net: Fix skb_set_peeked use-after-free bug

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@plumgrid.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>
Tested-by: Brenden Blanco <bblanco@plumgrid.com>
Reviewed-by: Konstantin Khlebnikov <khlebnikov@yandex-team.ru>
Signed-off-by: David S. Miller <davem@davemloft.net>

1 files changed, 7 insertions, 6 deletions

diff --git a/net/core/datagram.c b/net/core/datagram.c
index 4967262b2707..617088aee21d 100644
--- a/net/core/datagram.c
+++ b/net/core/datagram.c
@@ -131,12 +131,12 @@ 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))
@@ -144,7 +144,7 @@ static int skb_set_peeked(struct sk_buff *skb)
 
         nskb = skb_clone(skb, GFP_ATOMIC);
         if (!nskb)
-                return -ENOMEM;
+                return ERR_PTR(-ENOMEM);
 
         skb->prev->next = nskb;
         skb->next->prev = nskb;
@@ -157,7 +157,7 @@ static int skb_set_peeked(struct sk_buff *skb)
 done:
         skb->peeked = 1;
 
-        return 0;
+        return skb;
 }
 
 /**
@@ -229,8 +229,9 @@ struct sk_buff *__skb_recv_datagram(struct sock *sk, unsigned int flags,
                                         continue;
                                 }
 
-                                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);