linux-kernel - Re: [regression] nf_iterate(), BUG: unable to handle kernel NULL pointer dereference

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Date:	Thu, 24 Jul 2008 14:49:53 +0200
From:	Patrick McHardy <kaber@...sh.net>
To:	Pekka Enberg <penberg@...helsinki.fi>
CC:	Ingo Molnar <mingo@...e.hu>, David Miller <davem@...emloft.net>,
	herbert@...dor.apana.org.au, w@....eu, davidn@...idnewall.com,
	torvalds@...ux-foundation.org, akpm@...ux-foundation.org,
	netdev@...r.kernel.org, linux-kernel@...r.kernel.org,
	stefanr@...6.in-berlin.de, rjw@...k.pl, ilpo.jarvinen@...sinki.fi,
	Dave Jones <davej@...hat.com>
Subject: Re: [regression] nf_iterate(), BUG: unable to handle kernel NULL
 pointer dereference

Pekka Enberg wrote:
> On Thu, Jul 24, 2008 at 3:03 PM, Patrick McHardy <kaber@...sh.net> wrote:
>> Ingo Molnar wrote:
>>> * Ingo Molnar <mingo@...e.hu> wrote:
>>>
>>>> here's a new type of crash a -tip testbox triggered today:
>>>>
>>>> BUG: unable to handle kernel NULL pointer dereference at 0000000000000000
>>>> IP: [<0000000000000000>] 0x0
>>>> ...
>>>> Call Trace:
>>>>  [<ffffffff80777f25>] ? ipv4_confirm+0x8d/0x122
>>>>  [<ffffffff80733107>] nf_iterate+0x43/0xa5
>> Does reverting 31d8519c fix this?
> 
> Hmm, why do you think it's related? It looks like elem->hook() is a
> NULL pointer but my patch shouldn't make any difference here...

No, its already in ipv4_confirm, so its most likely helper->help()
thats NULL, which is contained in an extension.

The reason why I think its this patch is (quoting from an old
email that I never got a response to):

---
Your patch introduced a use-after-free and double-free.
krealloc() frees the old pointer, but it is still used
for the ->move operations, then freed again.

To fix this I think we need a __krealloc() that doesn't
free the old memory, especially since it must not be
freed immediately because it may still be used in a RCU
read side (see the last part in the patch attached to
this mail (based on a kernel without your patch)).
---

I've attached that patch again.

View attachment "01.diff" of type "text/x-diff" (3307 bytes)