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Message-Id: <578601B3.3050903@de.ibm.com>
Date: Wed, 13 Jul 2016 10:54:11 +0200
From: Christian Borntraeger <borntraeger@...ibm.com>
To: Andy Lutomirski <luto@...nel.org>, x86@...nel.org,
linux-kernel@...r.kernel.org
Cc: linux-arch@...r.kernel.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>, Jann Horn <jann@...jh.net>,
Heiko Carstens <heiko.carstens@...ibm.com>,
linux-s390 <linux-s390@...r.kernel.org>
Subject: Re: [PATCH v5 00/32] virtually mapped stacks and thread_info cleanup
On 07/11/2016 10:53 PM, Andy Lutomirski wrote:
> Hi all-
>
> 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.
>
> This series (starting with v4) also extensively cleans up
> thread_info. thread_info has been partially redundant with
> thread_struct for a long time -- both are places for arch code to
> add additional per-task variables. thread_struct is much cleaner:
> it's always in task_struct, and there's nothing particularly magical
> about it. So this series contains a bunch of cleanups on x86 to
> move almost everything from thread_info to thread_struct (which,
> even by itself, deletes more code than it adds) and to remove x86's
> dependence on thread_info's position on the stack. Then it opts x86
> into a new config option THREAD_INFO_IN_TASK to get rid of
> arch-specific thread_info entirely and simply embed a defanged
> thread_info (containing only flags) and 'int cpu' into task_struct.
>
> Once thread_info stops being magical, there's another benefit: we
> can free the thread stack as soon as the task is dead (without
> waiting for RCU) and then, if vmapped stacks are in use, cache the
> entire stack for reuse on the same cpu.
>
> This seems to be an overall speedup of about 0.5-1 µs per
> pthread_create/join in a simple test -- a percpu cache of vmalloced
> stacks appears to be a bit faster than a high-order stack
> allocation, at least when the cache hits. (I expect that workloads
> with a low cache hit rate are likely to be dominated by other
> effects anyway.)
>
> This does not address interrupt stacks.
>
> It's worth noting that s390 has an arch-specific gcc feature that
> detects stack overflows by adjusting function prologues. Arches
> with features like that may wish to avoid using vmapped stacks to
> minimize the performance hit.
Yes, might not need this for stack overflow detection. What might
be interesting is the thread_info/thread_struct change, if we can
strip down thread_info.(CONFIG_THREAD_INFO_IN_TASK). Would it actually
make sense to separate these two changes to see what performance
impact CONFIG_THREAD_INFO_IN_TASK has on its own?
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