Re: [PATCH v4 0/3] Compile-time stack frame pointer validation

Josh Poimboeuf
2015-05-22T14:32:20Z

On Fri, May 22, 2015 at 12:01:58AM +0200, Borislav Petkov wrote:
> On Thu, May 21, 2015 at 03:54:25PM -0500, Josh Poimboeuf wrote:
> > stackvalidate: arch/x86/lib/memmove_64.o: return instruction outside of a function at .altinstr_replacement+0x5
>
> That must be something like this:
>
> 0000000000000000 <.altinstr_replacement>:
> 0: 48 89 d1 mov %rdx,%rcx
> 3: f3 a4 rep movsb %ds:(%rsi),%es:(%rdi)
> 5: c3 retq
>
> right?
>
> In any case, anything with alternatives is probably a false positive
> because even if instructions appear outside of the containing function,
> they get patched in and are actually inside. Jump offsets get fixed up
> properly too. Should, at least :-)

Hm, alternatives do complicate things a bit. It *is* a false positive,
but not necessarily because its part of an alternative instruction
block.

The above code would be patched into memmove(), which is a leaf function
because it doesn't call any other functions. Leaf functions don't need
frame pointer logic, so we can ignore them.

If instead the above code were patched into a non-leaf function, we'd
have to change it to restore the frame pointer before returning.

--
Josh