rxrpc: Fix deadlock between call creation and sendmsg/recvmsg

All the routines by which rxrpc is accessed from the outside are serialised
by means of the socket lock (sendmsg, recvmsg, bind,
rxrpc_kernel_begin_call(), ...) and this presents a problem:

 (1) If a number of calls on the same socket are in the process of
     connection to the same peer, a maximum of four concurrent live calls
     are permitted before further calls need to wait for a slot.

 (2) If a call is waiting for a slot, it is deep inside sendmsg() or
     rxrpc_kernel_begin_call() and the entry function is holding the socket
     lock.

 (3) sendmsg() and recvmsg() or the in-kernel equivalents are prevented
     from servicing the other calls as they need to take the socket lock to
     do so.

 (4) The socket is stuck until a call is aborted and makes its slot
     available to the waiter.

Fix this by:

 (1) Provide each call with a mutex ('user_mutex') that arbitrates access
     by the users of rxrpc separately for each specific call.

 (2) Make rxrpc_sendmsg() and rxrpc_recvmsg() unlock the socket as soon as
     they've got a call and taken its mutex.

     Note that I'm returning EWOULDBLOCK from recvmsg() if MSG_DONTWAIT is
     set but someone else has the lock.  Should I instead only return
     EWOULDBLOCK if there's nothing currently to be done on a socket, and
     sleep in this particular instance because there is something to be
     done, but we appear to be blocked by the interrupt handler doing its
     ping?

 (3) Make rxrpc_new_client_call() unlock the socket after allocating a new
     call, locking its user mutex and adding it to the socket's call tree.
     The call is returned locked so that sendmsg() can add data to it
     immediately.

     From the moment the call is in the socket tree, it is subject to
     access by sendmsg() and recvmsg() - even if it isn't connected yet.

 (4) Lock new service calls in the UDP data_ready handler (in
     rxrpc_new_incoming_call()) because they may already be in the socket's
     tree and the data_ready handler makes them live immediately if a user
     ID has already been preassigned.

     Note that the new call is locked before any notifications are sent
     that it is live, so doing mutex_trylock() *ought* to always succeed.
     Userspace is prevented from doing sendmsg() on calls that are in a
     too-early state in rxrpc_do_sendmsg().

 (5) Make rxrpc_new_incoming_call() return the call with the user mutex
     held so that a ping can be scheduled immediately under it.

     Note that it might be worth moving the ping call into
     rxrpc_new_incoming_call() and then we can drop the mutex there.

 (6) Make rxrpc_accept_call() take the lock on the call it is accepting and
     release the socket after adding the call to the socket's tree.  This
     is slightly tricky as we've dequeued the call by that point and have
     to requeue it.

     Note that requeuing emits a trace event.

 (7) Make rxrpc_kernel_send_data() and rxrpc_kernel_recv_data() take the
     new mutex immediately and don't bother with the socket mutex at all.

This patch has the nice bonus that calls on the same socket are now to some
extent parallelisable.

Note that we might want to move rxrpc_service_prealloc() calls out from the
socket lock and give it its own lock, so that we don't hang progress in
other calls because we're waiting for the allocator.

We probably also want to avoid calling rxrpc_notify_socket() from within
the socket lock (rxrpc_accept_call()).

Signed-off-by: David Howells <dhowells@redhat.com>
Tested-by: Marc Dionne <marc.c.dionne@auristor.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

1 files changed, 46 insertions, 11 deletions

diff --git a/net/rxrpc/sendmsg.c b/net/rxrpc/sendmsg.c
index 0a6ef217aa8a..31c1538c1a8d 100644
--- a/net/rxrpc/sendmsg.c
+++ b/net/rxrpc/sendmsg.c
@@ -59,9 +59,12 @@ static int rxrpc_wait_for_tx_window(struct rxrpc_sock *rx,
 		}
 
 		trace_rxrpc_transmit(call, rxrpc_transmit_wait);
-		release_sock(&rx->sk);
+		mutex_unlock(&call->user_mutex);
 		*timeo = schedule_timeout(*timeo);
-		lock_sock(&rx->sk);
+		if (mutex_lock_interruptible(&call->user_mutex) < 0) {
+			ret = sock_intr_errno(*timeo);
+			break;
+		}
 	}
 
 	remove_wait_queue(&call->waitq, &myself);
@@ -171,7 +174,7 @@ static void rxrpc_queue_packet(struct rxrpc_call *call, struct sk_buff *skb,
 /*
  * send data through a socket
  * - must be called in process context
- * - caller holds the socket locked
+ * - The caller holds the call user access mutex, but not the socket lock.
  */
 static int rxrpc_send_data(struct rxrpc_sock *rx,
 			   struct rxrpc_call *call,
@@ -437,10 +440,13 @@ static int rxrpc_sendmsg_cmsg(struct msghdr *msg,
 
 /*
  * Create a new client call for sendmsg().
+ * - Called with the socket lock held, which it must release.
+ * - If it returns a call, the call's lock will need releasing by the caller.
  */
 static struct rxrpc_call *
 rxrpc_new_client_call_for_sendmsg(struct rxrpc_sock *rx, struct msghdr *msg,
 				  unsigned long user_call_ID, bool exclusive)
+	__releases(&rx->sk.sk_lock.slock)
 {
 	struct rxrpc_conn_parameters cp;
 	struct rxrpc_call *call;
@@ -450,8 +456,10 @@ rxrpc_new_client_call_for_sendmsg(struct rxrpc_sock *rx, struct msghdr *msg,
 
