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

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Date:	Tue, 8 Mar 2016 14:49:35 -0700
From:	Scotty Bauer <sbauer@....utah.edu>
To:	Andy Lutomirski <luto@...capital.net>
Cc:	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"kernel-hardening@...ts.openwall.com" 
	<kernel-hardening@...ts.openwall.com>, X86 ML <x86@...nel.org>,
	wmealing@...hat.com, Andi Kleen <ak@...ux.intel.com>,
	Abhiram Balasubramanian <abhiram@...utah.edu>
Subject: Re: [PATCH v3 1/3] SROP Mitigation: Architecture independent code for
 signal cookies

On 03/08/2016 01:58 PM, Andy Lutomirski wrote:
> On Tue, Mar 8, 2016 at 12:47 PM, Scott Bauer <sbauer@....utah.edu> 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!