Date: Tue, 29 Mar 2016 14:38:36 -0700 From: Andy Lutomirski <luto@...capital.net> To: Scotty Bauer <sbauer@....utah.edu> 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 Tue, Mar 29, 2016 at 2:36 PM, Scotty Bauer <sbauer@....utah.edu> wrote: > > > 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. > > Then there's an unanswered question: is this patch acceptable given that it's an ABI break? Security fixes are sometimes an exception to the "no ABI breaks" rule, but it's by no means an automatic exception. --Andy
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