RDS: TCP: fix race windows in send-path quiescence by rds_tcp_accept_one()

The send path needs to be quiesced before resetting callbacks from
rds_tcp_accept_one(), and commit eb192840266f ("RDS:TCP: Synchronize
rds_tcp_accept_one with rds_send_xmit when resetting t_sock") achieves
this using the c_state and RDS_IN_XMIT bit following the pattern
used by rds_conn_shutdown(). However this leaves the possibility
of a race window as shown in the sequence below
    take t_conn_lock in rds_tcp_conn_connect
    send outgoing syn to peer
    drop t_conn_lock in rds_tcp_conn_connect
    incoming from peer triggers rds_tcp_accept_one, conn is
	marked CONNECTING
    wait for RDS_IN_XMIT to quiesce any rds_send_xmit threads
    call rds_tcp_reset_callbacks
    [.. race-window where incoming syn-ack can cause the conn
	to be marked UP from rds_tcp_state_change ..]
    lock_sock called from rds_tcp_reset_callbacks, and we set
	t_sock to null
As soon as the conn is marked UP in the race-window above, rds_send_xmit()
threads will proceed to rds_tcp_xmit and may encounter a null-pointer
deref on the t_sock.

Given that rds_tcp_state_change() is invoked in softirq context, whereas
rds_tcp_reset_callbacks() is in workq context, and testing for RDS_IN_XMIT
after lock_sock could result in a deadlock with tcp_sendmsg, this
commit fixes the race by using a new c_state, RDS_TCP_RESETTING, which
will prevent a transition to RDS_CONN_UP from rds_tcp_state_change().

Signed-off-by: Sowmini Varadhan <sowmini.varadhan@oracle.com>
Acked-by: Santosh Shilimkar <santosh.shilimkar@oracle.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

5 files changed, 27 insertions, 8 deletions

diff --git a/net/rds/rds.h b/net/rds/rds.h
index 80256b08eac0..387df5f32e49 100644
--- a/net/rds/rds.h
+++ b/net/rds/rds.h
@@ -74,6 +74,7 @@ enum {
         RDS_CONN_CONNECTING,
         RDS_CONN_DISCONNECTING,
         RDS_CONN_UP,
+        RDS_CONN_RESETTING,
         RDS_CONN_ERROR,
 };
 
@@ -813,6 +814,7 @@ void rds_connect_worker(struct work_struct *);
 void rds_shutdown_worker(struct work_struct *);
 void rds_send_worker(struct work_struct *);
 void rds_recv_worker(struct work_struct *);
+void rds_connect_path_complete(struct rds_connection *conn, int curr);
 void rds_connect_complete(struct rds_connection *conn);
 
 /* transport.c */
diff --git a/net/rds/tcp.c b/net/rds/tcp.c
index 7ab1b41ffc88..74ee126a6fe6 100644
--- a/net/rds/tcp.c
+++ b/net/rds/tcp.c
@@ -148,11 +148,23 @@ void rds_tcp_reset_callbacks(struct socket *sock,
          * potentially have transitioned to the RDS_CONN_UP state,
          * so we must quiesce any send threads before resetting
          * c_transport_data. We quiesce these threads by setting
-         * cp_state to something other than RDS_CONN_UP, and then
+         * c_state to something other than RDS_CONN_UP, and then
          * waiting for any existing threads in rds_send_xmit to
          * complete release_in_xmit(). (Subsequent threads entering
          * rds_send_xmit() will bail on !rds_conn_up().
+         *
+         * However an incoming syn-ack at this point would end up
+         * marking the conn as RDS_CONN_UP, and would again permit
+         * rds_send_xmi() threads through, so ideally we would
+         * synchronize on RDS_CONN_UP after lock_sock(), but cannot
+         * do that: waiting on !RDS_IN_XMIT after lock_sock() may
+         * end up deadlocking with tcp_sendmsg(), and the RDS_IN_XMIT
+         * would not get set. As a result, we set c_state to
+         * RDS_CONN_RESETTTING, to ensure that rds_tcp_state_change
+         * cannot mark rds_conn_path_up() in the window before lock_sock()
          */
+        atomic_set(&conn->c_state, RDS_CONN_RESETTING);
+        wait_event(conn->c_waitq, !test_bit(RDS_IN_XMIT, &conn->c_flags));
         lock_sock(osock->sk);
         /* reset receive side state for rds_tcp_data_recv() for osock  */
         if (tc->t_tinc) {
diff --git a/net/rds/tcp_connect.c b/net/rds/tcp_connect.c
index fb82e0a0bf89..fba13d0305fb 100644
--- a/net/rds/tcp_connect.c
+++ b/net/rds/tcp_connect.c
@@ -60,7 +60,7 @@ void rds_tcp_state_change(struct sock *sk)
                 case TCP_SYN_RECV:
                         break;
                 case TCP_ESTABLISHED:
-                        rds_connect_complete(conn);
+                        rds_connect_path_complete(conn, RDS_CONN_CONNECTING);
                         break;
                 case TCP_CLOSE_WAIT:
                 case TCP_CLOSE:
diff --git a/net/rds/tcp_listen.c b/net/rds/tcp_listen.c
index d9fe53675d95..686b1d03a558 100644
--- a/net/rds/tcp_listen.c
+++ b/net/rds/tcp_listen.c
@@ -135,16 +135,15 @@ int rds_tcp_accept_one(struct socket *sock)
                     !conn->c_outgoing) {
                         goto rst_nsk;
                 } else {
-                        atomic_set(&conn->c_state, RDS_CONN_CONNECTING);
-                        wait_event(conn->c_waitq,
-                                   !test_bit(RDS_IN_XMIT, &conn->c_flags));
                         rds_tcp_reset_callbacks(new_sock, conn);
                         conn->c_outgoing = 0;
+                        /* rds_connect_path_complete() marks RDS_CONN_UP */
+                        rds_connect_path_complete(conn, RDS_CONN_DISCONNECTING);
                 }
         } else {
                 rds_tcp_set_callbacks(new_sock, conn);
+                rds_connect_path_complete(conn, RDS_CONN_CONNECTING);
         }
-        rds_connect_complete(conn); /* marks RDS_CONN_UP */
         new_sock = NULL;
         ret = 0;
         goto out;
diff --git a/net/rds/threads.c b/net/rds/threads.c
index 454aa6d23327..4a323045719b 100644
--- a/net/rds/threads.c
+++ b/net/rds/threads.c
@@ -71,9 +71,9 @@
 struct workqueue_struct *rds_wq;
 EXPORT_SYMBOL_GPL(rds_wq);
 
-void rds_connect_complete(struct rds_connection *conn)
+void rds_connect_path_complete(struct rds_connection *conn, int curr)
 {
-        if (!rds_conn_transition(conn, RDS_CONN_CONNECTING, RDS_CONN_UP)) {
+        if (!rds_conn_transition(conn, curr, RDS_CONN_UP)) {
                 printk(KERN_WARNING "%s: Cannot transition to state UP, "
                                 "current state is %d\n",
                                 __func__,
@@ -90,6 +90,12 @@ void rds_connect_complete(struct rds_connection *conn)
         queue_delayed_work(rds_wq, &conn->c_send_w, 0);
         queue_delayed_work(rds_wq, &conn->c_recv_w, 0);
 }
+EXPORT_SYMBOL_GPL(rds_connect_path_complete);
+
+void rds_connect_complete(struct rds_connection *conn)
+{
+        rds_connect_path_complete(conn, RDS_CONN_CONNECTING);
+}
 EXPORT_SYMBOL_GPL(rds_connect_complete);
 
 /*