From: Michael Bommarito Date: Thu, 25 Jun 2026 18:21:39 +0000 (+0100) Subject: tcp: restore RCU grace period in tcp_ao_destroy_sock X-Git-Tag: v7.2-rc2~22^2~24^2~2 X-Git-Url: http://git.ipfire.org/?a=commitdiff_plain;h=8bc4d43bccbd60efe85d0a44d5bf41762f2f0c30;p=thirdparty%2Fkernel%2Flinux.git tcp: restore RCU grace period in tcp_ao_destroy_sock Commit 51e547e8c89c ("tcp: Free TCP-AO/TCP-MD5 info/keys without RCU") removed the call_rcu() callback from tcp_ao_destroy_sock(), arguing that "the destruction of info/keys is delayed until the socket destructor" and therefore "no one can discover it anymore". That argument does not hold for the call site in tcp_connect() (net/ipv4/tcp_output.c:4327-4332). At that point the socket is in TCP_SYN_SENT, has already been inserted into the inet ehash by inet_hash_connect() in tcp_v4_connect(), and is therefore very much discoverable: any softirq running tcp_v4_rcv() on another CPU can take the socket out of the ehash, walk into tcp_inbound_hash(), and load tp->ao_info via implicit RCU before bh_lock_sock_nested() is taken on the destroying CPU. The reader path then enters __tcp_ao_do_lookup() (net/ipv4/tcp_ao.c:208) which re-loads tp->ao_info via rcu_dereference_check(); the re-load can still observe the (about-to-be-freed) pointer because there is no synchronize_rcu() between rcu_assign_pointer(tp->ao_info, NULL) and tcp_ao_info_free() in tcp_ao_destroy_sock(). The captured pointer is then walked at line 223: hlist_for_each_entry_rcu(key, &ao->head, node, ...) The writer's synchronous kfree() is free to complete between the line 218 re-fetch and the line 223 hlist iteration. The slab is reused (or simply LIST_POISON1-stamped if not yet reused) and the iteration walks attacker-controlled or poison memory in softirq context. Reproducer (no debug shim, stock x86_64 v7.1-rc2 SMP+KASAN, QEMU+KVM): an unprivileged uid=1000 process inside CLONE_NEWUSER|CLONE_NEWNET installs TCP_MD5SIG + TCP_AO_ADD_KEY on a TCP socket, sprays forged TCP-AO segments toward its eventual 4-tuple via raw sockets, then calls connect(). The md5-wins reconciliation in tcp_connect() fires tcp_ao_destroy_sock(); the softirq backlog reader on the loopback NAPI path crashes on the freed ao->head.first walk: Oops: general protection fault, probably for non-canonical address 0xfbd59c000000002f KASAN: maybe wild-memory-access in range [0xdead000000000178-0xdead00000000017f] CPU: 0 UID: 1000 PID: 100 Comm: repro_userns RIP: 0010:__tcp_ao_do_lookup+0x107/0x1c0 Call Trace: __tcp_ao_do_lookup+0x107/0x1c0 tcp_ao_inbound_lookup.constprop.0+0x12a/0x200 tcp_inbound_ao_hash+0x5ea/0x1520 tcp_inbound_hash+0x7ce/0x1240 tcp_v4_rcv+0x1e7a/0x3e10 ... Restore the RCU grace period: re-add struct rcu_head to tcp_ao_info and replace the synchronous tcp_ao_info_free() with a call_rcu() callback. Readers that captured tp->ao_info before rcu_assign_pointer NULLed it now see the object remain valid until rcu_read_unlock(). With the patch applied the reproducer runs cleanly for 2000 iterations on the same kernel build. Fixes: 51e547e8c89c ("tcp: Free TCP-AO/TCP-MD5 info/keys without RCU") Cc: stable@vger.kernel.org # v6.18+ Reviewed-by: Dmitry Safonov Signed-off-by: Michael Bommarito Reviewed-by: Eric Dumazet Signed-off-by: Dmitry Safonov <0x7f454c46@gmail.com> Link: https://patch.msgid.link/20260625-tcp-md5-connect-v3-1-1fd313d6c1e0@gmail.com Signed-off-by: Jakub Kicinski --- diff --git a/include/net/tcp_ao.h b/include/net/tcp_ao.h index 29fd7b735afaf..9a2333e62e994 100644 --- a/include/net/tcp_ao.h +++ b/include/net/tcp_ao.h @@ -145,6 +145,7 @@ struct tcp_ao_info { u32 snd_sne; u32 rcv_sne; refcount_t refcnt; /* Protects twsk destruction */ + struct rcu_head rcu; }; #ifdef CONFIG_TCP_MD5SIG diff --git a/net/ipv4/tcp_ao.c b/net/ipv4/tcp_ao.c index a56bb79e15e0e..e4ec60a334963 100644 --- a/net/ipv4/tcp_ao.c +++ b/net/ipv4/tcp_ao.c @@ -371,8 +371,9 @@ static void tcp_ao_key_free_rcu(struct rcu_head *head) kfree_sensitive(key); } -static void tcp_ao_info_free(struct tcp_ao_info *ao) +static void tcp_ao_info_free_rcu(struct rcu_head *head) { + struct tcp_ao_info *ao = container_of(head, struct tcp_ao_info, rcu); struct tcp_ao_key *key; struct hlist_node *n; @@ -411,7 +412,7 @@ void tcp_ao_destroy_sock(struct sock *sk, bool twsk) if (!twsk) tcp_ao_sk_omem_free(sk, ao); - tcp_ao_info_free(ao); + call_rcu(&ao->rcu, tcp_ao_info_free_rcu); } void tcp_ao_time_wait(struct tcp_timewait_sock *tcptw, struct tcp_sock *tp)