mptcp: fix possible list corruption on passive MPJ

At passive MPJ time, if the msk socket lock is held by the user,
the new subflow is appended to the msk->join_list under the msk
data lock.

In mptcp_release_cb()/__mptcp_flush_join_list(), the subflows in
that list are moved from the join_list into the conn_list under the
msk socket lock.

Append and removal could race, possibly corrupting such list.
Address the issue splicing the join list into a temporary one while
still under the msk data lock.

Found by code inspection, the race itself should be almost impossible
to trigger in practice.

Fixes: 3e5014909b56 ("mptcp: cleanup MPJ subflow list handling")
Cc: stable@vger.kernel.org
Signed-off-by: Paolo Abeni <pabeni@redhat.com>
Reviewed-by: Matthieu Baerts <matthieu.baerts@tessares.net>
Signed-off-by: Matthieu Baerts <matthieu.baerts@tessares.net>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

diff --git a/net/mptcp/protocol.c b/net/mptcp/protocol.c
index ee357700b27b16aadf5f47216dee223bbe287ac4..9a40dae31cec24ffe6aec03f1f8ede48e1a68201 100644 (file)
--- a/net/mptcp/protocol.c
+++ b/net/mptcp/protocol.c
@@ -850,12 +850,12 @@ static bool __mptcp_finish_join(struct mptcp_sock *msk, struct sock *ssk)
        return true;
 }
 
-static void __mptcp_flush_join_list(struct sock *sk)
+static void __mptcp_flush_join_list(struct sock *sk, struct list_head *join_list)
 {
        struct mptcp_subflow_context *tmp, *subflow;
        struct mptcp_sock *msk = mptcp_sk(sk);
 
-       list_for_each_entry_safe(subflow, tmp, &msk->join_list, node) {
+       list_for_each_entry_safe(subflow, tmp, join_list, node) {
                struct sock *ssk = mptcp_subflow_tcp_sock(subflow);
                bool slow = lock_sock_fast(ssk);
 
@@ -3342,9 +3342,14 @@ static void mptcp_release_cb(struct sock *sk)
        for (;;) {
                unsigned long flags = (msk->cb_flags & MPTCP_FLAGS_PROCESS_CTX_NEED) |
                                      msk->push_pending;
+               struct list_head join_list;
+
                if (!flags)
                        break;
 
+               INIT_LIST_HEAD(&join_list);
+               list_splice_init(&msk->join_list, &join_list);
+
                /* the following actions acquire the subflow socket lock
                 *
                 * 1) can't be invoked in atomic scope
@@ -3355,8 +3360,9 @@ static void mptcp_release_cb(struct sock *sk)
                msk->push_pending = 0;
                msk->cb_flags &= ~flags;
                spin_unlock_bh(&sk->sk_lock.slock);
+
                if (flags & BIT(MPTCP_FLUSH_JOIN_LIST))
-                       __mptcp_flush_join_list(sk);
+                       __mptcp_flush_join_list(sk, &join_list);
                if (flags & BIT(MPTCP_PUSH_PENDING))
                        __mptcp_push_pending(sk, 0);
                if (flags & BIT(MPTCP_RETRANSMIT))