[PATCH net-next v5 0/2] tcp: Destroy TCP-AO, TCP-MD5 keys in .sk_destruct()

[PATCH net-next v5 0/2] tcp: Destroy TCP-AO, TCP-MD5 keys in .sk_destruct()

[Dmitry Safonov via B4 Relay]

On one side a minor/cosmetic issue, especially nowadays when
TCP-AO/TCP-MD5 signature verification failures aren't logged to dmesg.

Yet, I think worth addressing for two reasons:
- unsigned RST gets ignored by the peer and the connection is alive for
  longer (keep-alive interval)
- netstat counters increase and trace events report that trusted BGP peer
  is sending unsigned/incorrectly signed segments, which can ring alarm
  on monitoring.

Signed-off-by: Dmitry Safonov <dima@arista.com>
---
Changes in v5:
- Use EXPORT_IPV6_MOD_GPL() instead of EXPORT_SYMBOL_GPL()
  for tcp_md5_destruct_sock (thanks to Kuniyuki Iwashima)
- Re-worded the destructor conditions, the socket is in TCP_CLOSE
  in both cases (Kuniyuki Iwashima)
- Link to v4: https://lore.kernel.org/r/20250903-b4-tcp-ao-md5-rst-finwait2-v4-0-ef3a9eec3ef3@arista.com

Changes in v4:
- Remove struct tcp_ao_info::rcu as it's no longer used (Jakub)
- Link to v3: https://lore.kernel.org/r/20250830-b4-tcp-ao-md5-rst-finwait2-v3-0-9002fec37444@arista.com

Changes in v3:
- Assign NULL to md5sig_info before call_rcu() (catched by Eric Dumazet)
- Link to v2: https://lore.kernel.org/r/20250828-b4-tcp-ao-md5-rst-finwait2-v2-0-653099bea5c1@arista.com

Changes in v2:
- Fixed TCP-MD5 ifdeffery (Reported-by: Victor Nogueira)
- Call proper destructor for inet_ipv6 (Reported-by: syzbot@syzkaller.appspotmail.com)
- Link to v1: https://lore.kernel.org/r/20250822-b4-tcp-ao-md5-rst-finwait2-v1-0-25825d085dcb@arista.com

---
Dmitry Safonov (2):
      tcp: Destroy TCP-AO, TCP-MD5 keys in .sk_destruct()
      tcp: Free TCP-AO/TCP-MD5 info/keys without RCU

 include/net/tcp.h        |  4 ++++
 include/net/tcp_ao.h     |  1 -
 net/ipv4/tcp.c           | 16 ++++++++++++++++
 net/ipv4/tcp_ao.c        |  5 ++---
 net/ipv4/tcp_ipv4.c      | 37 ++++++++++---------------------------
 net/ipv4/tcp_minisocks.c | 19 +++++--------------
 net/ipv6/tcp_ipv6.c      |  8 ++++++++
 7 files changed, 45 insertions(+), 45 deletions(-)
---
base-commit: abcf9f662bc7ec72b3591d785eccd7dd8c239365
change-id: 20250822-b4-tcp-ao-md5-rst-finwait2-e632b4d8f58d

Best regards,
-- 
Dmitry Safonov <dima@arista.com>

[PATCH net-next v5 1/2] tcp: Destroy TCP-AO, TCP-MD5 keys in .sk_destruct()

[Dmitry Safonov via B4 Relay]

From: Dmitry Safonov <dima@arista.com>

Currently there are a couple of minor issues with destroying the keys
tcp_v4_destroy_sock():

1. The socket is yet in TCP bind buckets, making it reachable for
   incoming segments [on another CPU core], potentially available to send
   late FIN/ACK/RST replies.

2. There is at least one code path, where tcp_done() is called before
   sending RST [kudos to Bob for investigation]. This is a case of
   a server, that finished sending its data and just called close().

