[PATCH net v6 03/10] rxrpc: Fix update of call->tx_pending without holding lock

[David Howells <dhowells@redhat.com>]

Currently, rxrpc_send_data() updates call->tx_pending just before it
returns - but it won't be holding the call->lock when it does this if a
wait was interrupted by a signal.  This would allow a parallel sendmsg() to
race.

Further, both the callers of rxrpc_send_data() call it with the lock held,
and then it returns an indication through the parameter list to say whether
it has dropped the lock or not - after which the callers both just drop the
lock if it's still held.

Fix this by:

 (1) Moving the release of call->lock down into rxrpc_send_data() and get
     rid of the indicator parameter.  This makes it easier to see where the
     lock is held.

 (2) Make the wait_for_space path move the value in txb back into
     call->tx_pending before dropping the lock prior to the wait.

 (3) After waiting, if the attempt to reacquire the mutex is interrupted,
     just return directly there rather than going to out_unlock

Fixes: b0f571ecd794 ("rxrpc: Fix locking in rxrpc's sendmsg")
Closes: https://sashiko.dev/#/patchset/20260702144919.172295-1-dhowells%40redhat.com
Signed-off-by: David Howells <dhowells@redhat.com>
cc: Marc Dionne <marc.dionne@auristor.com>
cc: Eric Dumazet <edumazet@google.com>
cc: "David S. Miller" <davem@davemloft.net>
cc: Jakub Kicinski <kuba@kernel.org>
cc: Paolo Abeni <pabeni@redhat.com>
cc: Simon Horman <horms@kernel.org>
cc: linux-afs@lists.infradead.org
cc: stable@kernel.org
---
 net/rxrpc/sendmsg.c | 56 ++++++++++++++++++++++++---------------------
 1 file changed, 30 insertions(+), 26 deletions(-)

diff --git a/net/rxrpc/sendmsg.c b/net/rxrpc/sendmsg.c
index 5ea10dc497a7..79fd31b02d11 100644
--- a/net/rxrpc/sendmsg.c
+++ b/net/rxrpc/sendmsg.c
@@ -320,8 +320,8 @@ static int rxrpc_alloc_txqueue(struct sock *sk, struct rxrpc_call *call)
 static int rxrpc_send_data(struct rxrpc_sock *rx,
 			   struct rxrpc_call *call,
 			   struct msghdr *msg, size_t len,
-			   rxrpc_notify_end_tx_t notify_end_tx,
-			   bool *_dropped_lock)
+			   rxrpc_notify_end_tx_t notify_end_tx)
+	__releases(&call->user_mutex)
 {
 	struct rxrpc_txbuf *txb;
 	struct sock *sk = &rx->sk;
@@ -334,12 +334,12 @@ static int rxrpc_send_data(struct rxrpc_sock *rx,
 
 	ret = rxrpc_wait_to_be_connected(call, &timeo);
 	if (ret < 0)
-		return ret;
+		goto out_unlock;
 
 	if (call->conn->state == RXRPC_CONN_CLIENT_UNSECURED) {
 		ret = rxrpc_init_client_conn_security(call->conn);
 		if (ret < 0)
-			return ret;
+			goto out_unlock;
 	}
 
 	/* this should be in poll */
@@ -350,13 +350,15 @@ static int rxrpc_send_data(struct rxrpc_sock *rx,
 		trace_rxrpc_abort(call->debug_id, rxrpc_sendmsg_late_send,
 				  call->cid, call->call_id, call->rx_consumed,
 				  0, -EPROTO);
-		return -EPROTO;
+		ret = -EPROTO;
+		goto out_unlock;
 	}
 	if (unlikely(test_bit(RXRPC_CALL_TX_ERROR, &call->flags))) {
 		trace_rxrpc_abort(call->debug_id, rxrpc_sendmsg_tx_error,
 				  call->cid, call->call_id, call->rx_consumed,
 				  0, -EIO);
-		return -EIO;
+		ret = -EIO;
+		goto out_unlock;
 	}
 
 	txb = call->tx_pending;
@@ -456,7 +458,7 @@ static int rxrpc_send_data(struct rxrpc_sock *rx,
 				if (ret == -ENOMEM)
 					goto maybe_error_rewind;
 				set_bit(RXRPC_CALL_TX_ERROR, &call->flags);
-				goto out;
+				goto out_txb;
 			}
 
