From 3157fbc900bdb366b2186e5a6e506cc5e4697cf0 Mon Sep 17 00:00:00 2001 From: Ben Hutchings Date: Sat, 3 Aug 2019 17:11:42 +0100 Subject: [PATCH] tcp: Clear sk_send_head after purging the write queue Denis Andzakovic discovered a potential use-after-free in older kernel versions, using syzkaller. tcp_write_queue_purge() frees all skbs in the TCP write queue and can leave sk->sk_send_head pointing to freed memory. tcp_disconnect() clears that pointer after calling tcp_write_queue_purge(), but tcp_connect() does not. It is (surprisingly) possible to add to the write queue between disconnection and reconnection, so this needs to be done in both places. This bug was introduced by backports of commit 7f582b248d0a ("tcp: purge write queue in tcp_connect_init()") and does not exist upstream because of earlier changes in commit 75c119afe14f ("tcp: implement rb-tree based retransmit queue"). The latter is a major change that's not suitable for stable. Reported-by: Denis Andzakovic Bisected-by: Salvatore Bonaccorso Fixes: 7f582b248d0a ("tcp: purge write queue in tcp_connect_init()") Cc: # before 4.15 Cc: Eric Dumazet Signed-off-by: Ben Hutchings --- include/net/tcp.h | 3 +++ 1 file changed, 3 insertions(+) diff --git a/include/net/tcp.h b/include/net/tcp.h index 79762b662de3a..7667c9adc92a6 100644 --- a/include/net/tcp.h +++ b/include/net/tcp.h @@ -1352,6 +1352,8 @@ struct tcp_fastopen_context { struct rcu_head rcu; }; +static inline void tcp_init_send_head(struct sock *sk); + /* write queue abstraction */ static inline void tcp_write_queue_purge(struct sock *sk) { @@ -1359,6 +1361,7 @@ static inline void tcp_write_queue_purge(struct sock *sk) while ((skb = __skb_dequeue(&sk->sk_write_queue)) != NULL) sk_wmem_free_skb(sk, skb); + tcp_init_send_head(sk); sk_mem_reclaim(sk); tcp_clear_all_retrans_hints(tcp_sk(sk)); } -- 2.47.2