   The socket is in TCP_FIN_WAIT2 and has RCV_SHUTDOWN (set by
   __tcp_close())

   tcp_v4_do_rcv()/tcp_v6_do_rcv()
     tcp_rcv_state_process()            /* LINUX_MIB_TCPABORTONDATA */
       tcp_reset()
         tcp_done_with_error()
           tcp_done()
             inet_csk_destroy_sock()    /* Destroys AO/MD5 keys */
     /* tcp_rcv_state_process() returns SKB_DROP_REASON_TCP_ABORT_ON_DATA */
   tcp_v4_send_reset()                  /* Sends an unsigned RST segment */

   tcpdump:
> 22:53:15.399377 00:00:b2:1f:00:00 > 00:00:01:01:00:00, ethertype IPv4 (0x0800), length 74: (tos 0x0, ttl 64, id 33929, offset 0, flags [DF], proto TCP (6), length 60)
>     1.0.0.1.34567 > 1.0.0.2.49848: Flags [F.], seq 2185658590, ack 3969644355, win 502, options [nop,nop,md5 valid], length 0
> 22:53:15.399396 00:00:01:01:00:00 > 00:00:b2:1f:00:00, ethertype IPv4 (0x0800), length 86: (tos 0x0, ttl 64, id 51951, offset 0, flags [DF], proto TCP (6), length 72)
>     1.0.0.2.49848 > 1.0.0.1.34567: Flags [.], seq 3969644375, ack 2185658591, win 128, options [nop,nop,md5 valid,nop,nop,sack 1 {2185658590:2185658591}], length 0
> 22:53:16.429588 00:00:b2:1f:00:00 > 00:00:01:01:00:00, ethertype IPv4 (0x0800), length 60: (tos 0x0, ttl 64, id 0, offset 0, flags [DF], proto TCP (6), length 40)
>     1.0.0.1.34567 > 1.0.0.2.49848: Flags [R], seq 2185658590, win 0, length 0
> 22:53:16.664725 00:00:b2:1f:00:00 > 00:00:01:01:00:00, ethertype IPv4 (0x0800), length 74: (tos 0x0, ttl 64, id 0, offset 0, flags [DF], proto TCP (6), length 60)
>     1.0.0.1.34567 > 1.0.0.2.49848: Flags [R], seq 2185658591, win 0, options [nop,nop,md5 valid], length 0
> 22:53:17.289832 00:00:b2:1f:00:00 > 00:00:01:01:00:00, ethertype IPv4 (0x0800), length 74: (tos 0x0, ttl 64, id 0, offset 0, flags [DF], proto TCP (6), length 60)
>     1.0.0.1.34567 > 1.0.0.2.49848: Flags [R], seq 2185658591, win 0, options [nop,nop,md5 valid], length 0

  Note the signed RSTs later in the dump - those are sent by the server
  when the fin-wait socket gets removed from hash buckets, by
  the listener socket.

Instead of destroying AO/MD5 info and their keys in inet_csk_destroy_sock(),
slightly delay it until the actual socket .sk_destruct(). As shutdown'ed
socket can yet send non-data replies, they should be signed in order for
the peer to process them. Now it also matches how AO/MD5 gets destructed
for TIME-WAIT sockets (in tcp_twsk_destructor()).

This seems optimal for TCP-MD5, while for TCP-AO it seems to have an
open problem: once RST get sent and socket gets actually destructed,
there is no information on the initial sequence numbers. So, in case
this last RST gets lost in the network, the server's listener socket
won't be able to properly sign another RST. Nothing in RFC 1122
prescribes keeping any local state after non-graceful reset.
Luckily, BGP are known to use keep alive(s).

While the issue is quite minor/cosmetic, these days monitoring network
counters is a common practice and getting invalid signed segments from
a trusted BGP peer can get customers worried.

