vduse: validate virtqueue alignment

In the Linux kernel, the following vulnerability has been resolved:

vduse: validate virtqueue alignment

vduse_validate_config() only checks the upper bound of vq_align. Invalid
values can therefore reach vring_create_virtqueue_map(). The split-ring
helpers use align - 1 as a bit mask, so the alignment must be a non-zero
power of two. A zero value makes vring_size() drop the descriptor and
available-ring part and vring_init() leave the used ring pointer NULL.

The VIRTIO spec requires the used ring to start at an address
aligned to at least 4 bytes. Reject values below VRING_USED_ALIGN_SIZE as
well as non-power-of-two values before they reach the virtio ring helpers.

Opening a virtio-net device created with vq_align=0 triggered:

BUG: KASAN: null-ptr-deref in virtqueue_kick_prepare_split+0xe3/0x100
Read of size 2 at addr 0000000000000000 by task systemd-network/1062

Call Trace (relevant frames):
dump_stack_lvl
print_report
kasan_report
__asan_load2
virtqueue_kick_prepare_split+0xe3/0x100
virtqueue_kick_prepare+0x40/0x60
try_fill_recv+0x857/0x1250
virtnet_open+0x189/0x460
__dev_open+0x225/0x390
__dev_change_flags+0x368/0x3b0
netif_change_flags+0x56/0xc0
do_setlink.isra.0+0x68c/0x1e30

Validate the value before it reaches the virtio ring helpers.