Re: [PATCH v4 1/9] vhost/vsock: Avoid allocating arbitrarily-sized SKBs

[Jason Wang]

On Thu, Jul 17, 2025 at 5:01 PM Will Deacon <will@xxxxxxxxxx> wrote:
>
> vhost_vsock_alloc_skb() returns NULL for packets advertising a length
> larger than VIRTIO_VSOCK_MAX_PKT_BUF_SIZE in the packet header. However,
> this is only checked once the SKB has been allocated and, if the length
> in the packet header is zero, the SKB may not be freed immediately.

Can this be triggered from the guest? (I guess yes) Did we need to
consider it as a security issue?

>
> Hoist the size check before the SKB allocation so that an iovec larger
> than VIRTIO_VSOCK_MAX_PKT_BUF_SIZE + the header size is rejected
> outright. The subsequent check on the length field in the header can
> then simply check that the allocated SKB is indeed large enough to hold
> the packet.
>
> Cc: <stable@xxxxxxxxxxxxxxx>
> Fixes: 71dc9ec9ac7d ("virtio/vsock: replace virtio_vsock_pkt with sk_buff")
> Reviewed-by: Stefano Garzarella <sgarzare@xxxxxxxxxx>
> Signed-off-by: Will Deacon <will@xxxxxxxxxx>
> ---
>  drivers/vhost/vsock.c | 6 ++++--
>  1 file changed, 4 insertions(+), 2 deletions(-)
>
> diff --git a/drivers/vhost/vsock.c b/drivers/vhost/vsock.c
> index 802153e23073..66a0f060770e 100644
> --- a/drivers/vhost/vsock.c
> +++ b/drivers/vhost/vsock.c
> @@ -344,6 +344,9 @@ vhost_vsock_alloc_skb(struct vhost_virtqueue *vq,
>
>         len = iov_length(vq->iov, out);
>
> +       if (len > VIRTIO_VSOCK_MAX_PKT_BUF_SIZE + VIRTIO_VSOCK_SKB_HEADROOM)
> +               return NULL;
> +
>         /* len contains both payload and hdr */
>         skb = virtio_vsock_alloc_skb(len, GFP_KERNEL);
>         if (!skb)
> @@ -367,8 +370,7 @@ vhost_vsock_alloc_skb(struct vhost_virtqueue *vq,
>                 return skb;
>
>         /* The pkt is too big or the length in the header is invalid */
> -       if (payload_len > VIRTIO_VSOCK_MAX_PKT_BUF_SIZE ||
> -           payload_len + sizeof(*hdr) > len) {
> +       if (payload_len + sizeof(*hdr) > len) {
>                 kfree_skb(skb);
>                 return NULL;
>         }
> --
> 2.50.0.727.gbf7dc18ff4-goog
>

Thanks