Re: [PATCH v3] 9p/trans_virtio: reject mount tags that cannot fit a sysfs page

[Christian Schoenebeck <linux_oss@crudebyte.com>]

On Friday, 26 June 2026 13:19:06 CEST Michael Bommarito wrote:
> p9_virtio_probe() reads the mount tag length from the device's
> virtio_9p_config.tag_len, a 16-bit field, and accepts any value up to
> 65535 with no upper bound:
> 
> 	virtio_cread(vdev, struct virtio_9p_config, tag_len, &tag_len);
> 	...
> 	tag = kzalloc(tag_len + 1, GFP_KERNEL);
> 
> The tag is later emitted through the world-readable
> /sys/.../mount_tag attribute by p9_mount_tag_show(), which copies the
> whole NUL-terminated tag into the single PAGE_SIZE buffer that the
> sysfs core provides:
> 
> 	tag_len = strlen(chan->tag);
> 	memcpy(buf, chan->tag, tag_len + 1);
> 
> A tag longer than the page therefore overruns the sysfs buffer. Under
> the confidential-computing threat model, where the guest does not
> trust the host, a malicious or compromised host can advertise a
> ~64 KiB tag; the first read of mount_tag (udev reads it at probe) then
> copies host-controlled content past the end of the 4 KiB page, a slab
> out-of-bounds write.
[...]
>  net/9p/trans_virtio.c | 5 +++++
>  1 file changed, 5 insertions(+)
> 
> diff --git a/net/9p/trans_virtio.c b/net/9p/trans_virtio.c
> index b0d0094ec8e2c..15568f40509c4 100644
> --- a/net/9p/trans_virtio.c
> +++ b/net/9p/trans_virtio.c
> @@ -633,6 +633,11 @@ static int p9_virtio_probe(struct virtio_device *vdev)
>  		err = -EINVAL;
>  		goto out_free_vq;
>  	}
> +	if (tag_len >= PAGE_SIZE) {

That should be tag_len >= PAGE_SIZE - 1.

However you are still assuming and hard-coding an implementation specific
limit of seq_file. If that limit changes there, it breaks *silently* here
again.

I would rather handle this more aggressively by using NAME_MAX (255) as tag
length limit; safe and avoids complicated alternative solutions.

> +		dev_err(&vdev->dev, "mount tag too long (%u bytes)\n", tag_len);
> +		err = -EINVAL;
> +		goto out_free_vq;
> +	}
>  	tag = kzalloc(tag_len + 1, GFP_KERNEL);
>  	if (!tag) {
>  		err = -ENOMEM;
> 
> base-commit: 4edcdefd4083ae04b1a5656f4be6cd83ae919ef4