netdev - Re: [PATCH] net: Fix potential NULL pointer dereference in __skb_try_recv

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Date:	Wed, 30 Dec 2015 12:14:42 +0100
From:	Jacob Siverskog <jacob@...nage.engineering>
To:	David Miller <davem@...emloft.net>
Cc:	Rainer Weikusat <rweikusat@...ileactivedefense.com>,
	netdev@...r.kernel.org, Herbert Xu <herbert@...dor.apana.org.au>,
	Eric Dumazet <edumazet@...gle.com>,
	Konstantin Khlebnikov <khlebnikov@...dex-team.ru>,
	Al Viro <viro@...iv.linux.org.uk>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] net: Fix potential NULL pointer dereference in __skb_try_recv_datagram

On Tue, Dec 29, 2015 at 9:08 PM, David Miller <davem@...emloft.net> wrote:
> From: Rainer Weikusat <rweikusat@...ileactivedefense.com>
> Date: Tue, 29 Dec 2015 19:42:36 +0000
>
>> Jacob Siverskog <jacob@...nage.engineering> writes:
>>> This should fix a NULL pointer dereference I encountered (dump
>>> below). Since __skb_unlink is called while walking,
>>> skb_queue_walk_safe should be used.
>>
>> The code in question is:
>  ...
>> __skb_unlink is only called prior to returning from the function.
>> Consequently, it won't affect the skb_queue_walk code.
>
> Agreed, this patch doesn't fix anything.

Ok. Thanks for your feedback. How do you believe the issue could be
solved? Investigating it gives:

static inline void __skb_unlink(struct sk_buff *skb, struct sk_buff_head *list)
{
struct sk_buff *next, *prev;

list->qlen--;
     51c: e2433001 sub r3, r3, #1
     520: e58b3074 str r3, [fp, #116] ; 0x74
next   = skb->next;
prev   = skb->prev;
     524: e894000c ldm r4, {r2, r3}
skb->next  = skb->prev = NULL;
     528: e5841000 str r1, [r4]
     52c: e5841004 str r1, [r4, #4]
next->prev = prev;
     530: e5823004 str r3, [r2, #4]                          <--
trapping instruction (r2 NULL)

Register contents:
r7 : c58cfe1c  r6 : c06351d0  r5 : c77810ac  r4 : c583eac0
r3 : 00000000  r2 : 00000000  r1 : 00000000  r0 : 20000013

If I understand this correctly, then r4 = skb, r2 = next, r3 = prev.

Should there be a check for this in __skb_try_recv_datagram?
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