linux-kernel - Re: BUG: Bad page map in process udevd (anon

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Date:	Thu, 17 Feb 2011 20:30:42 -0800
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	"Eric W. Biederman" <ebiederm@...ssion.com>,
	Eric Dumazet <eric.dumazet@...il.com>,
	Octavian Purdila <opurdila@...acom.com>,
	David Miller <davem@...emloft.net>
Cc:	Ingo Molnar <mingo@...e.hu>, Michal Hocko <mhocko@...e.cz>,
	linux-mm@...ck.org, LKML <linux-kernel@...r.kernel.org>
Subject: Re: BUG: Bad page map in process udevd (anon_vma: (null)) in 2.6.38-rc4

On Thu, Feb 17, 2011 at 7:16 PM, Eric W. Biederman
<ebiederm@...ssion.com> wrote:
>
> Interesting.  I just got this with DEBUG_PAGEALLOC
> It looks like something in DEBUG_PAGEALLOC is interfering with taking a
> successful crashdump.

Hmm. I don't see why, but we don't care. Just the IP and the Code:
section is plenty good enough.

> BUG: unable to handle kernel paging request at ffff8801adf8d760
> IP: [<ffffffff8140c7ca>] unregister_netdevice_queue+0x3a/0xb0

Yup. That's the "list_move()". The disassembly is exactly what I'd
expect from __list_del():

  16:	48 8b 93 a0 00 00 00 	mov    0xa0(%rbx),%rdx
  1d:	48 8b 83 a8 00 00 00 	mov    0xa8(%rbx),%rax
  24:	48 8d bb a0 00 00 00 	lea    0xa0(%rbx),%rdi
  2b:*	48 89 42 08          	mov    %rax,0x8(%rdx)     <-- trapping instruction
  2f:	48 89 10             	mov    %rdx,(%rax)

So I think we can consider this confirmed: it really is the stale
queue left over on the stack (introduced by commit 443457242beb). With
CONFIG_DEBUG_PAGEALLOC, you get a page fault when it tries to update
the now stale pointers.

The patch from Eric Dumazet (which adds a few more cases to my patch
and hopefully catches them all) almost certainly fixes this rather
nasty memory corruption.

                           Linus
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