 	_enter("");
 
-	if (!msg->msg_name)
+	if (!msg->msg_name) {
+		release_sock(&rx->sk);
 		return ERR_PTR(-EDESTADDRREQ);
+	}
 
 	key = rx->key;
 	if (key && !rx->key->payload.data[0])
@@ -464,6 +472,7 @@ rxrpc_new_client_call_for_sendmsg(struct rxrpc_sock *rx, struct msghdr *msg,
 	cp.exclusive		= rx->exclusive | exclusive;
 	cp.service_id		= srx->srx_service;
 	call = rxrpc_new_client_call(rx, &cp, srx, user_call_ID, GFP_KERNEL);
+	/* The socket is now unlocked */
 
 	_leave(" = %p\n", call);
 	return call;
@@ -475,6 +484,7 @@ rxrpc_new_client_call_for_sendmsg(struct rxrpc_sock *rx, struct msghdr *msg,
  * - the socket may be either a client socket or a server socket
  */
 int rxrpc_do_sendmsg(struct rxrpc_sock *rx, struct msghdr *msg, size_t len)
+	__releases(&rx->sk.sk_lock.slock)
 {
 	enum rxrpc_command cmd;
 	struct rxrpc_call *call;
@@ -488,12 +498,14 @@ int rxrpc_do_sendmsg(struct rxrpc_sock *rx, struct msghdr *msg, size_t len)
 	ret = rxrpc_sendmsg_cmsg(msg, &user_call_ID, &cmd, &abort_code,
 				 &exclusive);
 	if (ret < 0)
-		return ret;
+		goto error_release_sock;
 
 	if (cmd == RXRPC_CMD_ACCEPT) {
+		ret = -EINVAL;
 		if (rx->sk.sk_state != RXRPC_SERVER_LISTENING)
-			return -EINVAL;
+			goto error_release_sock;
 		call = rxrpc_accept_call(rx, user_call_ID, NULL);
+		/* The socket is now unlocked. */
 		if (IS_ERR(call))
 			return PTR_ERR(call);
 		rxrpc_put_call(call, rxrpc_call_put);
@@ -502,12 +514,29 @@ int rxrpc_do_sendmsg(struct rxrpc_sock *rx, struct msghdr *msg, size_t len)
 
 	call = rxrpc_find_call_by_user_ID(rx, user_call_ID);
 	if (!call) {
+		ret = -EBADSLT;
 		if (cmd != RXRPC_CMD_SEND_DATA)
-			return -EBADSLT;
+			goto error_release_sock;
+		ret = -EBUSY;
+		if (call->state == RXRPC_CALL_UNINITIALISED ||
+		    call->state == RXRPC_CALL_CLIENT_AWAIT_CONN ||
+		    call->state == RXRPC_CALL_SERVER_PREALLOC ||
+		    call->state == RXRPC_CALL_SERVER_SECURING ||
+		    call->state == RXRPC_CALL_SERVER_ACCEPTING)
+			goto error_release_sock;
 		call = rxrpc_new_client_call_for_sendmsg(rx, msg, user_call_ID,
 							 exclusive);
+		/* The socket is now unlocked... */
 		if (IS_ERR(call))
 			return PTR_ERR(call);
+		/* ... and we have the call lock. */
+	} else {
+		ret = mutex_lock_interruptible(&call->user_mutex);
+		release_sock(&rx->sk);
+		if (ret < 0) {
+			ret = -ERESTARTSYS;
+			goto error_put;
+		}
 	}
 
 	_debug("CALL %d USR %lx ST %d on CONN %p",
@@ -535,9 +564,15 @@ int rxrpc_do_sendmsg(struct rxrpc_sock *rx, struct msghdr *msg, size_t len)
 		ret = rxrpc_send_data(rx, call, msg, len);
 	}
 
+	mutex_unlock(&call->user_mutex);
+error_put:
 	rxrpc_put_call(call, rxrpc_call_put);
 	_leave(" = %d", ret);
 	return ret;
+
+error_release_sock:
+	release_sock(&rx->sk);
+	return ret;
 }
 
 /**
@@ -562,7 +597,7 @@ int rxrpc_kernel_send_data(struct socket *sock, struct rxrpc_call *call,
 	ASSERTCMP(msg->msg_name, ==, NULL);
 	ASSERTCMP(msg->msg_control, ==, NULL);
 
-	lock_sock(sock->sk);
+	mutex_lock(&call->user_mutex);
 
 	_debug("CALL %d USR %lx ST %d on CONN %p",
 	       call->debug_id, call->user_call_ID, call->state, call->conn);
@@ -577,7 +612,7 @@ int rxrpc_kernel_send_data(struct socket *sock, struct rxrpc_call *call,
 		ret = rxrpc_send_data(rxrpc_sk(sock->sk), call, msg, len);
 	}
 
-	release_sock(sock->sk);
+	mutex_unlock(&call->user_mutex);
 	_leave(" = %d", ret);
 	return ret;
 }
@@ -598,12 +633,12 @@ void rxrpc_kernel_abort_call(struct socket *sock, struct rxrpc_call *call,
 {
 	_enter("{%d},%d,%d,%s", call->debug_id, abort_code, error, why);
 
-	lock_sock(sock->sk);
+	mutex_lock(&call->user_mutex);
 
 	if (rxrpc_abort_call(why, call, 0, abort_code, error))
 		rxrpc_send_abort_packet(call);
 
-	release_sock(sock->sk);
+	mutex_unlock(&call->user_mutex);
 	_leave("");
 }