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

[Mathieu Desnoyers]

* Linus Torvalds (torvalds@xxxxxxxxxxxxxxxxxxxx) wrote:
> 
> 
> On Mon, 15 Jun 2009, Ingo Molnar wrote:
> > 
> > A simple cr2 corruption would explain all those cc1 SIGSEGVs and 
> > other user-space crashes i saw, with sufficiently intense sampling - 
> > easily.
> 
> Note that we could work around the %cr2 issue, since any corruption is 
> always nicely "nested" (ie there are never any SMP issues with async 
> writes to the register).
> 
> So what we _could_ do is to have a magic value for %cr2, along with a "NMI 
> sequence count", and if we see that value, we just return (without doing 
> anything) from the page fault handler.
> 
> Then, the NMI handler would be changed to always write that value to %cr2 
> after it has done the operation that could fault, and do an atomic 
> increment of the NMI sequence count. Then, we can do something like this 
> in the page fault handler:
> 
> 	if (cr2 == MAGIC_CR2) {
> 		static unsigned long my_seqno = -1;
> 		if (my_seqno != nmi_seqno) {
> 			my_seqno = nmi_seqno;
> 			return;
> 		}
> 	}
> 
> where the whole (and only) point of that "seqno" is to protect against 
> user space doing something like
> 
> 	int i = *(int *)MAGIC_CR2;
> 
> and causing infinite faults.
> 
> If a real NMI happens, then nmi_seqno will always be different, and we'll 
> just retry the fault (the NMI handler would do something like
> 
> 	write_cr2(MAGIC_CR2);
> 	atomic_inc(&nmi_seqno);
> 
> to set it all up).
> 
> Anyway, I do think that the _correct_ solution is to not do page faults 
> from within NMI's, but the above is an outline of how we could _try_ to 
> handle it if we really really wanted to. IOW, the fact that cr2 gets 
> corrupted is not insurmountable, exactly because we _could_ always just 
> retrigger the page fault, and thus "re-create' the corrupted %cr2 value.
> 
> Hacky, hacky. And I'm not sure how happy CPU's even are to have %cr2 
> written to, so we could hit CPU issues.
> 

Hrm, would it be possible to save the c2 register upon nmi handler entry
and restore it before iret instead ? This would ensure a
nmi-interrupted page fault handler would continue what it was doing with
a non-corrupted cr2 register after returning from nmi.

Plus, this involves no modification to the page fault handler fast path.

But I fear I might be missing something totally obvious.

Mathieu


> 			Linus

-- 
Mathieu Desnoyers
OpenPGP key fingerprint: 8CD5 52C3 8E3C 4140 715F  BA06 3F25 A8FE 3BAE 9A68
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