Commit 738ac1eb authored by Herbert Xu's avatar Herbert Xu Committed by David S. Miller
Browse files

net: Clone skb before setting peeked flag 



Shared skbs must not be modified and this is crucial for broadcast
and/or multicast paths where we use it as an optimisation to avoid
unnecessary cloning.

The function skb_recv_datagram breaks this rule by setting peeked
without cloning the skb first.  This causes funky races which leads
to double-free.

This patch fixes this by cloning the skb and replacing the skb
in the list when setting skb->peeked.

Fixes: a59322be ("[UDP]: Only increment counter on first peek/recv")
Reported-by: default avatarKonstantin Khlebnikov <khlebnikov@yandex-team.ru>
Signed-off-by: default avatarHerbert Xu <herbert@gondor.apana.org.au>
Signed-off-by: default avatarDavid S. Miller <davem@davemloft.net>
parent 035d210f

net/core/datagram.c

+38 −3
Original line number Diff line number Diff line
@@ -131,6 +131,35 @@ out_noerr: 
	goto out;

}



static int skb_set_peeked(struct sk_buff *skb)

{

	struct sk_buff *nskb;



	if (skb->peeked)

		return 0;



	/* We have to unshare an skb before modifying it. */

	if (!skb_shared(skb))

		goto done;



	nskb = skb_clone(skb, GFP_ATOMIC);

	if (!nskb)

		return -ENOMEM;



	skb->prev->next = nskb;

	skb->next->prev = nskb;

	nskb->prev = skb->prev;

	nskb->next = skb->next;



	consume_skb(skb);

	skb = nskb;



done:

	skb->peeked = 1;



	return 0;

}



/**

 *	__skb_recv_datagram - Receive a datagram skbuff

 *	@sk: socket
@@ -165,7 +194,9 @@ out_noerr: 
struct sk_buff *__skb_recv_datagram(struct sock *sk, unsigned int flags,

				    int *peeked, int *off, int *err)

{

	struct sk_buff_head *queue = &sk->sk_receive_queue;

	struct sk_buff *skb, *last;

	unsigned long cpu_flags;

	long timeo;

	/*

	 * Caller is allowed not to check sk->sk_err before skb_recv_datagram()
@@ -184,8 +215,6 @@ struct sk_buff *__skb_recv_datagram(struct sock *sk, unsigned int flags, 
		 * Look at current nfs client by the way...

		 * However, this function was correct in any case. 8)

		 */

		unsigned long cpu_flags;

		struct sk_buff_head *queue = &sk->sk_receive_queue;

		int _off = *off;



		last = (struct sk_buff *)queue;
@@ -199,7 +228,11 @@ struct sk_buff *__skb_recv_datagram(struct sock *sk, unsigned int flags, 
					_off -= skb->len;

					continue;

				}

				skb->peeked = 1;



				error = skb_set_peeked(skb);

				if (error)

					goto unlock_err;



				atomic_inc(&skb->users);

			} else

				__skb_unlink(skb, queue);
@@ -223,6 +256,8 @@ struct sk_buff *__skb_recv_datagram(struct sock *sk, unsigned int flags, 


	return NULL;



unlock_err:

	spin_unlock_irqrestore(&queue->lock, cpu_flags);

no_packet:

	*err = error;

	return NULL;

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