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

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Date:	Wed, 30 Dec 2015 15:30:22 +0100
From:	Jacob Siverskog <jacob@...nage.engineering>
To:	Eric Dumazet <edumazet@...gle.com>
Cc:	David Miller <davem@...emloft.net>,
	Rainer Weikusat <rweikusat@...ileactivedefense.com>,
	netdev <netdev@...r.kernel.org>,
	Herbert Xu <herbert@...dor.apana.org.au>,
	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 Wed, Dec 30, 2015 at 2:26 PM, Eric Dumazet <edumazet@...gle.com> wrote:
> On Wed, Dec 30, 2015 at 6:14 AM, Jacob Siverskog
> <jacob@...nage.engineering> wrote:
>
>> 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?
>
> At this point corruption already happened.
> We can not possibly detect every possible corruption caused by bugs
> elsewhere in the kernel and just 'recover' at this point.
> We must indeed find the root cause and fix it, instead of trying to hide it.
>
> How often can you trigger this bug ?

Ok. I don't have a good repro to trigger it unfortunately, I've seen it just a
few times when bringing up/down network interfaces. Does the trace
give any clue?

[<c02fc0a8>] (__skb_recv_datagram) from [<c0398f1c>] (udpv6_recvmsg+0x1d0/0x6d0)
[<c0398f1c>] (udpv6_recvmsg) from [<c0367a2c>] (inet_recvmsg+0x38/0x4c)
[<c0367a2c>] (inet_recvmsg) from [<c02efff4>] (___sys_recvmsg+0x94/0x170)
[<c02efff4>] (___sys_recvmsg) from [<c02f0d74>] (__sys_recvmsg+0x3c/0x6c)
[<c02f0d74>] (__sys_recvmsg) from [<c000f1e0>] (ret_fast_syscall+0x0/0x3c)
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