Re: [Qemu-devel] [2.2 PATCH V2] virtio-net: fix unmap leak

["Michael S. Tsirkin" <mst@redhat.com>]

On Thu, Nov 27, 2014 at 02:54:47PM +0000, Stefano Stabellini wrote:
> On Thu, 27 Nov 2014, Peter Maydell wrote:
> > On 27 November 2014 at 12:42, Peter Maydell <peter.maydell@linaro.org> wrote:
> > > On 27 November 2014 at 12:33, Michael S. Tsirkin <mst@redhat.com> wrote:
> > >> On Thu, Nov 27, 2014 at 06:04:03PM +0800, Jason Wang wrote:
> > >>> virtio_net_handle_ctrl() and other functions that process control vq
> > >>> request call iov_discard_front() which will shorten the iov. This will
> > >>> lead unmapping in virtqueue_push() leaks mapping.
> > >>>
> > >>> Fixes this by keeping the original iov untouched and using a temp variable
> > >>> in those functions.
> > >>>
> > >>> Cc: Wen Congyang <wency@cn.fujitsu.com>
> > >>> Cc: Stefano Stabellini <stefano.stabellini@eu.citrix.com>
> > >>> Cc: qemu-stable@nongnu.org
> > >>> Signed-off-by: Jason Wang <jasowang@redhat.com>
> > >>
> > >> Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
> > >>
> > >> Peter, can you pick this up or do you want a pull request?
> > >
> > > I can pick it up. I was waiting a bit to check that everybody
> > > was happy that this is the correct way to fix the bug and the
> > > patch is ok...
> > 
> > ...but discussing this with Stefan H on IRC we realised that the same
> > issue also (at least potentially) affects virtio-blk, which suggests
> > that we should fix this by making the core virtio code cope with
> > backends which modify the sglists.
> 
> I think that a similar patch could be produced against
> hw/block/virtio-blk.c:virtio_blk_handle_request

Hmm, this is a data path operation. Adding malloc/free calls
there will slow things down measureably.

> Alternatively my series would help by introducing virtqueue_unmap_sg and
> moving it out of virtqueue_fill. We could call virtqueue_unmap_sg from
> the drivers (instead of calling it from virtqueue_push) and that would
> solve the issue.