mptcp: add annotations around msk->subflow accesses

The MPTCP can access the first subflow socket in a few spots
outside the socket lock scope. That is actually safe, as MPTCP
will delete the socket itself only after the msk sock close().

Still the such accesses causes a few KCSAN splats, as reported
by Christoph. Silence the harmless warning adding a few annotation
around the relevant accesses.

Fixes: 71ba088ce0aa ("mptcp: cleanup accept and poll")
Reported-by: Christoph Paasch <cpaasch@apple.com>
Closes: https://github.com/multipath-tcp/mptcp_net-next/issues/402
Reviewed-by: Mat Martineau <martineau@kernel.org>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>
Signed-off-by: Mat Martineau <martineau@kernel.org>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

diff --git a/net/mptcp/protocol.c b/net/mptcp/protocol.c
index 9cafd3b..ce9de2c 100644 (file)
--- a/net/mptcp/protocol.c
+++ b/net/mptcp/protocol.c
@@ -91,7 +91,7 @@ static int __mptcp_socket_create(struct mptcp_sock *msk)
                return err;
 
        msk->first = ssock->sk;
-       msk->subflow = ssock;
+       WRITE_ONCE(msk->subflow, ssock);
        subflow = mptcp_subflow_ctx(ssock->sk);
        list_add(&subflow->node, &msk->conn_list);
        sock_hold(ssock->sk);
@@ -2282,7 +2282,7 @@ static void mptcp_dispose_initial_subflow(struct mptcp_sock *msk)
 {
        if (msk->subflow) {
                iput(SOCK_INODE(msk->subflow));
-               msk->subflow = NULL;
+               WRITE_ONCE(msk->subflow, NULL);
        }
 }
 
@@ -3136,7 +3136,7 @@ struct sock *mptcp_sk_clone(const struct sock *sk,
        msk = mptcp_sk(nsk);
        msk->local_key = subflow_req->local_key;
        msk->token = subflow_req->token;
-       msk->subflow = NULL;
+       WRITE_ONCE(msk->subflow, NULL);
        msk->in_accept_queue = 1;
        WRITE_ONCE(msk->fully_established, false);
        if (mp_opt->suboptions & OPTION_MPTCP_CSUMREQD)
@@ -3184,7 +3184,7 @@ static struct sock *mptcp_accept(struct sock *sk, int flags, int *err,
        struct socket *listener;
        struct sock *newsk;
 
-       listener = msk->subflow;
+       listener = READ_ONCE(msk->subflow);
        if (WARN_ON_ONCE(!listener)) {
                *err = -EINVAL;
                return NULL;
@@ -3736,10 +3736,10 @@ static int mptcp_stream_accept(struct socket *sock, struct socket *newsock,
 
        pr_debug("msk=%p", msk);
 
-       /* buggy applications can call accept on socket states other then LISTEN
+       /* Buggy applications can call accept on socket states other then LISTEN
         * but no need to allocate the first subflow just to error out.
         */
-       ssock = msk->subflow;
+       ssock = READ_ONCE(msk->subflow);
        if (!ssock)
                return -EINVAL;
 
@@ -3813,10 +3813,12 @@ static __poll_t mptcp_poll(struct file *file, struct socket *sock,
        state = inet_sk_state_load(sk);
        pr_debug("msk=%p state=%d flags=%lx", msk, state, msk->flags);
        if (state == TCP_LISTEN) {
-               if (WARN_ON_ONCE(!msk->subflow || !msk->subflow->sk))
+               struct socket *ssock = READ_ONCE(msk->subflow);
+
+               if (WARN_ON_ONCE(!ssock || !ssock->sk))
                        return 0;
 
-               return inet_csk_listen_poll(msk->subflow->sk);
+               return inet_csk_listen_poll(ssock->sk);
        }
 
        if (state != TCP_SYN_SENT && state != TCP_SYN_RECV) {

diff --git a/net/mptcp/protocol.h b/net/mptcp/protocol.h
index de4667d..7a1a3c3 100644 (file)
--- a/net/mptcp/protocol.h
+++ b/net/mptcp/protocol.h
@@ -305,7 +305,11 @@ struct mptcp_sock {
        struct list_head rtx_queue;
        struct mptcp_data_frag *first_pending;
        struct list_head join_list;
-       struct socket   *subflow; /* outgoing connect/listener/!mp_capable */
+       struct socket   *subflow; /* outgoing connect/listener/!mp_capable
+                                  * The mptcp ops can safely dereference, using suitable
+                                  * ONCE annotation, the subflow outside the socket
+                                  * lock as such sock is freed after close().
+                                  */
        struct sock     *first;
        struct mptcp_pm_data    pm;
        struct {