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Subject: Re: [PATCH bpf-next 1/2] bpf, net: Support SO_REUSEPORT sockets with
 bpf_sk_assign
To:     Eric Dumazet <edumazet@google.com>,
        Lorenz Bauer <lmb@isovalent.com>
Cc:     "David S. Miller" <davem@davemloft.net>,
        Jakub Kicinski <kuba@kernel.org>,
        Paolo Abeni <pabeni@redhat.com>,
        Alexei Starovoitov <ast@kernel.org>,
        Andrii Nakryiko <andrii@kernel.org>,
        Martin KaFai Lau <martin.lau@linux.dev>,
        Song Liu <song@kernel.org>, Yonghong Song <yhs@fb.com>,
        John Fastabend <john.fastabend@gmail.com>,
        KP Singh <kpsingh@kernel.org>,
        Stanislav Fomichev <sdf@google.com>,
        Hao Luo <haoluo@google.com>, Jiri Olsa <jolsa@kernel.org>,
        David Ahern <dsahern@kernel.org>,
        Willem de Bruijn <willemdebruijn.kernel@gmail.com>,
        Joe Stringer <joe@wand.net.nz>, Joe Stringer <joe@cilium.io>,
        Martin KaFai Lau <kafai@fb.com>, netdev@vger.kernel.org,
        linux-kernel@vger.kernel.org, bpf@vger.kernel.org
References: <20230525081923.8596-1-lmb@isovalent.com>
 <CANn89iJx74gR7Xuahd0S3pLXYC8EX6+JRkbt6T_bemMX-8zyig@mail.gmail.com>
From:   Daniel Borkmann <daniel@iogearbox.net>
Message-ID: <a71b8941-1ffc-37fc-6676-d3b4cf44f149@iogearbox.net>
Date:   Thu, 25 May 2023 21:51:14 +0200
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On 5/25/23 3:24 PM, Eric Dumazet wrote:
> On Thu, May 25, 2023 at 10:19 AM Lorenz Bauer <lmb@isovalent.com> wrote:
>>
>> Currently the bpf_sk_assign helper in tc BPF context refuses SO_REUSEPORT
>> sockets. This means we can't use the helper to steer traffic to Envoy, which
>> configures SO_REUSEPORT on its sockets. In turn, we're blocked from removing
>> TPROXY from our setup.
>>
>> The reason that bpf_sk_assign refuses such sockets is that the bpf_sk_lookup
>> helpers don't execute SK_REUSEPORT programs. Instead, one of the
>> reuseport sockets is selected by hash. This could cause dispatch to the
>> "wrong" socket:
>>
>>      sk = bpf_sk_lookup_tcp(...) // select SO_REUSEPORT by hash
>>      bpf_sk_assign(skb, sk) // SK_REUSEPORT wasn't executed
>>
>> Fixing this isn't as simple as invoking SK_REUSEPORT from the lookup
>> helpers unfortunately. In the tc context, L2 headers are at the start
>> of the skb, while SK_REUSEPORT expects L3 headers instead.
>>
>> Instead, we execute the SK_REUSEPORT program when the assigned socket
>> is pulled out of the skb, further up the stack. This creates some
>> trickiness with regards to refcounting as bpf_sk_assign will put both
>> refcounted and RCU freed sockets in skb->sk. reuseport sockets are RCU
>> freed. We can infer that the sk_assigned socket is RCU freed if the
>> reuseport lookup succeeds, but convincing yourself of this fact isn't
>> straight forward. Therefore we defensively check refcounting on the
>> sk_assign sock even though it's probably not required in practice.
>>
>> Fixes: 8e368dc ("bpf: Fix use of sk->sk_reuseport from sk_assign")
>> Fixes: cf7fbe6 ("bpf: Add socket assign support")
>> Co-developed-by: Daniel Borkmann <daniel@iogearbox.net>
>> Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
>> Signed-off-by: Lorenz Bauer <lmb@isovalent.com>
>> Cc: Joe Stringer <joe@cilium.io>
>> Link: https://lore.kernel.org/bpf/CACAyw98+qycmpQzKupquhkxbvWK4OFyDuuLMBNROnfWMZxUWeA@mail.gmail.com/
>> ---
>>   include/net/inet6_hashtables.h | 36 +++++++++++++++++++++++++++++-----
>>   include/net/inet_hashtables.h  | 27 +++++++++++++++++++++++--
>>   include/net/sock.h             |  7 +++++--
>>   include/uapi/linux/bpf.h       |  3 ---
>>   net/core/filter.c              |  2 --
>>   net/ipv4/inet_hashtables.c     | 15 +++++++-------
>>   net/ipv4/udp.c                 | 23 +++++++++++++++++++---
>>   net/ipv6/inet6_hashtables.c    | 19 +++++++++---------
>>   net/ipv6/udp.c                 | 23 +++++++++++++++++++---
>>   tools/include/uapi/linux/bpf.h |  3 ---
>>   10 files changed, 119 insertions(+), 39 deletions(-)
> 
> 
>> diff --git a/net/ipv4/inet_hashtables.c b/net/ipv4/inet_hashtables.c
>> index e7391bf310a7..920131e4a65d 100644
>> --- a/net/ipv4/inet_hashtables.c
>> +++ b/net/ipv4/inet_hashtables.c
>> @@ -332,10 +332,10 @@ static inline int compute_score(struct sock *sk, struct net *net,
>>          return score;
>>   }
>>
>> -static inline struct sock *lookup_reuseport(struct net *net, struct sock *sk,
>> -                                           struct sk_buff *skb, int doff,
>> -                                           __be32 saddr, __be16 sport,
>> -                                           __be32 daddr, unsigned short hnum)
>> +struct sock *inet_lookup_reuseport(struct net *net, struct sock *sk,
>> +                                  struct sk_buff *skb, int doff,
>> +                                  __be32 saddr, __be16 sport,
>> +                                  __be32 daddr, unsigned short hnum)
>>   {
>>          struct sock *reuse_sk = NULL;
>>          u32 phash;
>> @@ -346,6 +346,7 @@ static inline struct sock *lookup_reuseport(struct net *net, struct sock *sk,
>>          }
>>          return reuse_sk;
>>   }
>> +EXPORT_SYMBOL_GPL(inet_lookup_reuseport);
>>
>>   /*
>>    * Here are some nice properties to exploit here. The BSD API
>> @@ -369,8 +370,8 @@ static struct sock *inet_lhash2_lookup(struct net *net,
>>          sk_nulls_for_each_rcu(sk, node, &ilb2->nulls_head) {
>>                  score = compute_score(sk, net, hnum, daddr, dif, sdif);
>>                  if (score > hiscore) {
>> -                       result = lookup_reuseport(net, sk, skb, doff,
>> -                                                 saddr, sport, daddr, hnum);
>> +                       result = inet_lookup_reuseport(net, sk, skb, doff,
>> +                                                      saddr, sport, daddr, hnum);
>>                          if (result)
>>                                  return result;
>>
> 
> Please split in a series.
> 
> First a patch renaming lookup_reuseport() to inet_lookup_reuseport()
> and inet6_lookup_reuseport()
> (cleanup, no change in behavior)
> 
> This would ease review and future bug hunting quite a bit.

Makes sense and should reduce the churn on the actual change.

I think Lorenz is planning to flush out a v2 next week with this split.

Thanks,
Daniel