[PATCH v1 4/6] virtio console: Harden control message handling

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

In handle_control_message(), we look at the ->event field twice, which
gives a malicious VMM a window in which to switch it from PORT_ADD to
PORT_REMOVE, triggering a null dereference further down the line:

RIP: 0010:spin_lock_irq ./include/linux/spinlock.h:388
RIP: 0010:unplug_port+0x9/0x150 drivers/char/virtio_console.c:1512
Call Trace:
 handle_control_message+0x108/0x2c0 drivers/char/virtio_console.c:1600
 elfcorehdr_read+0x40/0x40 ??:?
 process_one_work+0x1b4/0x310 kernel/workqueue.c:2297
 worker_thread+0x5c/0x3a0 kernel/workqueue.c:2444
 kthread+0x120/0x140 kernel/kthread.c:319
 ret_from_fork+0x1f/0x30 arch/x86/entry/entry_64.S:295

Read the event code once instead, basing all following decisions on the
same value.

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, 5 insertions(+), 5 deletions(-)

diff --git a/drivers/char/virtio_console.c b/drivers/char/virtio_console.c
index 6599c2956ba4..62f69f949cb7 100644
--- a/drivers/char/virtio_console.c
+++ b/drivers/char/virtio_console.c
@@ -1563,22 +1563,22 @@ static void handle_control_message(struct virtio_device *vdev,
 	struct port *port;
 	size_t name_size;
 	int err;
-	unsigned id;
+	unsigned id, event;
 
 	cpkt = (struct virtio_console_control *)(buf->buf + buf->offset);
 
-	/* Make sure the host cannot change id under us */
+	/* Make sure the host cannot change id or event under us */
 	id = virtio32_to_cpu(vdev, READ_ONCE(cpkt->id));
+	event = virtio16_to_cpu(vdev, cpkt->event);
 	port = find_port_by_id(portdev, id);
-	if (!port &&
-	    cpkt->event != cpu_to_virtio16(vdev, VIRTIO_CONSOLE_PORT_ADD)) {
+	if (!port && event != VIRTIO_CONSOLE_PORT_ADD) {
 		/* No valid header at start of buffer.  Drop it. */
 		dev_dbg(&portdev->vdev->dev,
 			"Invalid index %u in control packet\n", cpkt->id);
 		return;
 	}
 
-	switch (virtio16_to_cpu(vdev, cpkt->event)) {
+	switch (event) {
 	case VIRTIO_CONSOLE_PORT_ADD:
 		if (port) {
 			dev_dbg(&portdev->vdev->dev,
-- 
2.39.0