Date: Tue, 22 Nov 2016 20:37:20 +0100
From: Peter Zijlstra <peterz@infradead.org>
To: Andy Lutomirski <luto@amacapital.net>
Cc: Linus Torvalds <torvalds@linux-foundation.org>,
        Ingo Molnar <mingo@kernel.org>, Thomas Gleixner <tglx@linutronix.de>,
        "H. Peter Anvin" <hpa@zytor.com>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>
Subject: Re: [RFC][PATCH] x86: Verify access_ok() context
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On Tue, Nov 22, 2016 at 09:28:01AM -0800, Andy Lutomirski wrote:
> On Tue, Nov 22, 2016 at 1:57 AM, Peter Zijlstra <peterz@infradead.org> wrote:

> > +#define access_ok(type, addr, size)                                    \
> > +({                                                                     \
> > +       WARN_ON_ONCE(!in_task());                                       \
> 
> Should this be guarded by some debug option?  This may hurt
> performance on production systems quite a bit.

I suspected something like that; any suitable CONFIG come to mind? I'm
somewhat reluctant to create yet another one for this.

CONFIG_DEBUG_VM seems somehow inappropriate.

> For what it's worth, I think ARM recently started saving the address
> limit and resetting it to USER_DS on NMI entry.

Up to them of course, but doing less on interrupt entry/exit seems
better.