[PATCH net v2] mptcp: protect listener backlog reads in diagnostics

[Cen Zhang <zzzccc427@gmail.com>]

mptcp_diag_get_info() reports the accept queue of a listening MPTCP
socket by reading msk->first. The token based dump and the exact token
query only hold a reference on the MPTCP socket, which pins neither the
first subflow nor the listener hash bucket, and __mptcp_close_ssk()
drops the last reference on the first subflow before clearing
msk->first.

An application can disconnect a connected MPTCP socket with AF_UNSPEC,
which removes the token but retains the first subflow, listen on it
again and close it. A dump that obtained the MPTCP socket before the
disconnect then reads the backlog of a freed TCP listener:

  Token diagnostic                 Application
  mptcp_token_iter_next()
    retain MPTCP socket
                                   connect(AF_UNSPEC)
                                     remove token, retain first
                                   listen()
                                     reuse first
  mptcp_diag_get_info()
    observe TCP_LISTEN
    lsk = msk->first
                                   close()
                                     mptcp_destroy()
                                       __mptcp_close_ssk()
                                         release first
    read lsk->sk_ack_backlog

The report below was decoded with scripts/decode_stacktrace.sh against
the 7.2.0-rc5 based test kernel that produced it, with the line numbers
mapped to this tree; only the relevant frames are kept:

  BUG: KASAN: slab-use-after-free in mptcp_diag_get_info (net/mptcp/mptcp_diag.c:209)
  Read of size 4 at addr ffff888111763818 by task python3/499

  Call Trace:
   mptcp_diag_get_info (net/mptcp/mptcp_diag.c:209)
   inet_sk_diag_fill (net/ipv4/inet_diag.c:330)
   mptcp_diag_dump (net/mptcp/mptcp_diag.c:23 net/mptcp/mptcp_diag.c:177)
   __inet_diag_dump (net/ipv4/inet_diag.c:823)
   netlink_dump (net/netlink/af_netlink.c:2351)

  Allocated by task 497:
   sk_alloc (net/core/sock.c:2337)
   inet_create (net/ipv4/af_inet.c:333)
   __sock_create (net/socket.c:1679)
   mptcp_subflow_create_socket (net/mptcp/subflow.c:1836)
   __mptcp_nmpc_sk (net/mptcp/protocol.c:111 net/mptcp/protocol.c:143)
   mptcp_setsockopt (net/mptcp/sockopt.c:313 net/mptcp/sockopt.c:916)

  Freed by task 0:
   slab_free_after_rcu_debug (mm/slub.c:6571)
   rcu_core (kernel/rcu/tree.c:2919)

  Second to last potentially related work creation:
   __call_rcu_common.constprop.0 (kernel/rcu/tree.c:3287)
   sk_destruct (net/core/sock.c:2446)
   __sk_free (net/core/sock.c:2459)
   sk_free (net/core/sock.c:2470)
   __mptcp_close_ssk (include/net/sock.h:2024 net/mptcp/protocol.c:2721)
   mptcp_destroy_common (net/mptcp/protocol.c:3579)
   mptcp_destroy (net/mptcp/protocol.c:3815)
   __mptcp_destroy_sock (net/mptcp/protocol.c:3421)
   __mptcp_close (net/mptcp/protocol.c:3523)
   mptcp_close (net/mptcp/protocol.c:3538)
   inet_release (net/ipv4/af_inet.c:442)

The first subflow is a TCP socket: TCP listeners carry SOCK_RCU_FREE
and tcp_prot/tcpv6_prot sockets live in SLAB_TYPESAFE_BY_RCU caches, so
reading the two backlog counters under rcu_read_lock() is safe, and a
stale value is acceptable for diagnostics. mptcp_diag_dump_listeners()
already runs the formatter inside an RCU read-side section; only the
token dump and mptcp_diag_dump_one() reach it without one. Take
rcu_read_lock() around the msk->first snapshot and the backlog reads.

Fixes: 4fa39b701ce9 ("mptcp: listen diag dump support")
Suggested-by: Eric Dumazet <edumazet@kernel.org>
Signed-off-by: Cen Zhang <zzzccc427@gmail.com>
Assisted-by: LLM
---
Changes in v2:
- Take rcu_read_lock() around the first subflow reads instead of the MPTCP
  data lock plus an owner check, as suggested by Eric Dumazet; this avoids
  the new msk slock -> listener hash lock nesting and keeps ss reporting the
  accept queue while the socket is owned.
- Decode the stack trace with scripts/decode_stacktrace.sh, map the line
  numbers to this tree and keep only the relevant frames.

Link to v1: https://lore.kernel.org/r/pm-mptcp-objects-candidate-0066-v3-73df72b7a543d4cada0e@gmail.com

 net/mptcp/mptcp_diag.c | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

diff --git a/net/mptcp/mptcp_diag.c b/net/mptcp/mptcp_diag.c
index 136c2d05c0ee..28a42eee6651 100644
--- a/net/mptcp/mptcp_diag.c
+++ b/net/mptcp/mptcp_diag.c
@@ -200,8 +200,10 @@ static void mptcp_diag_get_info(struct sock *sk, struct inet_diag_msg *r,
 	r->idiag_wqueue = sk_wmem_alloc_get(sk);
 
 	if (inet_sk_state_load(sk) == TCP_LISTEN) {
-		struct sock *lsk = READ_ONCE(msk->first);
+		struct sock *lsk;
 
+		rcu_read_lock();
+		lsk = READ_ONCE(msk->first);
 		if (lsk) {
 			/* override with settings from tcp listener,
 			 * so Send-Q will show accept queue.
@@ -209,6 +211,7 @@ static void mptcp_diag_get_info(struct sock *sk, struct inet_diag_msg *r,
 			r->idiag_rqueue = READ_ONCE(lsk->sk_ack_backlog);
 			r->idiag_wqueue = READ_ONCE(lsk->sk_max_ack_backlog);
 		}
+		rcu_read_unlock();
 	}
 
 	if (!info)