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Subject[patch 55/59] TCP: skb is unexpectedly freed.
-stable review patch.  If anyone has any objections, please let us know.

From: Masayuki Nakagawa <>

I encountered a kernel panic with my test program, which is a very
simple IPv6 client-server program.

The server side sets IPV6_RECVPKTINFO on a listening socket, and the
client side just sends a message to the server. Then the kernel panic
occurs on the server. (If you need the test program, please let me
know. I can provide it.)

This problem happens because a skb is forcibly freed in

When a socket in listening state(TCP_LISTEN) receives a syn packet,
then tcp_v6_conn_request() will be called from
tcp_rcv_state_process(). If the tcp_v6_conn_request() successfully
returns, the skb would be discarded by __kfree_skb().

However, in case of a listening socket which was already set
IPV6_RECVPKTINFO, an address of the skb will be stored in
treq->pktopts and a ref count of the skb will be incremented in
tcp_v6_conn_request(). But, even if the skb is still in use, the skb
will be freed. Then someone still using the freed skb will cause the
kernel panic.

I suggest to use kfree_skb() instead of __kfree_skb().

Signed-off-by: Masayuki Nakagawa <>
Signed-off-by: David S. Miller <>
Signed-off-by: Chris Wright <>

net/ipv4/tcp_input.c | 6 ++++--
1 file changed, 4 insertions(+), 2 deletions(-)

--- linux-
+++ linux-
@@ -4411,9 +4411,11 @@ int tcp_rcv_state_process(struct sock *s
* But, this leaves one open to an easy denial of
* service attack, and SYN cookies can't defend
* against this problem. So, we drop the data
- * in the interest of security over speed.
+ * in the interest of security over speed unless
+ * it's still in use.
- goto discard;
+ kfree_skb(skb);
+ return 0;
goto discard;

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