From: Paolo Abeni <pabeni@redhat.com>

Christoph reported a possible deadlock while the TCP stack
destroys an unaccepted subflow due to an incoming reset: the
MPTCP socket error path tries to acquire the msk-level socket
lock while TCP still owns the listener socket accept queue
spinlock, and the reverse dependency already exists in the
TCP stack.

Note that the above is actually a lockdep false positive, as
the chain involves two separate sockets. A different per-socket
lockdep key will address the issue, but such a change will be
quite invasive.

Instead, we can simply stop earlier the socket error handling
for orphaned or unaccepted subflows, breaking the critical
lockdep chain. Error handling in such a scenario is a no-op.

Fixes: 15cc10453398 ("mptcp: deliver ssk errors to msk")
Cc: stable@vger.kernel.org
Reported-and-tested-by: Christoph Paasch <redacted>
Closes: https://github.com/multipath-tcp/mptcp_net-next/issues/355
Signed-off-by: Paolo Abeni <pabeni@redhat.com>
Reviewed-by: Matthieu Baerts <redacted>
Signed-off-by: Matthieu Baerts <redacted>
---
 net/mptcp/subflow.c | 7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/net/mptcp/subflow.c b/net/mptcp/subflow.c
index 4ae1a7304cf0..5070dc33675d 100644
--- a/net/mptcp/subflow.c
+++ b/net/mptcp/subflow.c

@@ -1432,6 +1432,13 @@ static void subflow_error_report(struct sock *ssk)
 {
 	struct sock *sk = mptcp_subflow_ctx(ssk)->conn;
 
+	/* bail early if this is a no-op, so that we avoid introducing a
+	 * problematic lockdep dependency between TCP accept queue lock
+	 * and msk socket spinlock
+	 */
+	if (!sk->sk_socket)
+		return;
+
 	mptcp_data_lock(sk);
 	if (!sock_owned_by_user(sk))
 		__mptcp_error_report(sk);

-- 
2.38.1