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From:   Eric Dumazet <edumazet@google.com>
Date:   Thu, 25 May 2023 15:24:20 +0200
Message-ID: <CANn89iJx74gR7Xuahd0S3pLXYC8EX6+JRkbt6T_bemMX-8zyig@mail.gmail.com>
Subject: Re: [PATCH bpf-next 1/2] bpf, net: Support SO_REUSEPORT sockets with bpf_sk_assign
To:     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>,
        Daniel Borkmann <daniel@iogearbox.net>,
        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
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On Thu, May 25, 2023 at 10:19=E2=80=AFAM Lorenz Bauer <lmb@isovalent.com> w=
rote:
>
> 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, wh=
ich
> configures SO_REUSEPORT on its sockets. In turn, we're blocked from remov=
ing
> TPROXY from our setup.
>
> The reason that bpf_sk_assign refuses such sockets is that the bpf_sk_loo=
kup
> 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 =3D 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+qycmpQzKupquhkxbvWK4OFyDuuLMBN=
ROnfWMZxUWeA@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, st=
ruct 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 =3D NULL;
>         u32 phash;
> @@ -346,6 +346,7 @@ static inline struct sock *lookup_reuseport(struct ne=
t *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 *ne=
t,
>         sk_nulls_for_each_rcu(sk, node, &ilb2->nulls_head) {
>                 score =3D compute_score(sk, net, hnum, daddr, dif, sdif);
>                 if (score > hiscore) {
> -                       result =3D lookup_reuseport(net, sk, skb, doff,
> -                                                 saddr, sport, daddr, hn=
um);
> +                       result =3D inet_lookup_reuseport(net, sk, skb, do=
ff,
> +                                                      saddr, sport, dadd=
r, 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.