virtio: console: Prevent userspace from submitting NULL buffers

A userspace could submit a buffer with 0 length to be written to the
host.  Prevent such a situation.

This was not needed previously, but recent changes in the way write()
works exposed this condition to trigger a virtqueue event to the host,
causing a NULL buffer to be sent across.

Signed-off-by: Amit Shah <amit.shah@redhat.com>
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
CC: stable@kernel.org

1 files changed, 4 insertions, 0 deletions

diff --git a/drivers/char/virtio_console.c b/drivers/char/virtio_console.c
index 2f2e31b58b34..c810481a5bc2 100644
--- a/drivers/char/virtio_console.c
+++ b/drivers/char/virtio_console.c
@@ -596,6 +596,10 @@ static ssize_t port_fops_write(struct file *filp, const char __user *ubuf,
         ssize_t ret;
         bool nonblock;
 
+        /* Userspace could be out to fool us */
+        if (!count)
+                return 0;
+
         port = filp->private_data;
 
         nonblock = filp->f_flags & O_NONBLOCK;