Re: [PATCH V2] vhost-scsi: unbreak any layout for response

[Stefan Hajnoczi]

On Thu, Jan 19, 2023 at 03:36:47PM +0800, Jason Wang wrote:
> Al Viro said:
> 
> """
> Since "vhost/scsi: fix reuse of &vq->iov[out] in response"
> we have this:
>                 cmd->tvc_resp_iov = vq->iov[vc.out];
>                 cmd->tvc_in_iovs = vc.in;
> combined with
>                 iov_iter_init(&iov_iter, ITER_DEST, &cmd->tvc_resp_iov,
>                               cmd->tvc_in_iovs, sizeof(v_rsp));
> in vhost_scsi_complete_cmd_work().  We used to have ->tvc_resp_iov
> _pointing_ to vq->iov[vc.out]; back then iov_iter_init() asked to
> set an iovec-backed iov_iter over the tail of vq->iov[], with
> length being the amount of iovecs in the tail.
> 
> Now we have a copy of one element of that array.  Fortunately, the members
> following it in the containing structure are two non-NULL kernel pointers,
> so copy_to_iter() will not copy anything beyond the first iovec - kernel
> pointer is not (on the majority of architectures) going to be accepted by
> access_ok() in copyout() and it won't be skipped since the "length" (in
> reality - another non-NULL kernel pointer) won't be zero.
> 
> So it's not going to give a guest-to-qemu escalation, but it's definitely
> a bug.  Frankly, my preference would be to verify that the very first iovec
> is long enough to hold rsp_size.  Due to the above, any users that try to
> give us vq->iov[vc.out].iov_len < sizeof(struct virtio_scsi_cmd_resp)
> would currently get a failure in vhost_scsi_complete_cmd_work()
> anyway.
> """
> 
> However, the spec doesn't say anything about the legacy descriptor
> layout for the respone. So this patch tries to not assume the response
> to reside in a single separate descriptor which is what commit
> 79c14141a487 ("vhost/scsi: Convert completion path to use") tries to
> achieve towards to ANY_LAYOUT.
> 
> This is done by allocating and using dedicate resp iov in the
> command. To be safety, start with UIO_MAXIOV to be consistent with the
> limitation that we advertise to the vhost_get_vq_desc().
> 
> Testing with the hacked virtio-scsi driver that use 1 descriptor for 1
> byte in the response.
> 
> Reported-by: Al Viro <viro@xxxxxxxxxxxxxxxxxx>
> Cc: Benjamin Coddington <bcodding@xxxxxxxxxx>
> Cc: Nicholas Bellinger <nab@xxxxxxxxxxxxxxx>
> Fixes: a77ec83a5789 ("vhost/scsi: fix reuse of &vq->iov[out] in response")
> Signed-off-by: Jason Wang <jasowang@xxxxxxxxxx>
> ---
> Changes since V1:
> - tweak the changelog
> - fix the allocation size for tvc_resp_iov (should be sizeof(struct iovec))
> ---
>  drivers/vhost/scsi.c | 21 +++++++++++++++++----
>  1 file changed, 17 insertions(+), 4 deletions(-)

Reviewed-by: Stefan Hajnoczi <stefanha@xxxxxxxxxx>

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