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

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Date:	Thu, 24 Jul 2008 15:31:32 +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, 2008-07-24 at 14:49 +0200, Patrick McHardy wrote:
>> Pekka Enberg wrote:
>>> 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):
> 
> Oh, I'm really sorry about that.

No problem :)

>> ---
>> 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)).
> 
> Agreed. Something like this, perhaps?
> 
> [PATCH] netfilter: fix double-free and use-after free
> 
> As suggested by Patrick McHardy, introduce a __krealloc() that doesn't
> free the original buffer to fix a double-free and use-after-free bug
> introduced by me in netfilter that uses RCU.
> 
> Reported-by: Patrick McHardy <kaber@...sh.net>
> Signed-off-by: Pekka Enberg <penberg@...helsinki.fi>

Looks good to me, thanks.
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