[PATCH 3.2 017/107] net: Fix skb_set_peeked use-after-free bug

[Ben Hutchings]

3.2.72-rc1 review patch.  If anyone has any objections, please let me know.

------------------

From: Herbert Xu <herbert@xxxxxxxxxxxxxxxxxxx>

commit a0a2a6602496a45ae838a96db8b8173794b5d398 upstream.

The commit 738ac1ebb96d02e0d23bc320302a6ea94c612dec ("net: Clone
skb before setting peeked flag") introduced a use-after-free bug
in skb_recv_datagram.  This is because skb_set_peeked may create
a new skb and free the existing one.  As it stands the caller will
continue to use the old freed skb.

This patch fixes it by making skb_set_peeked return the new skb
(or the old one if unchanged).

Fixes: 738ac1ebb96d ("net: Clone skb before setting peeked flag")
Reported-by: Brenden Blanco <bblanco@xxxxxxxxxxxx>
Signed-off-by: Herbert Xu <herbert@xxxxxxxxxxxxxxxxxxx>
Tested-by: Brenden Blanco <bblanco@xxxxxxxxxxxx>
Reviewed-by: Konstantin Khlebnikov <khlebnikov@xxxxxxxxxxxxxx>
Signed-off-by: David S. Miller <davem@xxxxxxxxxxxxx>
Signed-off-by: Ben Hutchings <ben@xxxxxxxxxxxxxxx>
---
 net/core/datagram.c | 13 +++++++------
 1 file changed, 7 insertions(+), 6 deletions(-)

--- a/net/core/datagram.c
+++ b/net/core/datagram.c
@@ -128,12 +128,12 @@ out_noerr:
 	goto out;
 }
 
-static int skb_set_peeked(struct sk_buff *skb)
+static struct sk_buff *skb_set_peeked(struct sk_buff *skb)
 {
 	struct sk_buff *nskb;
 
 	if (skb->peeked)
-		return 0;
+		return skb;
 
 	/* We have to unshare an skb before modifying it. */
 	if (!skb_shared(skb))
@@ -141,7 +141,7 @@ static int skb_set_peeked(struct sk_buff
 
 	nskb = skb_clone(skb, GFP_ATOMIC);
 	if (!nskb)
-		return -ENOMEM;
+		return ERR_PTR(-ENOMEM);
 
 	skb->prev->next = nskb;
 	skb->next->prev = nskb;
@@ -154,7 +154,7 @@ static int skb_set_peeked(struct sk_buff
 done:
 	skb->peeked = 1;
 
-	return 0;
+	return skb;
 }
 
 /**
@@ -216,8 +216,9 @@ struct sk_buff *__skb_recv_datagram(stru
 			*peeked = skb->peeked;
 			if (flags & MSG_PEEK) {
 
-				error = skb_set_peeked(skb);
-				if (error)
+				skb = skb_set_peeked(skb);
+				error = PTR_ERR(skb);
+				if (IS_ERR(skb))
 					goto unlock_err;
 
 				atomic_inc(&skb->users);

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