Re: [PATCH v3 1/3] SROP Mitigation: Architecture independent code for signal cookies

From: Scotty Bauer
Date: Tue Mar 08 2016 - 16:50:03 EST

On 03/08/2016 01:58 PM, Andy Lutomirski wrote:
> On Tue, Mar 8, 2016 at 12:47 PM, Scott Bauer <sbauer@xxxxxxxxxxxx> wrote:
>> This patch adds a per-process secret to the task struct which
>> will be used during signal delivery and during a sigreturn.
>> Also, logic is added in signal.c to generate, place, extract,
>> clear and verify the signal cookie.
> Potentially silly question: it's been a while since I read the SROP
> paper, but would the technique be effectively mitigated if sigreturn
> were to zero out the whole signal frame before returning to user mode?

I don't know if I fully understand your question, but I'll respond anyway.

SROP is possible because the kernel doesn't know whether or not the
incoming sigreturn syscall is in response from a legitimate signal that
the kernel had previously delivered and the program handled. So essentially
these patches are an attempt to give the kernel a way to verify whether or
not the the incoming sigreturn is a valid response or a exploit trying to
hijack control of the user program.

So no, zeroing out the frame wouldn't do much because if I understand your
question correctly once we call sigreturn the kernel is going to hand off
control to wherever the sigframe tells it to so I don't think zeroing would
do much.

The reason why I zero out the cookie is so if there is a stack leak bug or
something along those lines an attacker couldnt leak the cookie and try and
derive what the per-process kernel secret is.

Hope that clarifies!