Re: [Qemu-devel] [PATCH v2 3/9] virtio-9p: handle handle_9p_output() error

[Greg Kurz]

On Fri, 23 Sep 2016 13:54:17 +0100
Stefan Hajnoczi <address@hidden> wrote:

> On Wed, Sep 21, 2016 at 06:57:12PM +0200, Greg Kurz wrote:
> > A broken guest may send a request with only non-empty out buffers
> > or only non-empty in buffers, virtqueue_pop() will then return a
> > VirtQueueElement with out_num == 0 or in_num == 0 respectively.
> > 
> > All 9P requests are expected to start with the following 7-byte header:
> > 
> >             uint32_t size_le;
> >             uint8_t id;
> >             uint16_t tag_le;
> > 
> > If iov_to_buf() fails to return these 7 bytes, then something is wrong in
> > the guest.
> > 
> > In both cases, it is wrong to crash QEMU, since the root cause lies in the
> > guest. Let's switch the device to the broken state instead.
> > 
> > Signed-off-by: Greg Kurz <address@hidden>
> > ---
> > v2: - added out_free_pdu: label for errors or when virtqueue is empty
> > ---
> >  hw/9pfs/virtio-9p-device.c |   20 ++++++++++++++++----
> >  1 file changed, 16 insertions(+), 4 deletions(-)
> > 
> > diff --git a/hw/9pfs/virtio-9p-device.c b/hw/9pfs/virtio-9p-device.c
> > index e7ea0e45f3dd..5f3a67cfc717 100644
> > --- a/hw/9pfs/virtio-9p-device.c
> > +++ b/hw/9pfs/virtio-9p-device.c
> > @@ -52,17 +52,24 @@ static void handle_9p_output(VirtIODevice *vdev, 
> > VirtQueue *vq)
> >  
> >          elem = virtqueue_pop(vq, sizeof(VirtQueueElement));
> >          if (!elem) {
> > -            pdu_free(pdu);
> > -            break;
> > +            goto out_free_pdu;
> >          }
> >  
> > -        BUG_ON(elem->out_num == 0 || elem->in_num == 0);
> > +        if (elem->out_num == 0 || elem->in_num == 0) {
> > +            virtio_error(vdev,
> > +                         "The guest sent a VirtFS request without 
> > headers");
> > +            goto out_free_pdu;
> > +        }  
> 
> I'm not sure this check is even necessary since we check
> iov_to_buf(elem->out_sg[]) and pdu_marshall() avoids overflowing
> elem->in_sg[].
> 

I agree that the difference between absent and empty out headers is quite
subtile indeed. It would probably be acceptable to rely on iov_to_buf(),
the same way virtio-net relies on iov_size() I guess.

The case of elem->in_sg[] seems different though: unless I missed something,
v9fs_packunpack() will simply return -ENOBUFS. The error will be propagated
to pdu_complete(), which does not even check for errors (see the comment).
And then we'll push a broken element to the guest... I'd rather raise the
flag here.

> Either way, your change is correct:
> 
> Reviewed-by: Stefan Hajnoczi <address@hidden>

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