Investigated-by: Bob Gilligan <gilligan@arista.com>
Reviewed-by: Eric Dumazet <edumazet@google.com>
Reviewed-by: Kuniyuki Iwashima <kuniyu@google.com>
Signed-off-by: Dmitry Safonov <dima@arista.com>
---
 include/net/tcp.h   |  4 ++++
 net/ipv4/tcp.c      | 27 +++++++++++++++++++++++++++
 net/ipv4/tcp_ipv4.c | 33 ++++++++-------------------------
 net/ipv6/tcp_ipv6.c |  8 ++++++++
 4 files changed, 47 insertions(+), 25 deletions(-)

diff --git a/include/net/tcp.h b/include/net/tcp.h
index 0fb7923b8367d7733f22bdb48add75a81dccc2ea..277914c4d067a83c958af00028cccb46956c693e 100644
--- a/include/net/tcp.h
+++ b/include/net/tcp.h
@@ -1941,6 +1941,7 @@ tcp_md5_do_lookup_any_l3index(const struct sock *sk,
 }
 
 #define tcp_twsk_md5_key(twsk)	((twsk)->tw_md5_key)
+void tcp_md5_destruct_sock(struct sock *sk);
 #else
 static inline struct tcp_md5sig_key *
 tcp_md5_do_lookup(const struct sock *sk, int l3index,
@@ -1957,6 +1958,9 @@ tcp_md5_do_lookup_any_l3index(const struct sock *sk,
 }
 
 #define tcp_twsk_md5_key(twsk)	NULL
+static inline void tcp_md5_destruct_sock(struct sock *sk)
+{
+}
 #endif
 
 int tcp_md5_alloc_sigpool(void);
diff --git a/net/ipv4/tcp.c b/net/ipv4/tcp.c
index 588932c3cf1d549973ccfa095a8de1be87421a63..adf5e1e8ffef69391318f2af0b3ea221eaa4afab 100644
--- a/net/ipv4/tcp.c
+++ b/net/ipv4/tcp.c
@@ -412,6 +412,33 @@ static u64 tcp_compute_delivery_rate(const struct tcp_sock *tp)
 	return rate64;
 }
 
+#ifdef CONFIG_TCP_MD5SIG
+static void tcp_md5sig_info_free_rcu(struct rcu_head *head)
+{
+	struct tcp_md5sig_info *md5sig;
+
+	md5sig = container_of(head, struct tcp_md5sig_info, rcu);
+	kfree(md5sig);
+	static_branch_slow_dec_deferred(&tcp_md5_needed);
+	tcp_md5_release_sigpool();
+}
+
+void tcp_md5_destruct_sock(struct sock *sk)
+{
+	struct tcp_sock *tp = tcp_sk(sk);
+
+	if (tp->md5sig_info) {
+		struct tcp_md5sig_info *md5sig;
+
+		md5sig = rcu_dereference_protected(tp->md5sig_info, 1);
+		tcp_clear_md5_list(sk);
+		rcu_assign_pointer(tp->md5sig_info, NULL);
+		call_rcu(&md5sig->rcu, tcp_md5sig_info_free_rcu);
+	}
+}
+EXPORT_IPV6_MOD_GPL(tcp_md5_destruct_sock);
+#endif
+
 /* Address-family independent initialization for a tcp_sock.
  *
  * NOTE: A lot of things set to zero explicitly by call to
diff --git a/net/ipv4/tcp_ipv4.c b/net/ipv4/tcp_ipv4.c
index 1e58a8a9ff7ae373c1e5ff11d3ec687f0fa744dc..17176a5d8638d5850898000db41bb123eff86857 100644
--- a/net/ipv4/tcp_ipv4.c
+++ b/net/ipv4/tcp_ipv4.c
@@ -2494,6 +2494,13 @@ static const struct tcp_sock_af_ops tcp_sock_ipv4_specific = {
 	.ao_calc_key_sk		= tcp_v4_ao_calc_key_sk,
 #endif
 };
+
+static void tcp4_destruct_sock(struct sock *sk)
+{
+	tcp_md5_destruct_sock(sk);
+	tcp_ao_destroy_sock(sk, false);
+	inet_sock_destruct(sk);
+}
 #endif
 
