net: Fix skb_set_peeked use-after-free bug

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@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>
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>
(cherry picked from commit e553622ccb6b6e06079f980f55cf04128db3420c)
Signed-off-by: Willy Tarreau <w@1wt.eu>

diff --git a/net/core/datagram.c b/net/core/datagram.c
index cbb3100c37dc8de7017db186681240dadbb694b5..c855336b3ebbeb7394a5c12730d41ee94495c527 100644 (file)
--- a/net/core/datagram.c
+++ b/net/core/datagram.c
@@ -127,12 +127,12 @@ 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))
@@ -140,7 +140,7 @@ 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;
@@ -153,7 +153,7 @@ static int skb_set_peeked(struct sk_buff *skb)
 done:
        skb->peeked = 1;
 
-       return 0;
+       return skb;
 }
 
 /**
@@ -214,8 +214,9 @@ struct sk_buff *__skb_recv_datagram(struct sock *sk, unsigned flags,
                        *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);