Re: [PATCH] net: Fix potential NULL pointer dereference in __skb_try_recv_datagram

From: Rainer Weikusat <rweikusat@mobileactivedefense.com>
To: Jacob Siverskog <jacob@teenage.engineering>
Cc: netdev@vger.kernel.org, "David S . Miller" <davem@davemloft.net>,
	Herbert Xu <herbert@gondor.apana.org.au>,
	Rainer Weikusat <rweikusat@mobileactivedefense.com>,
	Eric Dumazet <edumazet@google.com>,
	Konstantin Khlebnikov <khlebnikov@yandex-team.ru>,
	Al Viro <viro@zeniv.linux.org.uk>,
	linux-kernel@vger.kernel.org
Subject: Re: [PATCH] net: Fix potential NULL pointer dereference in __skb_try_recv_datagram
Date: Tue, 29 Dec 2015 19:42:36 +0000	[thread overview]
Message-ID: <87y4cdyrbn.fsf@doppelsaurus.mobileactivedefense.com> (raw)
In-Reply-To: <1451416224-15871-1-git-send-email-jacob@teenage.engineering> (Jacob Siverskog's message of "Tue, 29 Dec 2015 20:10:24 +0100")

Jacob Siverskog <jacob@teenage.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_queue_walk(queue, skb) {
	*last = skb;
	*peeked = skb->peeked;
	if (flags & MSG_PEEK) {
		if (_off >= skb->len && (skb->len || _off ||
					 skb->peeked)) {
			_off -= skb->len;
			continue;
		}

		skb = skb_set_peeked(skb);
		error = PTR_ERR(skb);
		if (IS_ERR(skb)) {
			spin_unlock_irqrestore(&queue->lock,
					       cpu_flags);
			goto no_packet;
		}

		atomic_inc(&skb->users);
	}  else
		__skb_unlink(skb, queue);

	spin_unlock_irqrestore(&queue->lock, cpu_flags);
	*off = _off;
	return skb;
}

__skb_unlink is only called prior to returning from the function.
Consequently, it won't affect the skb_queue_walk code.