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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