Re: perf_fuzzer compiled for x32 causes reboot

[Peter Zijlstra <peterz@infradead.org>]

On Fri, Feb 28, 2014 at 11:29:46AM -0500, Steven Rostedt wrote:
> On Fri, 28 Feb 2014 08:15:11 -0800
> "H. Peter Anvin" <hpa@zytor.com> wrote:
> 
> > Well, I was talking about the assumption spelled out in the comment
> > above copy_from_user_nmi() which pretty much states "cr2 is safe because
> > cr2 is saved/restored in the NMI wrappers."
> 
> Yeah, it seems that the name "copy_from_user_nmi()" is a misnomer. As
> it can be called outside of nmi context. Perhaps we should have a
> copy_from_user_trace() that does the save and restore of the cr2.
> 
> As that's the only place that faults, it may be the best answer.
> 
> Arnaldo,
> 
> Can you test this patch and see if it fixes the bug for you too. Vince
> already said it fixes it for him.
> 
> I'm attaching it below (it's from H. Peter).
> 
> Peter Z., as both Jiri's and my patch fixed the callers of the problem
> area, and as we have been discussing, there may be more problem areas,
> I'm thinking the best solution is to just use H. Peter's patch instead.
> And then we should rename it to copy_from_user_trace().

Like already said; _trace is an absolutely abysmal name. Also you
_really_ don't want an unconditional CR2 write in there, that's just
stupidly expensive.

Also, this function is called a _LOT_ under certain workloads, I don't
know how cheap a CR2 read is, but it had better be really cheap.