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

[Jason Wang]

On Thu, Jan 12, 2023 at 3:18 AM Stefan Hajnoczi <stefanha@xxxxxxxxxx> wrote:
>
> On Wed, Jan 11, 2023 at 02:07:30PM +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
> > vhost core.
> >
> > 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>
> > ---
> >  drivers/vhost/scsi.c | 21 +++++++++++++++++----
> >  1 file changed, 17 insertions(+), 4 deletions(-)
> >
> > diff --git a/drivers/vhost/scsi.c b/drivers/vhost/scsi.c
> > index dca6346d75b3..7d6d70072603 100644
> > --- a/drivers/vhost/scsi.c
> > +++ b/drivers/vhost/scsi.c
> > @@ -80,7 +80,7 @@ struct vhost_scsi_cmd {
> >       struct scatterlist *tvc_prot_sgl;
> >       struct page **tvc_upages;
> >       /* Pointer to response header iovec */
> > -     struct iovec tvc_resp_iov;
> > +     struct iovec *tvc_resp_iov;
> >       /* Pointer to vhost_scsi for our device */
> >       struct vhost_scsi *tvc_vhost;
> >       /* Pointer to vhost_virtqueue for the cmd */
> > @@ -563,7 +563,7 @@ static void vhost_scsi_complete_cmd_work(struct vhost_work *work)
> >               memcpy(v_rsp.sense, cmd->tvc_sense_buf,
> >                      se_cmd->scsi_sense_length);
> >
> > -             iov_iter_init(&iov_iter, ITER_DEST, &cmd->tvc_resp_iov,
> > +             iov_iter_init(&iov_iter, ITER_DEST, cmd->tvc_resp_iov,
> >                             cmd->tvc_in_iovs, sizeof(v_rsp));
> >               ret = copy_to_iter(&v_rsp, sizeof(v_rsp), &iov_iter);
> >               if (likely(ret == sizeof(v_rsp))) {
> > @@ -594,6 +594,7 @@ vhost_scsi_get_cmd(struct vhost_virtqueue *vq, struct vhost_scsi_tpg *tpg,
> >       struct vhost_scsi_cmd *cmd;
> >       struct vhost_scsi_nexus *tv_nexus;
> >       struct scatterlist *sg, *prot_sg;
> > +     struct iovec *tvc_resp_iov;
> >       struct page **pages;
> >       int tag;
> >
> > @@ -613,6 +614,7 @@ vhost_scsi_get_cmd(struct vhost_virtqueue *vq, struct vhost_scsi_tpg *tpg,
> >       sg = cmd->tvc_sgl;
> >       prot_sg = cmd->tvc_prot_sgl;
> >       pages = cmd->tvc_upages;
> > +     tvc_resp_iov = cmd->tvc_resp_iov;
> >       memset(cmd, 0, sizeof(*cmd));
> >       cmd->tvc_sgl = sg;
> >       cmd->tvc_prot_sgl = prot_sg;
> > @@ -625,6 +627,7 @@ vhost_scsi_get_cmd(struct vhost_virtqueue *vq, struct vhost_scsi_tpg *tpg,
> >       cmd->tvc_data_direction = data_direction;
> >       cmd->tvc_nexus = tv_nexus;
> >       cmd->inflight = vhost_scsi_get_inflight(vq);
> > +     cmd->tvc_resp_iov = tvc_resp_iov;
> >
> >       memcpy(cmd->tvc_cdb, cdb, VHOST_SCSI_MAX_CDB_SIZE);
> >
> > @@ -935,7 +938,7 @@ vhost_scsi_handle_vq(struct vhost_scsi *vs, struct vhost_virtqueue *vq)
> >       struct iov_iter in_iter, prot_iter, data_iter;
> >       u64 tag;
> >       u32 exp_data_len, data_direction;
> > -     int ret, prot_bytes, c = 0;
> > +     int ret, prot_bytes, i, c = 0;
> >       u16 lun;
> >       u8 task_attr;
> >       bool t10_pi = vhost_has_feature(vq, VIRTIO_SCSI_F_T10_PI);
> > @@ -1092,7 +1095,8 @@ vhost_scsi_handle_vq(struct vhost_scsi *vs, struct vhost_virtqueue *vq)
> >               }
> >               cmd->tvc_vhost = vs;
> >               cmd->tvc_vq = vq;
> > -             cmd->tvc_resp_iov = vq->iov[vc.out];
> > +             for (i = 0; i < vc.in ; i++)
> > +                     cmd->tvc_resp_iov[i] = vq->iov[vc.out + i];
>
> Where is the guarantee that vc.in < UIO_MAXIOV?
>

We limit it here in the vhost_virtqueue structure:

        struct iovec iov[UIO_MAXIOV];

And we pass sizeof(vq->iov) to vhost_get_vq_desc():

        vc->head = vhost_get_vq_desc(vq, vq->iov,
                                     ARRAY_SIZE(vq->iov), &vc->out, &vc->in,
                                     NULL, NULL);

> >               cmd->tvc_in_iovs = vc.in;
> >
> >               pr_debug("vhost_scsi got command opcode: %#02x, lun: %d\n",
> > @@ -1461,6 +1465,7 @@ static void vhost_scsi_destroy_vq_cmds(struct vhost_virtqueue *vq)
> >               kfree(tv_cmd->tvc_sgl);
> >               kfree(tv_cmd->tvc_prot_sgl);
> >               kfree(tv_cmd->tvc_upages);
> > +             kfree(tv_cmd->tvc_resp_iov);
> >       }
> >
> >       sbitmap_free(&svq->scsi_tags);
> > @@ -1508,6 +1513,14 @@ static int vhost_scsi_setup_vq_cmds(struct vhost_virtqueue *vq, int max_cmds)
> >                       goto out;
> >               }
> >
> > +             tv_cmd->tvc_resp_iov = kcalloc(UIO_MAXIOV,
> > +                                            sizeof(struct page *),
> > +                                            GFP_KERNEL);
>
> Should sizeof(struct page *) be sizeof(struct iovec)?

Yes, I will fix it.

Thanks

>
> > +             if (!tv_cmd->tvc_resp_iov) {
> > +                     pr_err("Unable to allocate tv_cmd->tvc_resp_iov\n");
> > +                     goto out;
> > +             }
> > +
> >               tv_cmd->tvc_prot_sgl = kcalloc(VHOST_SCSI_PREALLOC_PROT_SGLS,
> >                                              sizeof(struct scatterlist),
> >                                              GFP_KERNEL);
> > --
> > 2.25.1
> >