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Date: Thu, 20 Nov 2014 00:13:21 +0100
From: Frederic Weisbecker <fweisbec@gmail.com>
To: Andy Lutomirski <luto@amacapital.net>
Cc: Linus Torvalds <torvalds@linux-foundation.org>,
        Dave Jones <davej@redhat.com>, Don Zickus <dzickus@redhat.com>,
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        the arch/x86 maintainers <x86@kernel.org>,
        Peter Zijlstra <peterz@infradead.org>,
        Arnaldo Carvalho de Melo <acme@ghostprotocols.net>
Subject: Re: frequent lockups in 3.18rc4
Message-ID: <20141119231319.GD11386@lerouge>
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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@gmail.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.