Willem de Bruijn
2023-Oct-09 10:32 UTC
[RFC PATCH 5/7] tun: Introduce virtio-net hashing feature
On Mon, Oct 9, 2023 at 3:11?AM Akihiko Odaki <akihiko.odaki at daynix.com> wrote:> > On 2023/10/09 19:07, Willem de Bruijn wrote: > > On Mon, Oct 9, 2023 at 3:05?AM Akihiko Odaki <akihiko.odaki at daynix.com> wrote: > >> > >> > >> > >> On 2023/10/09 18:54, Willem de Bruijn wrote: > >>> On Mon, Oct 9, 2023 at 3:44?AM Akihiko Odaki <akihiko.odaki at daynix.com> wrote: > >>>> > >>>> On 2023/10/09 17:13, Willem de Bruijn wrote: > >>>>> On Sun, Oct 8, 2023 at 12:22?AM Akihiko Odaki <akihiko.odaki at daynix.com> 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 at daynix.com> > >>>>> > >>>>>> @@ -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.