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    Subject[PATCH bpf-next v3 6/7] bpf, net: Support SO_REUSEPORT sockets with bpf_sk_assign
    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: 8e368dc72e86 ("bpf: Fix use of sk->sk_reuseport from sk_assign")
    Fixes: cf7fbe660f2d ("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 | 59 ++++++++++++++++++++++++++++++++++++++----
    include/net/inet_hashtables.h | 52 +++++++++++++++++++++++++++++++++++--
    include/net/sock.h | 7 +++--
    include/uapi/linux/bpf.h | 3 ---
    net/core/filter.c | 2 --
    net/ipv4/udp.c | 8 ++++--
    net/ipv6/udp.c | 10 ++++---
    tools/include/uapi/linux/bpf.h | 3 ---
    8 files changed, 122 insertions(+), 22 deletions(-)

    diff --git a/include/net/inet6_hashtables.h b/include/net/inet6_hashtables.h
    index 4d2a1a3c0be7..4d300af6ccb6 100644
    --- a/include/net/inet6_hashtables.h
    +++ b/include/net/inet6_hashtables.h
    @@ -103,6 +103,49 @@ static inline struct sock *__inet6_lookup(struct net *net,
    daddr, hnum, dif, sdif);
    }

    +static inline
    +struct sock *inet6_steal_sock(struct net *net, struct sk_buff *skb, int doff,
    + const struct in6_addr *saddr, const __be16 sport,
    + const struct in6_addr *daddr, const __be16 dport,
    + bool *refcounted, inet6_ehashfn_t ehashfn)
    +{
    + struct sock *sk, *reuse_sk;
    + bool prefetched;
    +
    + sk = skb_steal_sock(skb, refcounted, &prefetched);
    + if (!sk)
    + return NULL;
    +
    + if (!prefetched)
    + return sk;
    +
    + if (sk->sk_protocol == IPPROTO_TCP) {
    + if (sk->sk_state != TCP_LISTEN)
    + return sk;
    + } else if (sk->sk_protocol == IPPROTO_UDP) {
    + if (sk->sk_state != TCP_CLOSE)
    + return sk;
    + } else {
    + return sk;
    + }
    +
    + reuse_sk = inet6_lookup_reuseport(net, sk, skb, doff,
    + saddr, sport, daddr, ntohs(dport),
    + ehashfn);
    + if (!reuse_sk || reuse_sk == sk)
    + return sk;
    +
    + /* We've chosen a new reuseport sock which is never refcounted.
    + * sk might be refcounted however, drop the reference if necessary.
    + */
    + if (*refcounted) {
    + sock_put(sk);
    + *refcounted = false;
    + }
    +
    + return reuse_sk;
    +}
    +
    static inline struct sock *__inet6_lookup_skb(struct inet_hashinfo *hashinfo,
    struct sk_buff *skb, int doff,
    const __be16 sport,
    @@ -110,14 +153,20 @@ static inline struct sock *__inet6_lookup_skb(struct inet_hashinfo *hashinfo,
    int iif, int sdif,
    bool *refcounted)
    {
    - struct sock *sk = skb_steal_sock(skb, refcounted);
    -
    + struct net *net = dev_net(skb_dst(skb)->dev);
    + const struct ipv6hdr *ip6h = ipv6_hdr(skb);
    + struct sock *sk;
    +
    + sk = inet6_steal_sock(net, skb, doff, &ip6h->saddr, sport, &ip6h->daddr, dport,
    + refcounted, inet6_ehashfn);
    + if (IS_ERR(sk))
    + return NULL;
    if (sk)
    return sk;

