tcp: fix use after free in tcp_xmit_retransmit_queue()

When tcp_sendmsg() allocates a fresh and empty skb, it puts it at the
tail of the write queue using tcp_add_write_queue_tail()

Then it attempts to copy user data into this fresh skb.

If the copy fails, we undo the work and remove the fresh skb.

Unfortunately, this undo lacks the change done to tp->highest_sack and
we can leave a dangling pointer (to a freed skb)

Later, tcp_xmit_retransmit_queue() can dereference this pointer and
access freed memory. For regular kernels where memory is not unmapped,
this might cause SACK bugs because tcp_highest_sack_seq() is buggy,
returning garbage instead of tp->snd_nxt, but with various debug
features like CONFIG_DEBUG_PAGEALLOC, this can crash the kernel.

This bug was found by Marco Grassi thanks to syzkaller.

Fixes: 6859d49475d4 ("[TCP]: Abstract tp->highest_sack accessing & point to next skb")
Reported-by: Marco Grassi <>
Signed-off-by: Eric Dumazet <>
Cc: Ilpo Järvinen <>
Cc: Yuchung Cheng <>
Cc: Neal Cardwell <>
Acked-by: Neal Cardwell <>
Reviewed-by: Cong Wang <>
Signed-off-by: David S. Miller <>
(cherry picked from commit bb1fceca22492109be12640d49f5ea5a544c6bb4)

Change-Id: Iad69bade5840fea19186c8e4ecbf9b3e5f49013b
diff --git a/include/net/tcp.h b/include/net/tcp.h
index cfb55e1..54285ba 100644
--- a/include/net/tcp.h
+++ b/include/net/tcp.h
@@ -1390,6 +1390,8 @@
 	if (sk->sk_send_head == skb_unlinked)
 		sk->sk_send_head = NULL;
+	if (tcp_sk(sk)->highest_sack == skb_unlinked)
+		tcp_sk(sk)->highest_sack = NULL;
 static inline void tcp_init_send_head(struct sock *sk)