Re: [tip:perfcounters/core] perf_counter: x86: Fix call-chainsupport to use NMI-safe methods

[Ingo Molnar]

* Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx> wrote:

> On Mon, 15 Jun 2009, tip-bot for Peter Zijlstra wrote:
> > 
> > __copy_from_user_inatomic() isn't NMI safe in that it can trigger
> > the page fault handler which is another trap and its return path
> > invokes IRET which will also close the NMI context.
> 
> That's not the only problem.
> 
> An even more fundamental problem is that the page fault handler is 
> not re-entrant because of simple the value in %cr2. So regardless 
> of any 'iret' issues, you *CANNOT* take a page fault in an NMI, 
> because the NMI might happen while we're in the critical region of 
> having taken another page fault, but before we've saved off the 
> value of %cr2 in that old page fault.
>
> If the NMI handler causes a page fault, it will corrupt the %cr2 
> of the outer page fault. That's why the page fault is done with an 
> interrupt gate, and why we have that conditional 
> local_irq_enable() in it.
> 
> So page faults are fundamentally only safe wrt normal interrupts, 
> not NMI.

ahhh ... a light goes up. Indeed.

I was suspecting something much more complex: like the CPU somehow 
having shadow state for attempted-fault which gets confused by 
NMI->fault.

A simple cr2 corruption would explain all those cc1 SIGSEGVs and 
other user-space crashes i saw, with sufficiently intense sampling - 
easily.

The thing is, that __copy_user_inatomic() has been in 
arch/x86/oprofile/backtrace.c for years, i didnt even suspect some 
simple, fundamental flaw like this. Apparently nobody uses it.

This is really good news in a sense: i really hate that additional 
entry*.S mucking in the exception path in the dont-IRET patch. We 
want less entry*.S magic, not more.

	Ingo
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