linux-kernel - Ugly rmap NULL ptr deref oopsie on hibernate (was Linux 2.6.34-rc3)

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Date:	Fri, 2 Apr 2010 19:59:37 +0200
From:	Borislav Petkov <bp@...en8.de>
To:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Andrew Morton <akpm@...ux-foundation.org>
Cc:	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Ugly rmap NULL ptr deref oopsie on hibernate (was Linux 2.6.34-rc3)

Hi,

I've got the following oopsie two times now when hibernating - this
means, I don't get it everytime I hibernate but only sometimes, say once
in a blue moon.

And yeah, I couldn't catch it over serial console so I had to make ugly
pictures. By the way, the numbers in the filenames increment as I scroll
down the whole oops (yep, it hadn't completely frozen and I still could
do Shift->PgUp or Shift->PgDn on the console):

http://www.kernel.org/pub/linux/kernel/people/bp/

So, here's what I could decipher from the oopsie, someone else who's
more knowledgeable in mm, rmap and anon_vma's list traversal should be
able to tell what goes wrong there.

EIP is at page_referenced+0xee

which is

<disasm>
    10c4:	41 01 c4             	add    %eax,%r12d
    10c7:	83 7d cc 00          	cmpl   $0x0,-0x34(%rbp)
    10cb:	74 19                	je     10e6 <page_referenced+0xff>
    10cd:	4d 8b 6d 20          	mov    0x20(%r13),%r13
    10d1:	49 83 ed 20          	sub    $0x20,%r13

    10d5:	49 8b 45 20          	mov    0x20(%r13),%rax		    <--------------

    10d9:	0f 18 08             	prefetcht0 (%rax)
    10dc:	49 8d 45 20          	lea    0x20(%r13),%rax
    10e0:	48 39 45 80          	cmp    %rax,-0x80(%rbp)
</disasm>


Corresponding asm:

<asm>
	.loc 1 496 0
	movq	32(%r13), %r13	# <variable>.same_anon_vma.next, __mptr.451
.LVL295:
	subq	$32, %r13	#, avc
.LVL296:
.L184:
.LBE1278:
	movq	32(%r13), %rax	# <variable>.same_anon_vma.next, <variable>.same_anon_vma.next			<----------------
	prefetcht0	(%rax)	# <variable>.same_anon_vma.next
	leaq	32(%r13), %rax	#, tmp97
	cmpq	%rax, -128(%rbp)	# tmp97, %sfp
	jne	.L187	#,
.L186:
	.loc 1 514 0
	movq	%r14, %rdi	# anon_vma,
	call	page_unlock_anon_vma	#
</asm>


and the NULL pointer in question is being written into %r13 and then 32
is subtracted from it (I'm guessing container_of()). This is consistent
with the register snapshot - %r13 contains 0xffffffffffffffe0 which is
-32 and with the code dump in the oops, in CIMG1640.JPG code points to
opcode 49 8b 45 20.

Which is the following piece of code in <mm/rmap.c:page_referenced_anon()>.

<source>

	mapcount = page_mapcount(page);
	list_for_each_entry(avc, &anon_vma->head, same_anon_vma) {
		struct vm_area_struct *vma = avc->vma;
		unsigned long address = vma_address(page, vma);
		if (address == -EFAULT)
			continue;

</source>

which tells us that same_anon_vma.next is NULL. Hmm...

-- 
Regards/Gruss,
    Boris.
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