Patch "net: Clone skb before setting peeked flag" has been added to the 4.1-stable tree

This is a note to let you know that I've just added the patch titled

net: Clone skb before setting peeked flag

to the 4.1-stable tree which can be found at:
http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary

The filename of the patch is:
net-clone-skb-before-setting-peeked-flag.patch
and it can be found in the queue-4.1 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree, please let <[email protected]> know about it.

From: Herbert Xu <[email protected]>
Date: Mon, 13 Jul 2015 16:04:13 +0800
Subject: net: Clone skb before setting peeked flag

From: Herbert Xu <[email protected]>

[ Upstream commit 738ac1ebb96d02e0d23bc320302a6ea94c612dec ]

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.

---
 net/core/datagram.c |   41 ++++++++++++++++++++++++++++++++++++++---
 1 file changed, 38 insertions(+), 3 deletions(-)

@@ -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(stru
 		 * 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(stru
 					_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(stru
 
 	return NULL;
 
+unlock_err:
+	spin_unlock_irqrestore(&queue->lock, cpu_flags);
 no_packet:
 	*err = error;
 	return NULL;

Patches currently in stable-queue which might be from [email protected] are

queue-4.1/powerpc-uncomment-and-make-enable_kernel_vsx-routine-available.patch queue-4.1/net-fix-skb_set_peeked-use-after-free-bug.patch queue-4.1/ipv6-lock-socket-in-ip6_datagram_connect.patch queue-4.1/net-fix-skb-csum-races-when-peeking.patch queue-4.1/rhashtable-fix-for-resize-events-during-table-walk.patch queue-4.1/net-clone-skb-before-setting-peeked-flag.patch queue-4.1/revert-sit-add-gro-callbacks-to-sit_offload.patch queue-4.1/crypto-vmx-adding-enable_kernel_vsx-to-access-vsx-instructions.patch queue-4.1/netlink-make-sure-ebusy-won-t-escape-from-netlink_insert.patch