[PATCH 3.16.y-ckt 070/130] net: Fix skb_set_peeked use-after-free bug

From: Luis Henriques
Date: Fri Sep 04 2015 - 09:39:04 EST

3.16.7-ckt17 -stable 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: Luis Henriques <luis.henriques@xxxxxxxxxxxxx>
net/core/datagram.c | 13 +++++++------
1 file changed, 7 insertions(+), 6 deletions(-)

diff --git a/net/core/datagram.c b/net/core/datagram.c
index 2b2ed1ae5b63..3c69a464595e 100644
--- a/net/core/datagram.c
+++ b/net/core/datagram.c
@@ -130,12 +130,12 @@ 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))
@@ -143,7 +143,7 @@ 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;
@@ -156,7 +156,7 @@ static int skb_set_peeked(struct sk_buff *skb)
skb->peeked = 1;

- return 0;
+ return skb;

@@ -228,8 +228,9 @@ struct sk_buff *__skb_recv_datagram(struct sock *sk, unsigned int flags,

- error = skb_set_peeked(skb);
- if (error)
+ skb = skb_set_peeked(skb);
+ error = PTR_ERR(skb);
+ if (IS_ERR(skb))
goto unlock_err;

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