linux-kernel - Re: [PATCH 00/13] Virtually mapped stacks with guard pages (x86, core)

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Date:	Thu, 16 Jun 2016 10:50:09 -0700
From:	Andy Lutomirski <luto@...capital.net>
To:	Heiko Carstens <heiko.carstens@...ibm.com>
Cc:	Andy Lutomirski <luto@...nel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	X86 ML <x86@...nel.org>, Borislav Petkov <bp@...en8.de>,
	Nadav Amit <nadav.amit@...il.com>,
	Kees Cook <keescook@...omium.org>,
	Brian Gerst <brgerst@...il.com>,
	"kernel-hardening@...ts.openwall.com" 
	<kernel-hardening@...ts.openwall.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Josh Poimboeuf <jpoimboe@...hat.com>
Subject: Re: [PATCH 00/13] Virtually mapped stacks with guard pages (x86, core)

On Wed, Jun 15, 2016 at 11:05 PM, Heiko Carstens
<heiko.carstens@...ibm.com> wrote:
> On Wed, Jun 15, 2016 at 05:28:22PM -0700, Andy Lutomirski wrote:
>> Since the dawn of time, a kernel stack overflow has been a real PITA
>> to debug, has caused nondeterministic crashes some time after the
>> actual overflow, and has generally been easy to exploit for root.
>>
>> With this series, arches can enable HAVE_ARCH_VMAP_STACK.  Arches
>> that enable it (just x86 for now) get virtually mapped stacks with
>> guard pages.  This causes reliable faults when the stack overflows.
>>
>> If the arch implements it well, we get a nice OOPS on stack overflow
>> (as opposed to panicing directly or otherwise exploding badly).  On
>> x86, the OOPS is nice, has a usable call trace, and the overflowing
>> task is killed cleanly.
>
> Do you have numbers which reflect the performance impact of this change?
>

Hmm.  My attempt to benchmark it caused some of the vmalloc core code
to hang.  I'll dig around.

FWIW, I expect some overhead on clone/fork (if it's high, then that
would be a good reason to improve vmalloc) and a small
workload-dependent overhead due to increased TLB pressure.

--Andy