linux-kernel - Re: [PATCH 0/2] Fix and re-enable vsyscall=emulate

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Message-ID: <CALCETrVxHvH61D-xY6vq4ymWXAVOdj8=XV_ENqLQUJ5rMieuEg@mail.gmail.com>
Date:	Fri, 2 Dec 2011 14:47:35 -0800
From:	Andy Lutomirski <luto@...capital.net>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	x86@...nel.org, Ingo Molnar <mingo@...e.hu>,
	richard -rw- weinberger <richard.weinberger@...il.com>,
	Adrian Bunk <bunk@...sta.de>,
	"H. Peter Anvin" <hpa@...ux.intel.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>, linux-kernel@...r.kernel.org,
	Andy Lutomirski <luto@...capital.net>
Subject: Re: [PATCH 0/2] Fix and re-enable vsyscall=emulate

On Mon, Nov 7, 2011 at 4:33 PM, Andy Lutomirski <luto@...capital.net> wrote:
> The really nice fix (wiring up access_ok failures to be able to raise
> signals) won't be ready on time for 3.2, so let's try the simpler fix
> for now.

I spoke to hpa about this a couple days ago, and he pointed out a
problem with making access_ok send signals.  Userspace expects signals
that come with full context information to be restartable, and many
system calls are not restartable.  read() and write() are the obvious
examples: once they're processed the beginning of the buffer, unless
they adjust their parameters, they can't safely be restarted.  So
without massive changes, I think allowing access_ok to raise a signal
with full context is asking for trouble.

I can still do the patch with two modes: signals without context via
arch_prctl and signals with context via vsyscall emulation, but that's
probably overkill for fixing this bug.  I'd say just apply these
patches as is (for 3.3).

--Andy
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