Re: Ugly rmap NULL ptr deref oopsie on hibernate (was Linux2.6.34-rc3)

From: Linus Torvalds
Date: Tue Apr 06 2010 - 19:32:25 EST

On Wed, 7 Apr 2010, Borislav Petkov wrote:
> Ok, I tried doing all you suggested and here's what came out. Please,
> take this with a grain of salt because I'm almost falling asleep - even
> the coffee is not working anymore so it could be just as well that I've
> made a mistake somewhere (the new OOPS is a #GP, by the way), just
> watch:

Hey ho, yeah.

The reason it's a #GP fault is that it's not a NULL pointer dereference
any more, but a wild pointer that is not in the legal region of pointers
on x86-64. That is also why your debugging code didn't catch it: the
pointer isn't NULL, so you got the #GP fault on the same old instruction:

2b:* 49 8b 45 20 mov 0x20(%r13),%rax <-- trapping instruction

for all the same old reasons.

But now %r13 has a non-zero value: 0x002e2e2e002e2e0e, which I do _not_
recognize as any of the normal poison values.

> and %r13 contains some funny stuff, could be some mangled SLUB debug
> poison or something: R13: 002e2e2e002e2e0e. Maybe this is the reason for
> the #GP.

Correct. You don't get a page fault if the pointer was totally bogus

> But yes, even if the oopsing instruction is
> movq 32(%r13), %rax # <variable>, <variable>
> this is not because we've come to the above
> instruction through the ".L186:" label, before which we have %r13
> already loaded with anon_vma->

No, you're mis-reading the asm. It's again the first iteration, and the
code above it is again the end of the loop. And %rax is once more a kernel
pointer, not the return value of 'page_referenced_one()'.

So it once more is 'anon_vma->' that is crap, but now it's not
NULL, it's that very odd 0x002e2e2e002e2e2e pattern (the %r13 has had 0x20
subtracted from it, so that LSB of "0x0e" is actually _also_ a 0x2e).

What does '0x2e' mean? It's ASCII '.', but that doesn't really mean
anything either.

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