* [Devel] [PATCH hci-8.1] drivers/vhost/blk: fail guest writes to a read-only backend
@ 2026-10-28 9:35 Alexander Ivanov
2026-10-28 9:39 ` no-reply
` (2 more replies)
0 siblings, 3 replies; 4+ messages in thread
From: Alexander Ivanov @ 2026-10-28 9:35 UTC (permalink / raw)
To: andrey.zhadchenko; +Cc: devel
When a disk is attached read-only, QEMU opens the backing block device
O_RDONLY and advertises VIRTIO_BLK_F_RO. vhost_blk_set_backend() checks
only that the fd is a block device, and requests are submitted as bios
straight to the bdev, so a guest that ignores VIRTIO_BLK_F_RO and sends
VIRTIO_BLK_T_OUT gets VIRTIO_BLK_S_OK and modifies the disk.
Complete VIRTIO_BLK_T_OUT with VIRTIO_BLK_S_IOERR when the backend file
lacks FMODE_WRITE or the bdev itself is read-only, as the userspace
virtio-blk device does. The bdev is checked per request since it can be
switched read-only at runtime. Flush stays allowed: it does not modify
data.
Fixes: d8722ff88c5d ("drivers/vhost: vhost-blk accelerator for virtio-blk guests")
https://virtuozzo.atlassian.net/browse/VSTOR-152127
Feature: vhost-blk: in-kernel accelerator for virtio-blk guests
Signed-off-by: Alexander Ivanov <alexander.ivanov@virtuozzo.com>
---
drivers/vhost/blk.c | 15 +++++++++++++++
1 file changed, 15 insertions(+)
diff --git a/drivers/vhost/blk.c b/drivers/vhost/blk.c
index b170439a1806..11ded3214c45 100644
--- a/drivers/vhost/blk.c
+++ b/drivers/vhost/blk.c
@@ -443,6 +443,12 @@ static int vhost_blk_req_submit(struct vhost_blk_req *req)
return ret;
}
+static bool vhost_blk_backend_ro(struct file *f)
+{
+ return !(f->f_mode & FMODE_WRITE) ||
+ bdev_read_only(I_BDEV(f->f_mapping->host));
+}
+
static int vhost_blk_req_handle(struct vhost_virtqueue *vq,
struct virtio_blk_outhdr *hdr,
u16 head, u16 out, u16 in)
@@ -480,6 +486,15 @@ static int vhost_blk_req_handle(struct vhost_virtqueue *vq,
switch (hdr->type) {
case VIRTIO_BLK_T_OUT:
+ if (vhost_blk_backend_ro(vhost_vq_get_backend(vq))) {
+ status = VIRTIO_BLK_S_IOERR;
+ ret = vhost_blk_set_status(req, status);
+ if (ret)
+ break;
+ vhost_add_used_and_signal(&blk->dev, vq, head,
+ sizeof(status));
+ break;
+ }
req->bi_opf = REQ_OP_WRITE;
ret = vhost_blk_req_submit(req);
break;
--
2.53.0
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^ permalink raw reply [flat|nested] 4+ messages in thread* Re: [Devel] [PATCH hci-8.1] drivers/vhost/blk: fail guest writes to a read-only backend
2026-10-28 9:35 [Devel] [PATCH hci-8.1] drivers/vhost/blk: fail guest writes to a read-only backend Alexander Ivanov
@ 2026-10-28 9:39 ` no-reply
2026-10-29 11:33 ` Andrey Zhadchenko
2026-10-29 12:42 ` Konstantin Khorenko
2 siblings, 0 replies; 4+ messages in thread
From: no-reply @ 2026-10-28 9:39 UTC (permalink / raw)
To: Alexander Ivanov, andrey.zhadchenko; +Cc: devel
> When a disk is attached read-only, QEMU opens the backing block device
> O_RDONLY and advertises VIRTIO_BLK_F_RO. vhost_blk_set_backend() checks
> only that the fd is a block device, and requests are submitted as bios
> straight to the bdev, so a guest that ignores VIRTIO_BLK_F_RO and sends
> VIRTIO_BLK_T_OUT gets VIRTIO_BLK_S_OK and modifies the disk.
>
> Complete VIRTIO_BLK_T_OUT with VIRTIO_BLK_S_IOERR when the backend file
> lacks FMODE_WRITE or the bdev itself is read-only, as the userspace
> virtio-blk device does. The bdev is checked per request since it can be
> switched read-only at runtime. Flush stays allowed: it does not modify
> data.
>
> Fixes: d8722ff88c5d ("drivers/vhost: vhost-blk accelerator for virtio-blk guests")
> https://virtuozzo.atlassian.net/browse/VSTOR-152127
> Feature: vhost-blk: in-kernel accelerator for virtio-blk guests
>
> Signed-off-by: Alexander Ivanov <alexander.ivanov@virtuozzo.com>
Sashiko has reviewed this patch and found no issues. It looks great!
