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Date: Wed, 18 Nov 2015 13:16:07 +0100
From: Florian Weimer <fweimer@...hat.com>
To: oss-security@...ts.openwall.com, Bernd Schmidt <bschmidt@...hat.com>
Cc: Jeff Law <law@...hat.com>
Subject: Re: Re: Fwd: x86 ROP mitigation
On 11/18/2015 02:57 AM, Solar Designer wrote:
> I'd like more detail on the plan of dealing with function epilogues, if
> there is a plan for that.
>
> I'm not sure if this fits under:
>
>> * Look into an idea Florian had for improving stack-protector
>> epilogues.
>
> or if that's (more likely) something entirely different.
It's about things like this:
1c6d3: 48 8b 4c 24 58 mov 0x58(%rsp),%rcx
1c6d8: 64 48 33 0c 25 28 00 xor %fs:0x28,%rcx
1c6df: 00 00
1c6e1: 75 31 jne 1c714
1c6e3: 48 83 c4 68 add $0x68,%rsp
1c6e7: 5b pop %rbx
1c6e8: 5d pop %rbp
1c6e9: 41 5c pop %r12
1c6eb: 41 5d pop %r13
1c6ed: 41 5e pop %r14
1c6ef: 41 5f pop %r15
1c6f1: c3 retq
…
1c714: e8 47 b7 ff ff callq <__stack_chk_fail@plt>
1c719: 0f 1f 80 00 00 00 00 nopl 0x0(%rax)
It seems to me that if the stack canary check happened directly before
the RET instruction, after restoring the registers, it would make it
more difficult to abuse the RET instruction. With the code above, you
can just jump to the address 1c6e7 and have access to quite a few useful
POP instructions.
Florian
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