 /* NOTE: A lot of things set to zero explicitly by call to
@@ -2509,23 +2516,12 @@ static int tcp_v4_init_sock(struct sock *sk)
 
 #if defined(CONFIG_TCP_MD5SIG) || defined(CONFIG_TCP_AO)
 	tcp_sk(sk)->af_specific = &tcp_sock_ipv4_specific;
+	sk->sk_destruct = tcp4_destruct_sock;
 #endif
 
 	return 0;
 }
 
-#ifdef CONFIG_TCP_MD5SIG
-static void tcp_md5sig_info_free_rcu(struct rcu_head *head)
-{
-	struct tcp_md5sig_info *md5sig;
-
-	md5sig = container_of(head, struct tcp_md5sig_info, rcu);
-	kfree(md5sig);
-	static_branch_slow_dec_deferred(&tcp_md5_needed);
-	tcp_md5_release_sigpool();
-}
-#endif
-
 static void tcp_release_user_frags(struct sock *sk)
 {
 #ifdef CONFIG_PAGE_POOL
@@ -2562,19 +2558,6 @@ void tcp_v4_destroy_sock(struct sock *sk)
 	/* Cleans up our, hopefully empty, out_of_order_queue. */
 	skb_rbtree_purge(&tp->out_of_order_queue);
 
-#ifdef CONFIG_TCP_MD5SIG
-	/* Clean up the MD5 key list, if any */
-	if (tp->md5sig_info) {
-		struct tcp_md5sig_info *md5sig;
-
-		md5sig = rcu_dereference_protected(tp->md5sig_info, 1);
-		tcp_clear_md5_list(sk);
-		call_rcu(&md5sig->rcu, tcp_md5sig_info_free_rcu);
-		rcu_assign_pointer(tp->md5sig_info, NULL);
-	}
-#endif
-	tcp_ao_destroy_sock(sk, false);
-
 	/* Clean up a referenced TCP bind bucket. */
 	if (inet_csk(sk)->icsk_bind_hash)
 		inet_put_port(sk);
diff --git a/net/ipv6/tcp_ipv6.c b/net/ipv6/tcp_ipv6.c
index 0562e939b2e316711004ce7d0da34ca249888746..08dabc47a6e7334b89b306af3a1e1c89c9935bb6 100644
--- a/net/ipv6/tcp_ipv6.c
+++ b/net/ipv6/tcp_ipv6.c
@@ -2110,6 +2110,13 @@ static const struct tcp_sock_af_ops tcp_sock_ipv6_mapped_specific = {
 	.ao_calc_key_sk	=	tcp_v4_ao_calc_key_sk,
 #endif
 };
+
+static void tcp6_destruct_sock(struct sock *sk)
+{
+	tcp_md5_destruct_sock(sk);
+	tcp_ao_destroy_sock(sk, false);
+	inet6_sock_destruct(sk);
+}
 #endif
 
 /* NOTE: A lot of things set to zero explicitly by call to
@@ -2125,6 +2132,7 @@ static int tcp_v6_init_sock(struct sock *sk)
 
 #if defined(CONFIG_TCP_MD5SIG) || defined(CONFIG_TCP_AO)
 	tcp_sk(sk)->af_specific = &tcp_sock_ipv6_specific;
+	sk->sk_destruct = tcp6_destruct_sock;
 #endif
 
 	return 0;

-- 
2.42.2

[PATCH net-next v5 2/2] tcp: Free TCP-AO/TCP-MD5 info/keys without RCU

[Dmitry Safonov via B4 Relay]

From: Dmitry Safonov <dima@arista.com>

Now that the destruction of info/keys is delayed until the socket
destructor, it's safe to use kfree() without an RCU callback.
The socket is in TCP_CLOSE state either because it never left it,
or it's already closed and the refcounter is zero. In any way,
no one can discover it anymore, it's safe to release memory
straight away.