--
Sashiko AI review · https://sashiko.vzint.dev/#/patchset/20261028093536.3072110-1-alexander.ivanov@virtuozzo.com?part=1
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^ permalink raw reply [flat|nested] 4+ messages in thread* Re: [Devel] [PATCH hci-8.1] drivers/vhost/blk: fail guest writes to a read-only backend
2026-10-28 9:35 [Devel] [PATCH hci-8.1] drivers/vhost/blk: fail guest writes to a read-only backend Alexander Ivanov
2026-10-28 9:39 ` no-reply
@ 2026-10-29 11:33 ` Andrey Zhadchenko
2026-10-29 12:42 ` Konstantin Khorenko
2 siblings, 0 replies; 4+ messages in thread
From: Andrey Zhadchenko @ 2026-10-29 11:33 UTC (permalink / raw)
To: Alexander Ivanov; +Cc: devel
Reviewed-by: Andrey Zhadchenko <andrey.zhadchenko@virtuozzo.com>
On 10/28/26 10:35, Alexander Ivanov wrote:
> When a disk is attached read-only, QEMU opens the backing block device
> O_RDONLY and advertises VIRTIO_BLK_F_RO. vhost_blk_set_backend() checks
> only that the fd is a block device, and requests are submitted as bios
> straight to the bdev, so a guest that ignores VIRTIO_BLK_F_RO and sends
> VIRTIO_BLK_T_OUT gets VIRTIO_BLK_S_OK and modifies the disk.
>
> Complete VIRTIO_BLK_T_OUT with VIRTIO_BLK_S_IOERR when the backend file
> lacks FMODE_WRITE or the bdev itself is read-only, as the userspace
> virtio-blk device does. The bdev is checked per request since it can be
> switched read-only at runtime. Flush stays allowed: it does not modify
> data.
>
> Fixes: d8722ff88c5d ("drivers/vhost: vhost-blk accelerator for virtio-blk guests")
> https://virtuozzo.atlassian.net/browse/VSTOR-152127
> Feature: vhost-blk: in-kernel accelerator for virtio-blk guests
>
> Signed-off-by: Alexander Ivanov <alexander.ivanov@virtuozzo.com>
> ---
> drivers/vhost/blk.c | 15 +++++++++++++++
> 1 file changed, 15 insertions(+)
>
> diff --git a/drivers/vhost/blk.c b/drivers/vhost/blk.c
> index b170439a1806..11ded3214c45 100644
> --- a/drivers/vhost/blk.c
> +++ b/drivers/vhost/blk.c
> @@ -443,6 +443,12 @@ static int vhost_blk_req_submit(struct vhost_blk_req *req)
> return ret;
> }
>
> +static bool vhost_blk_backend_ro(struct file *f)
> +{
> + return !(f->f_mode & FMODE_WRITE) ||
> + bdev_read_only(I_BDEV(f->f_mapping->host));
> +}
> +
> static int vhost_blk_req_handle(struct vhost_virtqueue *vq,
> struct virtio_blk_outhdr *hdr,
> u16 head, u16 out, u16 in)
> @@ -480,6 +486,15 @@ static int vhost_blk_req_handle(struct vhost_virtqueue *vq,
>
> switch (hdr->type) {
> case VIRTIO_BLK_T_OUT:
> + if (vhost_blk_backend_ro(vhost_vq_get_backend(vq))) {
> + status = VIRTIO_BLK_S_IOERR;
> + ret = vhost_blk_set_status(req, status);
> + if (ret)
> + break;
> + vhost_add_used_and_signal(&blk->dev, vq, head,
> + sizeof(status));
> + break;
> + }
> req->bi_opf = REQ_OP_WRITE;
> ret = vhost_blk_req_submit(req);
> break;
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^ permalink raw reply [flat|nested] 4+ messages in thread* Re: [Devel] [PATCH hci-8.1] drivers/vhost/blk: fail guest writes to a read-only backend
2026-10-28 9:35 [Devel] [PATCH hci-8.1] drivers/vhost/blk: fail guest writes to a read-only backend Alexander Ivanov
2026-10-28 9:39 ` no-reply
2026-10-29 11:33 ` Andrey Zhadchenko
@ 2026-10-29 12:42 ` Konstantin Khorenko
2 siblings, 0 replies; 4+ messages in thread
From: Konstantin Khorenko @ 2026-10-29 12:42 UTC (permalink / raw)
To: andrey.zhadchenko, Alexander Ivanov; +Cc: devel
On Wed, 28 Oct 2026 10:35:36 +0100, Alexander Ivanov wrote:
> drivers/vhost/blk: fail guest writes to a read-only backend
Applied to branch-rh10-6.12.0-211.52.1.22.x.vz10-ovz, will be released in rh10-6.12.0-211.52.1.22.12.vz10, thanks!
[1/1] drivers/vhost/blk: fail guest writes to a read-only backend
https://bitbucket.org/virtuozzocore/vzkernel.vzs/commits/8445ee9f5e09
--
Konstantin Khorenko,
Virtuozzo Linux Kernel Team
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2026-10-28 9:35 [Devel] [PATCH hci-8.1] drivers/vhost/blk: fail guest writes to a read-only backend Alexander Ivanov
2026-10-28 9:39 ` no-reply
2026-10-29 11:33 ` Andrey Zhadchenko
2026-10-29 12:42 ` Konstantin Khorenko
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