Virtualization - Jun 2022 - [External] Re: [PATCH] virtio_ring : fix vring_packed_desc memory out of bounds bug

On Sat, Jun 11, 2022 at 12:38:10AM +0800, ?? wrote:
> > This pattern was always iffy, but I don't think the patch
> > improves the situation much. last_used_idx and
vq->packed.used_wrap_counter
> > can still get out of sync.
> 
> Yes? You are absolutely correct, thanks for pointing out this issue, I
> didn't take that into consideration,
> how about disabling interrupts before this code below:
> 
> > vq->last_used_idx += vq->packed.desc_state[id].num;
> > if (unlikely(vq->last_used_idx >= vq->packed.vring.num)) {
> >          vq->last_used_idx -= vq->packed.vring.num;
> >          vq->packed.used_wrap_counter ^= 1;
> > }
> 
> it seems to be fine to just turn off the interrupts of the current vring.
> 
> BR
That would make datapath significantly slower.
> 
> Michael S. Tsirkin <mst at redhat.com> ?2022?6?10??? 22:50???
> >
> > On Fri, Jun 10, 2022 at 06:33:14PM +0800, huangjie.albert wrote:
> > > ksoftirqd may consume the packet and it will call:
> > > virtnet_poll
> > >       -->virtnet_receive
> > >               -->virtqueue_get_buf_ctx
> > >                       -->virtqueue_get_buf_ctx_packed
> > > and in virtqueue_get_buf_ctx_packed:
> > >
> > > vq->last_used_idx += vq->packed.desc_state[id].num;
> > > if (unlikely(vq->last_used_idx >= vq->packed.vring.num))
{
> > >          vq->last_used_idx -= vq->packed.vring.num;
> > >          vq->packed.used_wrap_counter ^= 1;
> > > }
> > >
> > > if at the same time, there comes a vring interrupt?in
vring_interrupt:
> > > we will call:
> > > vring_interrupt
> > >       -->more_used
> > >               -->more_used_packed
> > >                       -->is_used_desc_packed
> > > in is_used_desc_packed, the last_used_idx maybe >=
vq->packed.vring.num.
> > > so this could case a memory out of bounds bug.
> > >
> > > this patch is to fix this.
> > >
> > > Signed-off-by: huangjie.albert <huangjie.albert at
bytedance.com>
> >
> >
> > This pattern was always iffy, but I don't think the patch
> > improves the situation much. last_used_idx and
vq->packed.used_wrap_counter
> > can still get out of sync.
> >
> > Maybe refactor code to keep everything in vq->last_used_idx?
> >
> > Jason what is your take?
> >
> >
> >
> >
> >
> > > ---
> > >  drivers/virtio/virtio_ring.c | 3 +++
> > >  1 file changed, 3 insertions(+)
> > >
> > > diff --git a/drivers/virtio/virtio_ring.c
b/drivers/virtio/virtio_ring.c
> > > index 13a7348cedff..d2abbb3a8187 100644
> > > --- a/drivers/virtio/virtio_ring.c
> > > +++ b/drivers/virtio/virtio_ring.c
> > > @@ -1397,6 +1397,9 @@ static inline bool
is_used_desc_packed(const struct vring_virtqueue *vq,
> > >       bool avail, used;
> > >       u16 flags;
> > >
> > > +     if (idx >= vq->packed.vring.num)
> > > +             return false;
> > > +
> > >       flags = le16_to_cpu(vq->packed.vring.desc[idx].flags);
> > >       avail = !!(flags & (1 <<
VRING_PACKED_DESC_F_AVAIL));
> > >       used = !!(flags & (1 <<
VRING_PACKED_DESC_F_USED));
> > > --
> > > 2.27.0
> >