Re: [PATCH net v2] 9p/xen : Fix use after free bug in xen_9pfs_front_remove due to race condition

[Michal Swiatkowski <michal.swiatkowski@linux.intel.com>]

On Mon, Mar 13, 2023 at 10:01:21PM +0800, Zheng Hacker wrote:
> Michal Swiatkowski <michal.swiatkowski@linux.intel.com> 于2023年3月13日周一 21:54写道：
> >
> > On Mon, Mar 13, 2023 at 05:00:02PM +0800, Zheng Wang wrote:
> > > In xen_9pfs_front_probe, it calls xen_9pfs_front_alloc_dataring
> > > to init priv->rings and bound &ring->work with p9_xen_response.
> > >
> > > When it calls xen_9pfs_front_event_handler to handle IRQ requests,
> > > it will finally call schedule_work to start the work.
> > >
> > > When we call xen_9pfs_front_remove to remove the driver, there
> > > may be a sequence as follows:
> > >
> > > Fix it by finishing the work before cleanup in xen_9pfs_front_free.
> > >
> > > Note that, this bug is found by static analysis, which might be
> > > false positive.
> > >
> > > CPU0                  CPU1
> > >
> > >                      |p9_xen_response
> > > xen_9pfs_front_remove|
> > >   xen_9pfs_front_free|
> > > kfree(priv)          |
> > > //free priv          |
> > >                      |p9_tag_lookup
> > >                      |//use priv->client
> > >
> > > Fixes: 71ebd71921e4 ("xen/9pfs: connect to the backend")
> > > Signed-off-by: Zheng Wang <zyytlz.wz@163.com>
> > > ---
> > > v2:
> > > - fix type error of ring found by kernel test robot
> > > ---
> > >  net/9p/trans_xen.c | 5 +++++
> > >  1 file changed, 5 insertions(+)
> > >
> > > diff --git a/net/9p/trans_xen.c b/net/9p/trans_xen.c
> > > index c64050e839ac..83764431c066 100644
> > > --- a/net/9p/trans_xen.c
> > > +++ b/net/9p/trans_xen.c
> > > @@ -274,12 +274,17 @@ static const struct xenbus_device_id xen_9pfs_front_ids[] = {
> > >  static void xen_9pfs_front_free(struct xen_9pfs_front_priv *priv)
> > >  {
> > >       int i, j;
> > > +     struct xen_9pfs_dataring *ring = NULL;
> > Move it before int i, j to have RCT.
> >
Sorry I didn't notice it before, move the definition to for loop.

> > >
> > >       write_lock(&xen_9pfs_lock);
> > >       list_del(&priv->list);
> > >       write_unlock(&xen_9pfs_lock);
> > >
> > >       for (i = 0; i < priv->num_rings; i++) {
Here:
struct xen_9pfs_dataring *ring = &priv->rings[i];

> > > +             /*cancel work*/
> > It isn't needed I think, the function cancel_work_sync() tells everything
> > here.
> >
> 
> Get it, will remove it in the next version of patch.
> 
> Best regards,
> Zheng