Subject: Re: [PATCH] virtio-blk: clamp zone report to the report buffer capacity

From: "Michael S. Tsirkin" <[email-masked]>
Date: Sat, 6 Jun 2026 22:23:22 -0400

On Sat, Jun 06, 2026 at 01:04:15PM -0400, Michael Bommarito wrote:
> 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: 95bfec41bd3d ("virtio-blk: add support for zoned block devices")
> Assisted-by: Claude:claude-opus-4-8
> Signed-off-by: Michael Bommarito <[email-masked]>
> ---
>  drivers/block/virtio_blk.c | 8 ++++++++
>  1 file changed, 8 insertions(+)
> 
> diff --git a/drivers/block/virtio_blk.c b/drivers/block/virtio_blk.c
> index b1c9a27fe00f3..d50aaf956d558 100644
> --- a/drivers/block/virtio_blk.c
> +++ b/drivers/block/virtio_blk.c
> @@ -689,6 +689,14 @@ static int virtblk_report_zones(struct gendisk *disk, sector_t sector,
>  
>  		nz = min_t(u64, virtio64_to_cpu(vblk->vdev, report->nr_zones),

I think nr_zones should have been le64, bot virtio64.

>  			   nr_zones);
> +		/*
> +		 * The device-reported nr_zones is untrusted;

this part depends on the config. just drop it.

> clamp it to the
> +		 * number of descriptors that actually fit in the report buffer
> +		 * so a malicious or buggy device cannot drive the parse loop
> +		 * past the allocation.
> +		 */
> +		nz = min_t(u64, nz,
> +			   (buflen - sizeof(*report)) / sizeof(report->zones[0]));
>  		if (!nz)
>  			break;
>  
> 
> base-commit: 5200f5f493f79f14bbdc349e402a40dfb32f23c8
> -- 
> 2.53.0