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Message-Id: <57868E36.7050605@de.ibm.com>
Date: Wed, 13 Jul 2016 20:53:42 +0200
From: Christian Borntraeger <borntraeger@...ibm.com>
To: Andy Lutomirski <luto@...capital.net>
Cc: Andy Lutomirski <luto@...nel.org>, X86 ML <x86@...nel.org>,
"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
linux-arch <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 07/13/2016 08:36 PM, Andy Lutomirski wrote:
> On Wed, Jul 13, 2016 at 1:54 AM, Christian Borntraeger
> <borntraeger@...ibm.com> wrote:
>> 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?
>>
>
> They're already separated.
>
> CONFIG_THREAD_INFO_IN_TASK should have basically no performance impact
> unless there are arch-dependent (percpu?) issues involved. It does
> enable immediate thread stack deallocation, though, and it would be
> straightforward to make CONFIG_THREAD_INFO_IN_TASK cache stacks even
> if CONFIG_VMAP_STACK=n. That should be a moderate clone() speedup.
Yes. My point was more of having two patch series in case the discussion goes
on regarding virtual stack.
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