Patch "tcp: fix use after free in tcp_xmit_retransmit_queue()" has been added to the 4.7-stable tree

This is a note to let you know that I've just added the patch titled

tcp: fix use after free in tcp_xmit_retransmit_queue()

to the 4.7-stable tree which can be found at:
http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary

The filename of the patch is:
tcp-fix-use-after-free-in-tcp_xmit_retransmit_queue.patch
and it can be found in the queue-4.7 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree, please let <[email protected]> know about it.

From: Eric Dumazet <[email protected]>
Date: Wed, 17 Aug 2016 05:56:26 -0700
Subject: tcp: fix use after free in tcp_xmit_retransmit_queue()

From: Eric Dumazet <[email protected]>

[ Upstream commit bb1fceca22492109be12640d49f5ea5a544c6bb4 ]

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.

---
 include/net/tcp.h |    2 ++
 1 file changed, 2 insertions(+)

@@ -1522,6 +1522,8 @@ static inline void tcp_check_send_head(s
 {
 	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)

Patches currently in stable-queue which might be from [email protected] are

queue-4.7/bonding-fix-bonding-crash.patch queue-4.7/tcp-properly-scale-window-in-tcp_v_reqsk_send_ack.patch queue-4.7/tcp-fastopen-avoid-negative-sk_forward_alloc.patch queue-4.7/udp-fix-poll-issue-with-zero-sized-packets.patch queue-4.7/tcp-fix-use-after-free-in-tcp_xmit_retransmit_queue.patch queue-4.7/tun-fix-transmit-timestamp-support.patch