Patch "tcp: fix tcp_mtu_probe() vs highest_sack" has been added to the 4.9-stable tree

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

tcp: fix tcp_mtu_probe() vs highest_sack

to the 4.9-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-tcp_mtu_probe-vs-highest_sack.patch
and it can be found in the queue-4.9 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: Mon, 30 Oct 2017 23:08:20 -0700
Subject: tcp: fix tcp_mtu_probe() vs highest_sack

From: Eric Dumazet <[email protected]>

[ Upstream commit 2b7cda9c35d3b940eb9ce74b30bbd5eb30db493d ]

Based on SNMP values provided by Roman, Yuchung made the observation that some crashes in tcp_sacktag_walk() might be caused by MTU probing.

Looking at tcp_mtu_probe(), I found that when a new skb was placed in front of the write queue, we were not updating tcp highest sack.

If one skb is freed because all its content was copied to the new skb (for MTU probing), then tp->highest_sack could point to a now freed skb.

Bad things would then happen, including infinite loops.

This patch renames tcp_highest_sack_combine() and uses it from tcp_mtu_probe() to fix the bug.

Note that I also removed one test against tp->sacked_out, since we want to replace tp->highest_sack regardless of whatever condition, since keeping a stale pointer to freed skb is a recipe for disaster.

---
 include/net/tcp.h     |    6 +++---
 net/ipv4/tcp_output.c |    3 ++-
 2 files changed, 5 insertions(+), 4 deletions(-)

@@ -1681,12 +1681,12 @@ static inline void tcp_highest_sack_rese
 	tcp_sk(sk)->highest_sack = tcp_write_queue_head(sk);
 }
 
-/* Called when old skb is about to be deleted (to be combined with new skb) */
-static inline void tcp_highest_sack_combine(struct sock *sk,
+/* Called when old skb is about to be deleted and replaced by new skb */
+static inline void tcp_highest_sack_replace(struct sock *sk,
 					    struct sk_buff *old,
 					    struct sk_buff *new)
 {
-	if (tcp_sk(sk)->sacked_out && (old == tcp_sk(sk)->highest_sack))
+	if (old == tcp_highest_sack(sk))
 		tcp_sk(sk)->highest_sack = new;
 }
 

@@ -1996,6 +1996,7 @@ static int tcp_mtu_probe(struct sock *sk
 	nskb->ip_summed = skb->ip_summed;
 
 	tcp_insert_write_queue_before(nskb, skb, sk);
+	tcp_highest_sack_replace(sk, skb, nskb);
 
 	len = 0;
 	tcp_for_write_queue_from_safe(skb, next, sk) {

@@ -2535,7 +2536,7 @@ static void tcp_collapse_retrans(struct
 
 	BUG_ON(tcp_skb_pcount(skb) != 1 || tcp_skb_pcount(next_skb) != 1);
 
-	tcp_highest_sack_combine(sk, next_skb, skb);
+	tcp_highest_sack_replace(sk, next_skb, skb);
 
 	tcp_unlink_write_queue(next_skb, sk);
 

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

queue-4.9/net-call-cgroup_sk_alloc-earlier-in-sk_clone_lock.patch queue-4.9/tcp-dccp-fix-ireq-opt-races.patch queue-4.9/tcp-fix-tcp_mtu_probe-vs-highest_sack.patch queue-4.9/ipv6-addrconf-increment-ifp-refcount-before-ipv6_del_addr.patch queue-4.9/ipv6-flowlabel-do-not-leave-opt-tot_len-with-garbage.patch queue-4.9/packet-avoid-panic-in-packet_getsockopt.patch queue-4.9/sctp-add-the-missing-sock_owned_by_user-check-in-sctp_icmp_redirect.patch queue-4.9/net_sched-avoid-matching-qdisc-with-zero-handle.patch queue-4.9/tun-tap-sanitize-tunsetsndbuf-input.patch queue-4.9/tcp-dccp-fix-lockdep-splat-in-inet_csk_route_req.patch queue-4.9/tcp-dccp-fix-other-lockdep-splats-accessing-ireq_opt.patch