Similar thing was possible for twsk already.

Reviewed-by: Kuniyuki Iwashima <kuniyu@google.com>
Signed-off-by: Dmitry Safonov <dima@arista.com>
---
 include/net/tcp_ao.h     |  1 -
 net/ipv4/tcp.c           | 17 +++--------------
 net/ipv4/tcp_ao.c        |  5 ++---
 net/ipv4/tcp_ipv4.c      |  4 ++--
 net/ipv4/tcp_minisocks.c | 19 +++++--------------
 5 files changed, 12 insertions(+), 34 deletions(-)

diff --git a/include/net/tcp_ao.h b/include/net/tcp_ao.h
index df655ce6987d3730fea7a6ef0db09c2e27b34f21..1e9e27d6e06ba1be185910d7618ef6453e72ba38 100644
--- a/include/net/tcp_ao.h
+++ b/include/net/tcp_ao.h
@@ -130,7 +130,6 @@ 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.c b/net/ipv4/tcp.c
index adf5e1e8ffef69391318f2af0b3ea221eaa4afab..77eb10273fa824403464a0fd2036abc1a703acb9 100644
--- a/net/ipv4/tcp.c
+++ b/net/ipv4/tcp.c
@@ -413,27 +413,16 @@ static u64 tcp_compute_delivery_rate(const struct tcp_sock *tp)
 }
 
 #ifdef CONFIG_TCP_MD5SIG
-static void tcp_md5sig_info_free_rcu(struct rcu_head *head)
-{
-	struct tcp_md5sig_info *md5sig;
-
-	md5sig = container_of(head, struct tcp_md5sig_info, rcu);
-	kfree(md5sig);
-	static_branch_slow_dec_deferred(&tcp_md5_needed);
-	tcp_md5_release_sigpool();
-}
-
 void tcp_md5_destruct_sock(struct sock *sk)
 {
 	struct tcp_sock *tp = tcp_sk(sk);
 
 	if (tp->md5sig_info) {
-		struct tcp_md5sig_info *md5sig;
 
-		md5sig = rcu_dereference_protected(tp->md5sig_info, 1);
 		tcp_clear_md5_list(sk);
-		rcu_assign_pointer(tp->md5sig_info, NULL);
-		call_rcu(&md5sig->rcu, tcp_md5sig_info_free_rcu);
+		kfree(rcu_replace_pointer(tp->md5sig_info, NULL, 1));
+		static_branch_slow_dec_deferred(&tcp_md5_needed);
+		tcp_md5_release_sigpool();
 	}
 }
 EXPORT_IPV6_MOD_GPL(tcp_md5_destruct_sock);
diff --git a/net/ipv4/tcp_ao.c b/net/ipv4/tcp_ao.c
index bbb8d5f0eae7d3d8887da3fa4d68e248af9060ad..31302be78bc4450b56fa23a390b6d03b2262741d 100644
--- a/net/ipv4/tcp_ao.c
+++ b/net/ipv4/tcp_ao.c
@@ -268,9 +268,8 @@ static void tcp_ao_key_free_rcu(struct rcu_head *head)
 	kfree_sensitive(key);
 }
 
-static void tcp_ao_info_free_rcu(struct rcu_head *head)
+static void tcp_ao_info_free(struct tcp_ao_info *ao)
 {
-	struct tcp_ao_info *ao = container_of(head, struct tcp_ao_info, rcu);
 	struct tcp_ao_key *key;
 	struct hlist_node *n;
 
