linux-kernel - Re: [PATCH v4 0/4] SROP Mitigation: Sigreturn Cookies

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Date:	Tue, 29 Mar 2016 15:36:49 -0600
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>,
	Andi Kleen <ak@...ux.intel.com>,
	Ingo Molnar <mingo@...hat.com>,
	Thomas Gleixner <tglx@...utronix.de>, wmealing@...hat.com,
	Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: [PATCH v4 0/4] SROP Mitigation: Sigreturn Cookies



On 03/29/2016 03:29 PM, Andy Lutomirski wrote:
> On Tue, Mar 29, 2016 at 12:53 PM, Scott Bauer <sbauer@....utah.edu> wrote:
>> Sigreturn-oriented programming is a new attack vector in userland
>> where an attacker crafts a fake signal frame on the stack and calls
>> sigreturn. The kernel will extract the fake signal frame, which
>> contains attacker controlled "saved" registers. The kernel will then
>> transfer control to the attacker controlled userland instruction pointer.
>>
>> To prevent SROP attacks the kernel needs to know or be able to dervive
>> whether a sigreturn it is processing is in response to a legitimate
>> signal the kernel previously delivered.
>>
>> Further information and test code can be found in Documentation/security
>> and this excellent article:
>> http://lwn.net/Articles/676803/
>>
>> These patches implement the necessary changes to generate a cookie
>> which will be placed above signal frame upon signal delivery to userland.
>> The cookie is generated using a per-process random value xor'd with
>> the address where the cookie will be stored on the stack.
>>
>> Upon a sigreturn the kernel will extract the cookie from userland,
>> recalculate what the original cookie should be and verify that the two
>> do not differ. If the two differ the kernel will terminate the process
>> with a SIGSEGV.
>>
>> This prevents SROP by adding a value that the attacker cannot guess,
>> but the kernel can verify. Therefore an attacker cannot use sigreturn as
>> a method to control the flow of a process.
>>
> 
> Has anyone verified that this doesn't break CRIU cross-machine (or
> cross-boot) migration and that this doesn't break dosemu?  You're
> changing the ABI here.
> 

I haven't yet I'll do that to verify it breaks -- I'm pretty sure under some
conditions it will break CRIU. That's why we added the sysctl to turn it off. 
Should I have mentioned this in the main commit that it possibly breaks CRIU/DOSEMU?
I went ahead and added that to the Documentation.