[PATCH net-next] rxrpc: Fix ability to add more data to a call once MSG_MORE deasserted

[David Howells <dhowells@redhat.com>]

When userspace is adding data to an RPC call for transmission, it must pass
MSG_MORE to sendmsg() if it intends to add more data in future calls to
sendmsg().  Calling sendmsg() without MSG_MORE being asserted closes the
transmission phase of the call (assuming sendmsg() adds all the data
presented) and further attempts to add more data should be rejected.

However, this is no longer the case.  The change of call state that was
previously the guard got bumped over to the I/O thread, which leaves a
window for a repeat sendmsg() to insert more data.  This previously went
unnoticed, but the more recent patch that changed the structures behind the
Tx queue added a warning:

        WARNING: CPU: 3 PID: 6639 at net/rxrpc/sendmsg.c:296 rxrpc_send_data+0x3f2/0x860

and rejected the additional data, returning error EPROTO.

Fix this by adding a guard flag to the call, setting the flag when we queue
the final packet and then rejecting further attempts to add data with
EPROTO.

Fixes: 2d689424b618 ("rxrpc: Move call state changes from sendmsg to I/O thread")
Reported-by: syzbot+ff11be94dfcd7a5af8da@syzkaller.appspotmail.com
Closes: https://lore.kernel.org/r/6757fb68.050a0220.2477f.005f.GAE@google.com/
Signed-off-by: David Howells <dhowells@redhat.com>
Tested-by: syzbot+ff11be94dfcd7a5af8da@syzkaller.appspotmail.com
cc: Marc Dionne <marc.dionne@auristor.com>
cc: Jakub Kicinski <kuba@kernel.org>
cc: "David S. Miller" <davem@davemloft.net>
cc: Eric Dumazet <edumazet@google.com>
cc: Paolo Abeni <pabeni@redhat.com>
cc: linux-afs@lists.infradead.org
cc: netdev@vger.kernel.org
---
 net/rxrpc/ar-internal.h |    1 +
 net/rxrpc/sendmsg.c     |    8 ++++++++
 2 files changed, 9 insertions(+)

diff --git a/net/rxrpc/ar-internal.h b/net/rxrpc/ar-internal.h
index 0c0a3c89dba3..718193df9d2e 100644
--- a/net/rxrpc/ar-internal.h
+++ b/net/rxrpc/ar-internal.h
@@ -571,6 +571,7 @@ enum rxrpc_call_flag {
 	RXRPC_CALL_RX_LAST,		/* Received the last packet (at rxtx_top) */
 	RXRPC_CALL_TX_LAST,		/* Last packet in Tx buffer (at rxtx_top) */
 	RXRPC_CALL_TX_ALL_ACKED,	/* Last packet has been hard-acked */
+	RXRPC_CALL_TX_NO_MORE,		/* No more data to transmit (MSG_MORE deasserted) */
 	RXRPC_CALL_SEND_PING,		/* A ping will need to be sent */
 	RXRPC_CALL_RETRANS_TIMEOUT,	/* Retransmission due to timeout occurred */
 	RXRPC_CALL_BEGAN_RX_TIMER,	/* We began the expect_rx_by timer */
diff --git a/net/rxrpc/sendmsg.c b/net/rxrpc/sendmsg.c
index c4c8b718cafa..0e8da909d4f2 100644
--- a/net/rxrpc/sendmsg.c
+++ b/net/rxrpc/sendmsg.c
@@ -266,6 +266,7 @@ static void rxrpc_queue_packet(struct rxrpc_sock *rx, struct rxrpc_call *call,
 	/* Order send_top after the queue->next pointer and txb content. */
 	smp_store_release(&call->send_top, seq);
 	if (last) {
+		set_bit(RXRPC_CALL_TX_NO_MORE, &call->flags);
 		rxrpc_notify_end_tx(rx, call, notify_end_tx);
 		call->send_queue = NULL;
 	}
@@ -329,6 +330,13 @@ static int rxrpc_send_data(struct rxrpc_sock *rx,
 	bool more = msg->msg_flags & MSG_MORE;
 	int ret, copied = 0;
 
+	if (test_bit(RXRPC_CALL_TX_NO_MORE, &call->flags)) {
+		trace_rxrpc_abort(call->debug_id, rxrpc_sendmsg_late_send,
+				  call->cid, call->call_id, call->rx_consumed,
+				  0, -EPROTO);
+		return -EPROTO;
+	}
+
 	timeo = sock_sndtimeo(sk, msg->msg_flags & MSG_DONTWAIT);
 
 	ret = rxrpc_wait_to_be_connected(call, &timeo);