Re: [RFC PATCH 5/7] tun: Introduce virtio-net hashing feature

[Willem de Bruijn]

On Mon, Oct 9, 2023 at 3:11 AM Akihiko Odaki <akihiko.odaki@xxxxxxxxxx> wrote:
>
> On 2023/10/09 19:07, Willem de Bruijn wrote:
> > On Mon, Oct 9, 2023 at 3:05 AM Akihiko Odaki <akihiko.odaki@xxxxxxxxxx> wrote:
> >>
> >>
> >>
> >> On 2023/10/09 18:54, Willem de Bruijn wrote:
> >>> On Mon, Oct 9, 2023 at 3:44 AM Akihiko Odaki <akihiko.odaki@xxxxxxxxxx> wrote:
> >>>>
> >>>> On 2023/10/09 17:13, Willem de Bruijn wrote:
> >>>>> On Sun, Oct 8, 2023 at 12:22 AM Akihiko Odaki <akihiko.odaki@xxxxxxxxxx> wrote:
> >>>>>>
> >>>>>> virtio-net have two usage of hashes: one is RSS and another is hash
> >>>>>> reporting. Conventionally the hash calculation was done by the VMM.
> >>>>>> However, computing the hash after the queue was chosen defeats the
> >>>>>> purpose of RSS.
> >>>>>>
> >>>>>> Another approach is to use eBPF steering program. This approach has
> >>>>>> another downside: it cannot report the calculated hash due to the
> >>>>>> restrictive nature of eBPF.
> >>>>>>
> >>>>>> Introduce the code to compute hashes to the kernel in order to overcome
> >>>>>> thse challenges. An alternative solution is to extend the eBPF steering
> >>>>>> program so that it will be able to report to the userspace, but it makes
> >>>>>> little sense to allow to implement different hashing algorithms with
> >>>>>> eBPF since the hash value reported by virtio-net is strictly defined by
> >>>>>> the specification.
> >>>>>>
> >>>>>> The hash value already stored in sk_buff is not used and computed
> >>>>>> independently since it may have been computed in a way not conformant
> >>>>>> with the specification.
> >>>>>>
> >>>>>> Signed-off-by: Akihiko Odaki <akihiko.odaki@xxxxxxxxxx>
> >>>>>
> >>>>>> @@ -2116,31 +2172,49 @@ static ssize_t tun_put_user(struct tun_struct *tun,
> >>>>>>            }
> >>>>>>
> >>>>>>            if (vnet_hdr_sz) {
> >>>>>> -               struct virtio_net_hdr gso;
> >>>>>> +               union {
> >>>>>> +                       struct virtio_net_hdr hdr;
> >>>>>> +                       struct virtio_net_hdr_v1_hash v1_hash_hdr;
> >>>>>> +               } hdr;
> >>>>>> +               int ret;
> >>>>>>
> >>>>>>                    if (iov_iter_count(iter) < vnet_hdr_sz)
> >>>>>>                            return -EINVAL;
> >>>>>>
> >>>>>> -               if (virtio_net_hdr_from_skb(skb, &gso,
> >>>>>> -                                           tun_is_little_endian(tun), true,
> >>>>>> -                                           vlan_hlen)) {
> >>>>>> +               if ((READ_ONCE(tun->vnet_hash.flags) & TUN_VNET_HASH_REPORT) &&
> >>>>>> +                   vnet_hdr_sz >= sizeof(hdr.v1_hash_hdr) &&
> >>>>>> +                   skb->tun_vnet_hash) {
> >>>>>
> >>>>> Isn't vnet_hdr_sz guaranteed to be >= hdr.v1_hash_hdr, by virtue of
> >>>>> the set hash ioctl failing otherwise?
> >>>>>
> >>>>> Such checks should be limited to control path where possible
> >>>>
> >>>> There is a potential race since tun->vnet_hash.flags and vnet_hdr_sz are
> >>>> not read at once.
> >>>
> >>> It should not be possible to downgrade the hdr_sz once v1 is selected.
> >>
> >> I see nothing that prevents shrinking the header size.
> >>
> >> tun->vnet_hash.flags is read after vnet_hdr_sz so the race can happen
> >> even for the case the header size grows though this can be fixed by
> >> reordering the two reads.
> >
> > One option is to fail any control path that tries to re-negotiate
> > header size once this hash option is enabled?
> >
> > There is no practical reason to allow feature re-negotiation at any
> > arbitrary time.
>
> I think it's a bit awkward interface design since tun allows to
> reconfigure any of its parameters, but it's certainly possible.

If this would be the only exception to that rule, and this is the only
place that needs a datapath check, then it's fine to leave as is.

In general, this runtime configurability serves little purpose but to
help syzbot exercise code paths no real application would attempt. But
I won't ask to diverge from whatever tun already does. We just have to
be more careful about the possible races it brings.