virtio-blk: clamp zone report to the report buffer capacity

virtblk_report_zones() trusts the device-reported number of zones when
walking the report buffer:

	nz = min_t(u64, virtio64_to_cpu(vblk->vdev, report->nr_zones),
		   nr_zones);
	...
	for (i = 0; i < nz && zone_idx < nr_zones; i++) {
		ret = virtblk_parse_zone(vblk, &report->zones[i], ...);

The buffer is allocated by virtblk_alloc_report_buffer(), whose size is
capped by the queue's max hardware sectors and max segments and can
therefore hold fewer descriptors than nr_zones. nz is bounded only by
the device-supplied report->nr_zones and the requested nr_zones, never
by the buffer's descriptor capacity. At probe time the request count is
unbounded (blk_revalidate_disk_zones() calls report_zones() with
nr_zones == UINT_MAX), so the device-supplied report->nr_zones is the
sole gate: a device that reports more zones than fit in the buffer
drives the loop to read report->zones[i] past the end of the allocation.

A malicious or buggy virtio-blk device that reports an inflated nr_zones
triggers this during zone revalidation at probe. KASAN reports a
vmalloc-out-of-bounds read in virtblk_report_zones() against the report
buffer allocated a few lines earlier.

Clamp nz to the number of descriptors that actually fit in the report
buffer.

Fixes: 95bfec41bd ("virtio-blk: add support for zoned block devices")
Assisted-by: Claude:claude-opus-4-8
Signed-off-by: Michael Bommarito <michael.bommarito@gmail.com>
Acked-by: Michael S. Tsirkin <mst@redhat.com>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Link: https://patch.msgid.link/20260607124834.3059944-1-michael.bommarito@gmail.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>

drivers/block/virtio_blk.c

@@ -689,6 +689,8 @@ static int virtblk_report_zones(struct gendisk *disk, sector_t sector,
 
 		nz = min_t(u64, virtio64_to_cpu(vblk->vdev, report->nr_zones),
 			   nr_zones);
+		nz = min_t(u64, nz,
+			   (buflen - sizeof(*report)) / sizeof(report->zones[0]));
 		if (!nz)
 			break;