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

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Date: Mon, 9 Oct 2023 19:05:17 +0900
From: Akihiko Odaki <akihiko.odaki@...nix.com>
To: Willem de Bruijn <willemdebruijn.kernel@...il.com>
Cc: Jason Wang <jasowang@...hat.com>, "Michael S. Tsirkin" <mst@...hat.com>,
 netdev@...r.kernel.org, linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
 virtualization@...ts.linux-foundation.org, linux-kselftest@...r.kernel.org,
 bpf@...r.kernel.org, davem@...emloft.net, kuba@...nel.org, ast@...nel.org,
 daniel@...earbox.net, andrii@...nel.org, kafai@...com,
 songliubraving@...com, yhs@...com, john.fastabend@...il.com,
 kpsingh@...nel.org, rdunlap@...radead.org, willemb@...gle.com,
 gustavoars@...nel.org, herbert@...dor.apana.org.au,
 steffen.klassert@...unet.com, nogikh@...gle.com, pablo@...filter.org,
 decui@...rosoft.com, cai@....pw, jakub@...udflare.com, elver@...gle.com,
 pabeni@...hat.com, Yuri Benditovich <yuri.benditovich@...nix.com>
Subject: Re: [RFC PATCH 5/7] tun: Introduce virtio-net hashing feature

On 2023/10/09 18:54, Willem de Bruijn wrote:
> On Mon, Oct 9, 2023 at 3:44 AM Akihiko Odaki <akihiko.odaki@...nix.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@...nix.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@...nix.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.