lkml.org 
[lkml]   [2024]   [Mar]   [28]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
    Patch in this message
    /
    From
    Subject[PATCH] objtool/x86: objtool can confuse memory and stack access
    Date
    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

    \
     
     \ /
      Last update: 2024-05-27 16:13    [W:2.671 / U:0.004 seconds]
    ©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site