linux-kernel - Re: frequent lockups in 3.18rc4

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Date:	Thu, 20 Nov 2014 00:13:21 +0100
From:	Frederic Weisbecker <fweisbec@...il.com>
To:	Andy Lutomirski <luto@...capital.net>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Dave Jones <davej@...hat.com>, Don Zickus <dzickus@...hat.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Linux Kernel <linux-kernel@...r.kernel.org>,
	the arch/x86 maintainers <x86@...nel.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Arnaldo Carvalho de Melo <acme@...stprotocols.net>
Subject: Re: frequent lockups in 3.18rc4

On Wed, Nov 19, 2014 at 03:07:17PM -0800, Andy Lutomirski wrote:
> On Wed, Nov 19, 2014 at 3:00 PM, Frederic Weisbecker <fweisbec@...il.com> wrote:
> > Note that's non-sleeping faults. So probably most places are fine except
> > a few of them that really don't want exception to mess up some state. I
> > can imagine some entry code that really don't want that.
> 
> Any non-IST fault at all on the kernel_stack reference in system_call
> is instant root on non-SMAP systems and instant double-fault or more
> challenging root on SMAP systems.  The issue is that rsp is
> user-controlled, so the CPU cannot deliver a non-IST fault safely.

Heh.

> >
> > Is kernel stack allocated by vmalloc or alloc_percpu()?
> 
> DEFINE_PER_CPU(unsigned long, kernel_stack)
> 
> Note that I'm talking about kernel_stack, not the kernel stack itself.

Ah. Note, static allocation like DEFINE_PER_CPU() is probably fine. The
issue is on dynamic allocations: alloc_percpu().

> The actual stack is regular linearly-mapped memory, although I plan on
> trying to change that, complete with all kinds of care to avoid double
> faults.

If you do so, you must really ensure that the resulting memory will never
fault.
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