Re: [PATCH] objtool/x86: objtool can confuse memory and stack access

[Alexandre Chartre]

On 3/29/24 02:39, Josh Poimboeuf wrote:
> On Thu, Mar 28, 2024 at 02:46:34PM +0100, Alexandre Chartre wrote:
> > 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@xxxxxxxxx>
> > Closes: https://lore.kernel.org/oe-kbuild-all/202402220435.MGN0EV6l-lkp@xxxxxxxxx/
> > Signed-off-by: Alexandre Chartre <alexandre.chartre@xxxxxxxxxx>
>
> Nice, thanks for finding and debugging this.
>
> Would it make sense to make the check more generic by putting it into
> rm_is()?

Yes. Making the change.

Thanks,

alex.