,14 @@ int arch_decode_instruction(struct objtool_file *file, const struct section *sec if (!rex_w) break; + /* + * If the SIB base is 5, and ModRM mod is 0 then there + * is no base. But SIB decoding will set sib_base to + * CFI_BP (register 5). + */ + if (have_SIB() && sib_base == CFI_BP && modrm_mod == 0) + break; + if (rm_is_mem(CFI_BP)) { /* mov disp(%rbp), reg */ -- 2.39.3
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