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 <denis.andzakovic@pulsesecurity.co.nz> Bisected-by: Salvatore Bonaccorso <carnil@debian.org> Fixes: 7f582b248d0a ("tcp: purge write queue in tcp_connect_init()") Cc: <stable@vger.kernel.org> # before 4.15 Cc: Eric Dumazet <edumazet@google.com> Signed-off-by: Ben Hutchings <ben@decadent.org.uk> CVE-2020-0040 Signed-off-by: Kevin F. Haggerty <haggertk@lineageos.org> Change-Id: I3c20d87056584d59741e8c94137c495e37581d84
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@ -1256,6 +1256,8 @@ extern int tcp_md5_hash_skb_data(struct tcp_md5sig_pool *, const struct sk_buff
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extern int tcp_md5_hash_key(struct tcp_md5sig_pool *hp,
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const struct tcp_md5sig_key *key);
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static inline void tcp_init_send_head(struct sock *sk);
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/* write queue abstraction */
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static inline void tcp_write_queue_purge(struct sock *sk)
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{
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@ -1263,6 +1265,7 @@ static inline void tcp_write_queue_purge(struct sock *sk)
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while ((skb = __skb_dequeue(&sk->sk_write_queue)) != NULL)
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sk_wmem_free_skb(sk, skb);
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tcp_init_send_head(sk);
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sk_mem_reclaim(sk);
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tcp_clear_all_retrans_hints(tcp_sk(sk));
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}
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