IPV6_ADDRFORM turns an AF_INET6 TCP socket into an AF_INET one. It requires the socket to be established, and a listener can get there with connect(AF_UNSPEC) followed by connect(). Request sockets queued while it was listening are still there: inet_csk_listen_stop() leaves them in the ehash, and their timers only drop them while the socket is not listening, so making it listen again keeps them alive. A request that arrived over IPv6 was hashed with inet6_ehashfn(). Its child is cloned from the converted socket and hashed with inet_ehashfn(), so it belongs in a different bucket. inet_ehash_insert() locks the child's bucket, warns about the mismatching hashes, and replaces the request with the child in the request's own bucket anyway. reqsk_queue_unlink() locks the bucket the request is really in, so there is no synchronization between the two. Both can see the request still hashed and both can drop the reference the ehash holds. The extra put takes the request's refcount to zero too early, so it is freed while it is still on the listener's accept queue. The listener is then closed, and inet_csk_listen_stop() reads the freed request and writes to it in reqsk_put(). In short: socket(AF_INET6) -> setsockopt(TCP_DEFER_ACCEPT, 120) -> bind -> listen // a native IPv6 client connects and sends nothing // the request stays in the bucket inet6_ehashfn() picked connect(AF_UNSPEC) // stop listening connect(a v4-mapped peer) // become established setsockopt(IPV6_ADDRFORM, PF_INET) // become an AF_INET socket connect(AF_UNSPEC), listen() // listen again // the client sends data and the leftover request completes close() // use-after-free KASAN log: BUG: KASAN: slab-use-after-free in inet_csk_listen_stop+0x1c2/0x760 Write of size 4 at addr ffff888010b7cbe0 by task init/1 ... Call Trace: inet_csk_listen_stop+0x1c2/0x760 __tcp_close+0x6c1/0x7b0 tcp_close+0x23/0x90 inet_release+0x93/0x100 __sock_release+0x66/0x130 sock_close+0x18/0x20 __fput+0x1f0/0x4c0 fput_close_sync+0xd2/0x170 __x64_sys_close+0x55/0x90 ... Allocated by task 0: inet_reqsk_alloc+0x8c/0x320 tcp_conn_request+0x324/0x11f0 tcp_rcv_state_process+0x2ff/0x2cf0 tcp_v6_do_rcv+0x326/0xc30 tcp_v6_rcv+0x1e07/0x1e90 ... Freed by task 0: slab_free_after_rcu_debug+0xc5/0x200 rcu_core+0x4dc/0xd20 ... Last potentially related work creation: kmem_cache_free+0x11d/0x5f0 tcp_v6_rcv+0xb30/0x1e90 ... The buggy address belongs to the object at ffff888010b7cb60 which belongs to the cache request_sock_TCPv6 of size 352 Refuse the conversion if inet_csk_reqsk_queue_len() is not zero. Nothing clears that counter when a socket stops listening or listens again, so it still accounts for the requests left in the ehash. A socket that never listened is not affected. Reading the counter once is not enough. inet_csk_reqsk_queue_hash_add() puts the request in the ehash first and bumps the counter second, so a setsockopt that lands in between misses a request that is already reachable. No new request is created once the socket stops listening, and a SYN that found it while it was still listening is handled inside the RCU read-side critical section the receive path holds. Waiting for one grace period before reading again therefore leaves no request that is reachable but not yet counted. Fixes: 079096f103fa ("tcp/dccp: install syn_recv requests into ehash table") Cc: stable@vger.kernel.org Signed-off-by: Hyunwoo Kim --- Changes in v2: - Read inet_csk_reqsk_queue_len() again after synchronize_rcu(). A request is put in the ehash before the counter is bumped, so v1 could be raced. - Move the check below the TCP_ESTABLISHED and v4-mapped checks. In v1 a listener returned -EBUSY or -ENOTCONN depending on whether a peer had a half-open connection at that moment. - Add the trigger sequence and the KASAN log to the commit message. - v1: https://lore.kernel.org/all/20260817090319.3897799-2-imv4bel@gmail.com/ --- net/ipv6/ipv6_sockglue.c | 15 +++++++++++++++ 1 file changed, 15 insertions(+) diff --git a/net/ipv6/ipv6_sockglue.c b/net/ipv6/ipv6_sockglue.c index b4c977434c2e0a..6d2dd9ae46a964 100644 --- a/net/ipv6/ipv6_sockglue.c +++ b/net/ipv6/ipv6_sockglue.c @@ -587,6 +587,21 @@ int do_ipv6_setsockopt(struct sock *sk, int level, int optname, break; } + if (sk->sk_protocol == IPPROTO_TCP) { + if (inet_csk_reqsk_queue_len(sk)) { + retv = -EBUSY; + break; + } + /* A SYN that found this socket while it was + * still listening may not be counted yet. + */ + synchronize_rcu(); + if (inet_csk_reqsk_queue_len(sk)) { + retv = -EBUSY; + break; + } + } + __ipv6_sock_mc_close(sk); __ipv6_sock_ac_close(sk); -- 2.43.0