From: Michael Bommarito Date: Thu, 25 Jun 2026 18:21:40 +0000 (+0100) Subject: tcp: defer md5sig_info kfree past RCU grace period in tcp_connect X-Git-Tag: v7.2-rc2~22^2~24^2~1 X-Git-Url: http://git.ipfire.org/?a=commitdiff_plain;h=b74cd55038905d5e74c1de109ab78a30b2ea0e1f;p=thirdparty%2Fkernel%2Flinux.git tcp: defer md5sig_info kfree past RCU grace period in tcp_connect The md5+ao reconciliation in tcp_connect() (net/ipv4/tcp_output.c) has two symmetric branches: if (needs_md5) { tcp_ao_destroy_sock(sk, false); } else if (needs_ao) { tcp_clear_md5_list(sk); kfree(rcu_replace_pointer(tp->md5sig_info, NULL, ...)); } Both branches free a per-socket auth-info object while the socket is in TCP_SYN_SENT and is already on the inet ehash (inserted by inet_hash_connect() in tcp_v4_connect()). Both branches are reachable by softirq RX-path readers that load the corresponding info pointer via implicit RCU before bh_lock_sock_nested() is taken. The needs_md5 branch is fixed in the prior patch by re-introducing the call_rcu() free in tcp_ao_destroy_sock(): the equivalent per-key loop runs inside tcp_ao_info_free_rcu(), the RCU callback, so by the time it frees each tcp_ao_key all softirq readers that captured the container have already completed rcu_read_unlock(). The needs_ao branch is not symmetric in the same way. The container free can be deferred via kfree_rcu(md5sig, rcu) -- struct tcp_md5sig_info already has the required rcu member (include/net/tcp.h:1999-2002), and the rest of the tree already does this in the tcp_md5sig_info_add() rollback paths (net/ipv4/tcp_ipv4.c:1410, 1436). But the per-key teardown is done by tcp_clear_md5_list() in process context BEFORE the container's RCU grace period: it walks &md5sig->head and frees each tcp_md5sig_key with bare hlist_del + kfree. A concurrent softirq reader in __tcp_md5_do_lookup() / __tcp_md5_do_lookup_exact() (tcp_ipv4.c:1253, 1298) walks the same list via hlist_for_each_entry_rcu() and races with that bare kfree on the keys themselves -- a per-key slab use-after-free of the same class as the TCP-AO bug, on the same race window. Fix this in two halves: 1. Convert the bare kfree() in tcp_connect() to kfree_rcu() so the md5sig_info container joins the rest of the md5sig lifecycle. The local-variable lift is mechanical and required because kfree_rcu() is a macro that expects an lvalue. 2. Make tcp_clear_md5_list() RCU-safe by replacing hlist_del + kfree(key) with hlist_del_rcu + kfree_rcu(key, rcu). struct tcp_md5sig_key already carries the rcu member (include/net/tcp.h:1995) and tcp_md5_do_del() (net/ipv4/tcp_ipv4.c:1456) already uses kfree_rcu, so this restores the lifecycle invariant the rest of the file follows rather than introducing a one-off. The other caller of tcp_clear_md5_list() is tcp_md5_destruct_sock() (net/ipv4/tcp.c:412), which runs from the sock destructor when the socket is already unhashed and unreachable; the extra grace period there is unnecessary but harmless. Making the helper unconditionally RCU-safe is the cleaner contract. The needs_ao branch is not reachable by the userns reproducer used to demonstrate the AO-side splat (the repro installs both keys but ends up in the needs_md5 branch because the connect peer matches the MD5 key, not the AO key); however the symmetric race exists and a maintainer touching this code should not have to think about which branch escapes RCU and which one does not. Fixes: 51e547e8c89c ("tcp: Free TCP-AO/TCP-MD5 info/keys without RCU") Cc: stable@vger.kernel.org # v6.18+ Suggested-by: Eric Dumazet Signed-off-by: Michael Bommarito Reviewed-by: Dmitry Safonov Reviewed-by: Eric Dumazet [also credits to Qihang, who found that this races with tcp-diag] Reported-by: Qihang Signed-off-by: Dmitry Safonov <0x7f454c46@gmail.com> Link: https://patch.msgid.link/20260625-tcp-md5-connect-v3-2-1fd313d6c1e0@gmail.com Signed-off-by: Jakub Kicinski --- diff --git a/net/ipv4/tcp_ipv4.c b/net/ipv4/tcp_ipv4.c index ec09f97cc9e6..209ef7522508 100644 --- a/net/ipv4/tcp_ipv4.c +++ b/net/ipv4/tcp_ipv4.c @@ -1467,9 +1467,9 @@ void tcp_clear_md5_list(struct sock *sk) md5sig = rcu_dereference_protected(tp->md5sig_info, 1); hlist_for_each_entry_safe(key, n, &md5sig->head, node) { - hlist_del(&key->node); + hlist_del_rcu(&key->node); atomic_sub(sizeof(*key), &sk->sk_omem_alloc); - kfree(key); + kfree_rcu(key, rcu); } } diff --git a/net/ipv4/tcp_output.c b/net/ipv4/tcp_output.c index 00ec4b5900f2..bc03809ca3af 100644 --- a/net/ipv4/tcp_output.c +++ b/net/ipv4/tcp_output.c @@ -4329,9 +4329,13 @@ int tcp_connect(struct sock *sk) if (needs_md5) { tcp_ao_destroy_sock(sk, false); } else if (needs_ao) { + struct tcp_md5sig_info *md5sig; + tcp_clear_md5_list(sk); - kfree(rcu_replace_pointer(tp->md5sig_info, NULL, - lockdep_sock_is_held(sk))); + md5sig = rcu_replace_pointer(tp->md5sig_info, NULL, + lockdep_sock_is_held(sk)); + if (md5sig) + kfree_rcu(md5sig, rcu); } } #endif