virtio console: Harden multiport against invalid host input

It's possible for the host to set the multiport flag, but pass in
0 multiports, which results in:

BUG: KASAN: slab-out-of-bounds in init_vqs+0x244/0x6c0 drivers/char/virtio_console.c:1878
Write of size 8 at addr ffff888001cc24a0 by task swapper/1

CPU: 0 PID: 1 Comm: swapper Not tainted 5.15.0-rc1-140273-gaab0bb9fbaa1-dirty torvalds#588
Call Trace:
 init_vqs+0x244/0x6c0 drivers/char/virtio_console.c:1878
 virtcons_probe+0x1a3/0x5b0 drivers/char/virtio_console.c:2042
 virtio_dev_probe+0x2b9/0x500 drivers/virtio/virtio.c:263
 call_driver_probe drivers/base/dd.c:515
 really_probe+0x1c9/0x5b0 drivers/base/dd.c:601
 really_probe_debug drivers/base/dd.c:694
 __driver_probe_device+0x10d/0x1f0 drivers/base/dd.c:754
 driver_probe_device+0x68/0x150 drivers/base/dd.c:786
 __driver_attach+0xca/0x200 drivers/base/dd.c:1145
 bus_for_each_dev+0x108/0x190 drivers/base/bus.c:301
 driver_attach+0x30/0x40 drivers/base/dd.c:1162
 bus_add_driver+0x325/0x3c0 drivers/base/bus.c:618
 driver_register+0xf3/0x1d0 drivers/base/driver.c:171
...

Add a suitable sanity check.

Signed-off-by: Andi Kleen <[email protected]>
Signed-off-by: Alexander Shishkin <[email protected]>
Cc: Amit Shah <[email protected]>
Cc: Arnd Bergmann <[email protected]>
Cc: Greg Kroah-Hartman <[email protected]>

drivers/char/virtio_console.c

@@ -1843,6 +1843,9 @@ static int init_vqs(struct ports_device *portdev)
 	int err;
 
 	nr_ports = portdev->max_nr_ports;
+	if (use_multiport(portdev) && nr_ports < 1)
+		return -EINVAL;
+
 	nr_queues = use_multiport(portdev) ? (nr_ports + 1) * 2 : 2;
 
 	vqs = kmalloc_array(nr_queues, sizeof(struct virtqueue *), GFP_KERNEL);