@@ -310,7 +309,7 @@ void tcp_ao_destroy_sock(struct sock *sk, bool twsk)
 
 	if (!twsk)
 		tcp_ao_sk_omem_free(sk, ao);
-	call_rcu(&ao->rcu, tcp_ao_info_free_rcu);
+	tcp_ao_info_free(ao);
 }
 
 void tcp_ao_time_wait(struct tcp_timewait_sock *tcptw, struct tcp_sock *tp)
diff --git a/net/ipv4/tcp_ipv4.c b/net/ipv4/tcp_ipv4.c
index 17176a5d8638d5850898000db41bb123eff86857..2a06020357297a9c0b2dbd79900be8911a52a17e 100644
--- a/net/ipv4/tcp_ipv4.c
+++ b/net/ipv4/tcp_ipv4.c
@@ -1503,9 +1503,9 @@ 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_rcu(&key->node);
+		hlist_del(&key->node);
 		atomic_sub(sizeof(*key), &sk->sk_omem_alloc);
-		kfree_rcu(key, rcu);
+		kfree(key);
 	}
 }
 
diff --git a/net/ipv4/tcp_minisocks.c b/net/ipv4/tcp_minisocks.c
index d1c9e40886463ca308f9f3682c4039f491e7555f..7c2ae07d8d5d2a18d6ce3210cc09ee5d9850ea29 100644
--- a/net/ipv4/tcp_minisocks.c
+++ b/net/ipv4/tcp_minisocks.c
@@ -377,26 +377,17 @@ void tcp_time_wait(struct sock *sk, int state, int timeo)
 }
 EXPORT_SYMBOL(tcp_time_wait);
 
-#ifdef CONFIG_TCP_MD5SIG
-static void tcp_md5_twsk_free_rcu(struct rcu_head *head)
-{
-	struct tcp_md5sig_key *key;
-
-	key = container_of(head, struct tcp_md5sig_key, rcu);
-	kfree(key);
-	static_branch_slow_dec_deferred(&tcp_md5_needed);
-	tcp_md5_release_sigpool();
-}
-#endif
-
 void tcp_twsk_destructor(struct sock *sk)
 {
 #ifdef CONFIG_TCP_MD5SIG
 	if (static_branch_unlikely(&tcp_md5_needed.key)) {
 		struct tcp_timewait_sock *twsk = tcp_twsk(sk);
 
-		if (twsk->tw_md5_key)
-			call_rcu(&twsk->tw_md5_key->rcu, tcp_md5_twsk_free_rcu);
+		if (twsk->tw_md5_key) {
+			kfree(twsk->tw_md5_key);
+			static_branch_slow_dec_deferred(&tcp_md5_needed);
+			tcp_md5_release_sigpool();
+		}
 	}
 #endif
 	tcp_ao_destroy_sock(sk, true);

-- 
2.42.2

Re: [PATCH net-next v5 0/2] tcp: Destroy TCP-AO, TCP-MD5 keys in .sk_destruct()

[patchwork-bot+netdevbpf]

Hello:

This series was applied to netdev/net-next.git (main)
by Jakub Kicinski <kuba@kernel.org>:

On Tue, 09 Sep 2025 02:18:49 +0100 you wrote:
> On one side a minor/cosmetic issue, especially nowadays when
> TCP-AO/TCP-MD5 signature verification failures aren't logged to dmesg.
> 
> Yet, I think worth addressing for two reasons:
> - unsigned RST gets ignored by the peer and the connection is alive for
>   longer (keep-alive interval)
> - netstat counters increase and trace events report that trusted BGP peer
>   is sending unsigned/incorrectly signed segments, which can ring alarm
>   on monitoring.
> 
> [...]

Here is the summary with links:
  - [net-next,v5,1/2] tcp: Destroy TCP-AO, TCP-MD5 keys in .sk_destruct()
    https://git.kernel.org/netdev/net-next/c/9e472d9e84b1
  - [net-next,v5,2/2] tcp: Free TCP-AO/TCP-MD5 info/keys without RCU
    https://git.kernel.org/netdev/net-next/c/51e547e8c89c

You are awesome, thank you!
-- 
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