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Message-ID: <6a698c99-6f02-4cfb-a709-ba02296a05f7@daynix.com>
Date:   Mon, 9 Oct 2023 19:05:17 +0900
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Subject: Re: [RFC PATCH 5/7] tun: Introduce virtio-net hashing feature
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To:     Willem de Bruijn <willemdebruijn.kernel@gmail.com>
Cc:     Jason Wang <jasowang@redhat.com>,
        "Michael S. Tsirkin" <mst@redhat.com>, netdev@vger.kernel.org,
        linux-kernel@vger.kernel.org, kvm@vger.kernel.org,
        virtualization@lists.linux-foundation.org,
        linux-kselftest@vger.kernel.org, bpf@vger.kernel.org,
        davem@davemloft.net, kuba@kernel.org, ast@kernel.org,
        daniel@iogearbox.net, andrii@kernel.org, kafai@fb.com,
        songliubraving@fb.com, yhs@fb.com, john.fastabend@gmail.com,
        kpsingh@kernel.org, rdunlap@infradead.org, willemb@google.com,
        gustavoars@kernel.org, herbert@gondor.apana.org.au,
        steffen.klassert@secunet.com, nogikh@google.com,
        pablo@netfilter.org, decui@microsoft.com, cai@lca.pw,
        jakub@cloudflare.com, elver@google.com, pabeni@redhat.com,
        Yuri Benditovich <yuri.benditovich@daynix.com>
References: <20231008052101.144422-1-akihiko.odaki@daynix.com>
 <20231008052101.144422-6-akihiko.odaki@daynix.com>
 <CAF=yD-K2MQt4nnfwJrx6h6Nii_rho7j1o6nb_jYaSwcWY45pPw@mail.gmail.com>
 <48e20be1-b658-4117-8856-89ff1df6f48f@daynix.com>
 <CAF=yD-K4bCBpUVtDR_cv=bagRL+vM4Rusez+uHFTb4_kR8XkpA@mail.gmail.com>
From:   Akihiko Odaki <akihiko.odaki@daynix.com>
In-Reply-To: <CAF=yD-K4bCBpUVtDR_cv=bagRL+vM4Rusez+uHFTb4_kR8XkpA@mail.gmail.com>
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On 2023/10/09 18:54, Willem de Bruijn wrote:
> On Mon, Oct 9, 2023 at 3:44 AM Akihiko Odaki <akihiko.odaki@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@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@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.