 			if (msg_data_left(msg) == 0 && !more)
@@ -468,15 +470,18 @@ static int rxrpc_send_data(struct rxrpc_sock *rx,
 
 success:
 	ret = copied;
-out:
+out_txb:
 	call->tx_pending = txb;
+out_unlock:
+	mutex_unlock(&call->user_mutex);
 	_leave(" = %d", ret);
 	return ret;
 
 call_terminated:
 	rxrpc_put_txbuf(txb, rxrpc_txbuf_put_send_aborted);
-	_leave(" = %d", call->error);
-	return call->error;
+	call->tx_pending = NULL;
+	ret = call->error;
+	goto out_unlock;
 
 maybe_error_rewind:
 	/* If we got a retryable error after copying all the supplied data into
@@ -499,36 +504,40 @@ static int rxrpc_send_data(struct rxrpc_sock *rx,
 		if (rxrpc_call_is_complete(call) &&
 		    call->error < 0) {
 			ret = call->error;
-			goto out;
+			goto out_txb;
 		}
 		goto success;
 	}
-	goto out;
+	goto out_txb;
 
 efault:
 	ret = -EFAULT;
-	goto out;
+	goto out_txb;
 
 wait_for_space:
 	ret = -EAGAIN;
 	if (msg->msg_flags & MSG_DONTWAIT)
 		goto maybe_error;
+	call->tx_pending = txb;
+	txb = NULL;
 	mutex_unlock(&call->user_mutex);
-	*_dropped_lock = true;
+
 	ret = rxrpc_wait_for_tx_window(rx, call, &timeo,
 				       msg->msg_flags & MSG_WAITALL);
 	if (ret < 0)
-		goto maybe_error;
+		goto out_nolock;
 	if (call->interruptibility == RXRPC_INTERRUPTIBLE) {
 		if (mutex_lock_interruptible(&call->user_mutex) < 0) {
 			ret = sock_intr_errno(timeo);
-			goto maybe_error;
+			goto out_nolock;
 		}
 	} else {
 		mutex_lock(&call->user_mutex);
 	}
-	*_dropped_lock = false;
 	goto reload;
+out_nolock:
+	_leave(" = %d [intr]", ret);
+	return copied ?: ret;
 }
 
 /*
@@ -694,7 +703,6 @@ rxrpc_new_client_call_for_sendmsg(struct rxrpc_sock *rx, struct msghdr *msg,
 int rxrpc_do_sendmsg(struct rxrpc_sock *rx, struct msghdr *msg, size_t len)
 {
 	struct rxrpc_call *call;
-	bool dropped_lock = false;
 	int ret;
 
 	struct rxrpc_send_params p = {
@@ -803,16 +811,15 @@ int rxrpc_do_sendmsg(struct rxrpc_sock *rx, struct msghdr *msg, size_t len)
 		ret = 0;
 		break;
 	case RXRPC_CMD_SEND_DATA:
-		ret = rxrpc_send_data(rx, call, msg, len, NULL, &dropped_lock);
-		break;
+		ret = rxrpc_send_data(rx, call, msg, len, NULL);
+		goto error_put;
 	default:
 		ret = -EINVAL;
 		break;
 	}
 
 out_put_unlock:
-	if (!dropped_lock)
-		mutex_unlock(&call->user_mutex);
+	mutex_unlock(&call->user_mutex);
 error_put:
 	rxrpc_put_call(call, rxrpc_call_put_sendmsg);
 	_leave(" = %d", ret);
@@ -842,7 +849,6 @@ int rxrpc_kernel_send_data(struct socket *sock, struct rxrpc_call *call,
 			   struct msghdr *msg, size_t len,
 			   rxrpc_notify_end_tx_t notify_end_tx)
 {
-	bool dropped_lock = false;
 	int ret;
 
 	_enter("{%d},", call->debug_id);
@@ -853,12 +859,10 @@ int rxrpc_kernel_send_data(struct socket *sock, struct rxrpc_call *call,
 	mutex_lock(&call->user_mutex);
 
 	ret = rxrpc_send_data(rxrpc_sk(sock->sk), call, msg, len,
-			      notify_end_tx, &dropped_lock);
+			      notify_end_tx);
 	if (ret == -ESHUTDOWN)
 		ret = call->error;
 
-	if (!dropped_lock)
-		mutex_unlock(&call->user_mutex);
 	_leave(" = %d", ret);
 	return ret;
 }