i386/prologues: ROP mitigation for normal function epilogues

[Bernd Schmidt]

On 06/17/2016 12:37 PM, Jakub Jelinek wrote:

> Do you really need to require frame pointer for this?
> I mean, couldn't you instead use what you do if a function needs frame
> pointer and otherwise just replace the original ret with
> 	pushq	%rbp
> 	movq	%rsp, %rbp
> 	jmp	__rop_ret
> ?  Or would that defeat the purpose of the mitigation?

Yes, kind of, because then you can jump into code before this little 
sequence and the whole pushq/movq/jmp/leave/ret would just behave like a 
normal ret. This is admittedly a concern for smaller functions that look 
a lot like this; maybe we need to pad function entry points as well.

> As for __rop_ret, if you are non-PLT jmp to it, I bet it must be in the same
> executable or shared library as the code branching to it, so should be
> .hidden.  Is libgcc.a really the best place for it though?

I declare myself agnostic.

> Looking at nop; nop; 1: jmp 1b; leave; ret
> if you branch into the middle of the jmp insn (0x3 below), there is:
>    0:	90                   	nop
>    1:	90                   	nop
>    2:	eb fe                	jmp    0x2
>    4:	c9                   	leaveq
>    5:	c3                   	retq
> and thus:
>    3:	fe c9                	dec    %cl
>    5:	c3                   	retq
> and thus if you don't mind decreasing %cl, you still have retq without leave
> before it.  But I very likely just don't understand the ROP threat stuff
> enough.

You'd also have to find useful code before this sequence, and in any 
case it's just a single ret where we used to have many. But maybe 
there's a one-byte trap that could be used instead.


Bernd