[PATCH 4/9] virtio_console: validate max_nr_ports before trying to use it

[Jason Wang <jasowang@redhat.com>]

We calculate nr_ports based on the max_nr_ports:

nr_queues = use_multiport(portdev) ? (nr_ports + 1) * 2 : 2;

If the device advertises a large max_nr_ports, we will end up with a
integer overflow. Fixing this by validating the max_nr_ports
advertised by the device in .validate() and clear the MULTIPORT is
it's greater than 0x8000 (which is guaranteed be safe).

Cc: Amit Shah <amit@kernel.org>
Signed-off-by: Jason Wang <jasowang@redhat.com>
---
 drivers/char/virtio_console.c | 10 ++++++++++
 1 file changed, 10 insertions(+)

diff --git a/drivers/char/virtio_console.c b/drivers/char/virtio_console.c
index daeed31df622..ef13763699c0 100644
--- a/drivers/char/virtio_console.c
+++ b/drivers/char/virtio_console.c
@@ -28,6 +28,7 @@
 #include "../tty/hvc/hvc_console.h"
 
 #define is_rproc_enabled IS_ENABLED(CONFIG_REMOTEPROC)
+#define VIRTCONS_MAX_PORTS 0x8000
 
 /*
  * This is a global struct for storing common data for all the devices
@@ -1981,6 +1982,8 @@ static void virtcons_remove(struct virtio_device *vdev)
 
 static int virtcons_validate(struct virtio_device *vdev)
 {
+	u32 max_nr_ports;
+
 	if (is_rproc_serial(vdev)) {
 		/* Don't test F_SIZE at all if we're rproc: not a
 		 * valid feature! */
@@ -1999,6 +2002,13 @@ static int virtcons_validate(struct virtio_device *vdev)
 		return -EINVAL;
 	}
 
+	if (virtio_cread_feature(vdev, VIRTIO_CONSOLE_F_MULTIPORT,
+				 struct virtio_console_config, max_nr_ports,
+				 &max_nr_ports) == 0) {
+		if (max_nr_ports == 0 || max_nr_ports > VIRTCONS_MAX_PORTS)
+			__virtio_clear_bit(vdev, VIRTIO_CONSOLE_F_MULTIPORT);
+	}
+
 	return 0;
 }
 
-- 
2.25.1