Re: [PATCH 6/6] vhost: don't do synchronize_rcu() in vhost_uninit_vq_maps()

[Jason Wang]

On 2019/7/23 äå5:16, Michael S. Tsirkin wrote:
> On Tue, Jul 23, 2019 at 03:57:18AM -0400, Jason Wang wrote:
> > There's no need for RCU synchronization in vhost_uninit_vq_maps()
> > since we've already serialized with readers (memory accessors). This
> > also avoid the possible userspace DOS through ioctl() because of the
> > possible high latency caused by synchronize_rcu().
> >
> > Reported-by: Michael S. Tsirkin <mst@xxxxxxxxxx>
> > Fixes: 7f466032dc9e ("vhost: access vq metadata through kernel virtual address")
> > Signed-off-by: Jason Wang <jasowang@xxxxxxxxxx>
> I agree synchronize_rcu in both mmu notifiers and ioctl
> is a problem we must fix.
>
> > ---
> >   drivers/vhost/vhost.c | 4 +++-
> >   1 file changed, 3 insertions(+), 1 deletion(-)
> >
> > diff --git a/drivers/vhost/vhost.c b/drivers/vhost/vhost.c
> > index 5b8821d00fe4..a17df1f4069a 100644
> > --- a/drivers/vhost/vhost.c
> > +++ b/drivers/vhost/vhost.c
> > @@ -334,7 +334,9 @@ static void vhost_uninit_vq_maps(struct vhost_virtqueue *vq)
> >   	}
> >   	spin_unlock(&vq->mmu_lock);
> >   
> > -	synchronize_rcu();
> > +	/* No need for synchronize_rcu() or kfree_rcu() since we are
> > +	 * serialized with memory accessors (e.g vq mutex held).
> > +	 */
> >   
> >   	for (i = 0; i < VHOST_NUM_ADDRS; i++)
> >   		if (map[i])
> > --
> > 2.18.1
> .. however we can not RCU with no synchronization in sight.
> Sometimes there are hacks like using a lock/unlock
> pair instead of sync, but here no one bothers.
>
> specifically notifiers call reset vq maps which calls
> uninit vq maps which is not under any lock.


Notifier did this:

ÂÂÂÂÂÂÂ if (map) {
ÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂ if (uaddr->write) {
ÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂ for (i = 0; i < map->npages; i++)
set_page_dirty(map->pages[i]);
}
ÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂ rcu_assign_pointer(vq->maps[index], NULL);
}
spin_unlock(&vq->mmu_lock);

ÂÂÂÂÂÂÂ if (map) {
synchronize_rcu();
vhost_map_unprefetch(map);
ÂÂÂÂÂÂÂ }

So it indeed have a synchronize_rcu() there.

Thanks


> You will get use after free when map is then accessed.
>
> If you always have a lock then just take that lock
> and no need for RCU.