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Date: Sat, 25 Jun 2016 16:30:14 -0700
From: Andy Lutomirski <luto@...capital.net>
To: Linus Torvalds <torvalds@...ux-foundation.org>
Cc: Josh Poimboeuf <jpoimboe@...hat.com>, Brian Gerst <brgerst@...il.com>,
Peter Zijlstra <peterz@...radead.org>, Oleg Nesterov <oleg@...hat.com>,
Andy Lutomirski <luto@...nel.org>, "the arch/x86 maintainers" <x86@...nel.org>,
Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
"linux-arch@...r.kernel.org" <linux-arch@...r.kernel.org>, Borislav Petkov <bp@...en8.de>,
Nadav Amit <nadav.amit@...il.com>, Kees Cook <keescook@...omium.org>,
"kernel-hardening@...ts.openwall.com" <kernel-hardening@...ts.openwall.com>, Jann Horn <jann@...jh.net>,
Heiko Carstens <heiko.carstens@...ibm.com>
Subject: Re: [PATCH v3 00/13] Virtually mapped stacks with guard pages (x86, core)
On Sat, Jun 25, 2016 at 4:19 PM, Andy Lutomirski <luto@...capital.net> wrote:
> I rebased my series onto your tree and then rebased this thing onto my
> series, tweaked it some, and split it up a bit. My version works a
> bit differently (thread_info has a single element if the new option is
> set) but is otherwise more or less your code. It seems to work.
>
> https://git.kernel.org/cgit/linux/kernel/git/luto/linux.git/commit/?h=x86/vmap_stack&id=01ac3242f314405c1bac0f820ec3575a850509d3
> https://git.kernel.org/cgit/linux/kernel/git/luto/linux.git/commit/?h=x86/vmap_stack&id=87194cac139aebecec89a68eff719ab44f0469a2
>
> On top of *that*, I taught the kernel to free stacks in release_task
> and to cache stacks if vmalloced. That still blows up: when
> cryptomgr_test calls do_exit during boot, do_exit calls exit_notify,
> which observes that the task state is TASK_DEAD and thus calls
> release_task on itself and goes boom.
>
> Linus, Oleg, help? How am I supposed to quickly free the stack if the
> task goes through this code path? In fact, why does this work on
> current kernels? After all, can't schedule() indicates an RCU grace
> period? In principle, what prevents delayed_put_task_struct from
> deleting the running stack before scheduling?
>
> My kludged up patch that only early-releases the stack if release_task
> is called from a different task is here:
>
> https://git.kernel.org/cgit/linux/kernel/git/luto/linux.git/commit/?h=x86/vmap_stack&id=2327ca2ab3d634d120ea185e737fef2e4e9cf012
Maybe I'm misunderstanding the role of release_task. It looks like
there's this path in the scheduler I can borrow:
if (unlikely(prev_state == TASK_DEAD)) {
With a kludge in place to free the stack in there and release_task and
__put_task_struct, whichever is first, I get a nice speedup.
Benchmarks coming later on. Can I rely on that code path always being
called?
--Andy
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