[PATCH v1 2/6] virtio console: Harden port adding

[Alexander Shishkin <alexander.shishkin@linux.intel.com>]

From: Andi Kleen <ak@linux.intel.com>

The ADD_PORT operation reads and sanity checks the port id multiple
times from the untrusted host. This is not safe because a malicious
host could change it between reads.

Read the port id only once and cache it for subsequent uses.

Signed-off-by: Andi Kleen <ak@linux.intel.com>
Signed-off-by: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: Amit Shah <amit@kernel.org>
Cc: Arnd Bergmann <arnd@arndb.de>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
---
 drivers/char/virtio_console.c | 10 ++++++----
 1 file changed, 6 insertions(+), 4 deletions(-)

diff --git a/drivers/char/virtio_console.c b/drivers/char/virtio_console.c
index f4fd5fe7cd3a..6599c2956ba4 100644
--- a/drivers/char/virtio_console.c
+++ b/drivers/char/virtio_console.c
@@ -1563,10 +1563,13 @@ static void handle_control_message(struct virtio_device *vdev,
 	struct port *port;
 	size_t name_size;
 	int err;
+	unsigned id;
 
 	cpkt = (struct virtio_console_control *)(buf->buf + buf->offset);
 
-	port = find_port_by_id(portdev, virtio32_to_cpu(vdev, cpkt->id));
+	/* Make sure the host cannot change id under us */
+	id = virtio32_to_cpu(vdev, READ_ONCE(cpkt->id));
+	port = find_port_by_id(portdev, id);
 	if (!port &&
 	    cpkt->event != cpu_to_virtio16(vdev, VIRTIO_CONSOLE_PORT_ADD)) {
 		/* No valid header at start of buffer.  Drop it. */
@@ -1583,15 +1586,14 @@ static void handle_control_message(struct virtio_device *vdev,
 			send_control_msg(port, VIRTIO_CONSOLE_PORT_READY, 1);
 			break;
 		}
-		if (virtio32_to_cpu(vdev, cpkt->id) >=
-		    portdev->max_nr_ports) {
+		if (id >= portdev->max_nr_ports) {
 			dev_warn(&portdev->vdev->dev,
 				"Request for adding port with "
 				"out-of-bound id %u, max. supported id: %u\n",
 				cpkt->id, portdev->max_nr_ports - 1);
 			break;
 		}
-		add_port(portdev, virtio32_to_cpu(vdev, cpkt->id));
+		add_port(portdev, id);
 		break;
 	case VIRTIO_CONSOLE_PORT_REMOVE:
 		unplug_port(port);
-- 
2.39.0