Re: [PATCH] vringh: Fix maximum number check for indirect descriptors

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On Thu, May 5, 2022 at 3:46 PM Jason Wang <jasowang@xxxxxxxxxx> wrote:
>
> On Wed, May 4, 2022 at 4:12 PM Xie Yongji <xieyongji@xxxxxxxxxxxxx> wrote:
> >
> > We should use size of descriptor chain to check the maximum
> > number of consumed descriptors in indirect case.
>
> AFAIK, it's a guard for loop descriptors.
>
> > And the
> > statistical counts should also be reset to zero each time
> > we get an indirect descriptor.
>
> What might happen if we don't have this patch?
>
> >
> > Fixes: f87d0fbb5798 ("vringh: host-side implementation of virtio rings.")
> > Signed-off-by: Xie Yongji <xieyongji@xxxxxxxxxxxxx>
> > Signed-off-by: Fam Zheng <fam.zheng@xxxxxxxxxxxxx>
> > ---
> >  drivers/vhost/vringh.c | 4 +++-
> >  1 file changed, 3 insertions(+), 1 deletion(-)
> >
> > diff --git a/drivers/vhost/vringh.c b/drivers/vhost/vringh.c
> > index 14e2043d7685..c1810b77a05e 100644
> > --- a/drivers/vhost/vringh.c
> > +++ b/drivers/vhost/vringh.c
> > @@ -344,12 +344,13 @@ __vringh_iov(struct vringh *vrh, u16 i,
> >                         addr = (void *)(long)(a + range.offset);
> >                         err = move_to_indirect(vrh, &up_next, &i, addr, &desc,
> >                                                &descs, &desc_max);
> > +                       count = 0;
>
> Then it looks to me we can detect a loop indirect descriptor chain?

I meant "can't" actually.

Thanks

>
> Thanks
>
> >                         if (err)
> >                                 goto fail;
> >                         continue;
> >                 }
> >
> > -               if (count++ == vrh->vring.num) {
> > +               if (count++ == desc_max) {
> >                         vringh_bad("Descriptor loop in %p", descs);
> >                         err = -ELOOP;
> >                         goto fail;
> > @@ -410,6 +411,7 @@ __vringh_iov(struct vringh *vrh, u16 i,
> >                         if (unlikely(up_next > 0)) {
> >                                 i = return_from_indirect(vrh, &up_next,
> >                                                          &descs, &desc_max);
> > +                               count = 0;
> >                                 slow = false;
> >                         } else
> >                                 break;
> > --
> > 2.20.1
> >

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