    - return __inet6_lookup(dev_net(skb_dst(skb)->dev), hashinfo, skb,
    - doff, &ipv6_hdr(skb)->saddr, sport,
    - &ipv6_hdr(skb)->daddr, ntohs(dport),
    + return __inet6_lookup(net, hashinfo, skb,
    + doff, &ip6h->saddr, sport,
    + &ip6h->daddr, ntohs(dport),
    iif, sdif, refcounted);
    }

    diff --git a/include/net/inet_hashtables.h b/include/net/inet_hashtables.h
    index aa02f1db1f86..2c405d9df300 100644
    --- a/include/net/inet_hashtables.h
    +++ b/include/net/inet_hashtables.h
    @@ -449,6 +449,49 @@ static inline struct sock *inet_lookup(struct net *net,
    return sk;
    }

    +static inline
    +struct sock *inet_steal_sock(struct net *net, struct sk_buff *skb, int doff,
    + const __be32 saddr, const __be16 sport,
    + const __be32 daddr, const __be16 dport,
    + bool *refcounted, inet_ehashfn_t ehashfn)
    +{
    + struct sock *sk, *reuse_sk;
    + bool prefetched;
    +
    + sk = skb_steal_sock(skb, refcounted, &prefetched);
    + if (!sk)
    + return NULL;
    +
    + if (!prefetched)
    + return sk;
    +
    + if (sk->sk_protocol == IPPROTO_TCP) {
    + if (sk->sk_state != TCP_LISTEN)
    + return sk;
    + } else if (sk->sk_protocol == IPPROTO_UDP) {
    + if (sk->sk_state != TCP_CLOSE)
    + return sk;
    + } else {
    + return sk;
    + }
    +
    + reuse_sk = inet_lookup_reuseport(net, sk, skb, doff,
    + saddr, sport, daddr, ntohs(dport),
    + ehashfn);
    + if (!reuse_sk || reuse_sk == sk)
    + return sk;
    +
    + /* We've chosen a new reuseport sock which is never refcounted.
    + * sk might be refcounted however, drop the reference if necessary.
    + */
    + if (*refcounted) {
    + sock_put(sk);
    + *refcounted = false;
    + }
    +
    + return reuse_sk;
    +}
    +
    static inline struct sock *__inet_lookup_skb(struct inet_hashinfo *hashinfo,
    struct sk_buff *skb,
    int doff,
    @@ -457,13 +500,18 @@ static inline struct sock *__inet_lookup_skb(struct inet_hashinfo *hashinfo,
    const int sdif,
    bool *refcounted)
    {
    - struct sock *sk = skb_steal_sock(skb, refcounted);
    + struct net *net = dev_net(skb_dst(skb)->dev);
    const struct iphdr *iph = ip_hdr(skb);
    + struct sock *sk;

    + sk = inet_steal_sock(net, skb, doff, iph->saddr, sport, iph->daddr, dport,
    + refcounted, inet_ehashfn);
    + if (IS_ERR(sk))
    + return NULL;
    if (sk)
    return sk;

    - return __inet_lookup(dev_net(skb_dst(skb)->dev), hashinfo, skb,
    + return __inet_lookup(net, hashinfo, skb,
    doff, iph->saddr, sport,
    iph->daddr, dport, inet_iif(skb), sdif,
    refcounted);
    diff --git a/include/net/sock.h b/include/net/sock.h
    index 656ea89f60ff..5645570c2a64 100644
    --- a/include/net/sock.h
    +++ b/include/net/sock.h
    @@ -2806,20 +2806,23 @@ sk_is_refcounted(struct sock *sk)
    * skb_steal_sock - steal a socket from an sk_buff
    * @skb: sk_buff to steal the socket from
    * @refcounted: is set to true if the socket is reference-counted
    + * @prefetched: is set to true if the socket was assigned from bpf
    */
    static inline struct sock *
    -skb_steal_sock(struct sk_buff *skb, bool *refcounted)
    +skb_steal_sock(struct sk_buff *skb, bool *refcounted, bool *prefetched)
    {
    if (skb->sk) {
    struct sock *sk = skb->sk;

    *refcounted = true;
    - if (skb_sk_is_prefetched(skb))
    + *prefetched = skb_sk_is_prefetched(skb);
    + if (*prefetched)
    *refcounted = sk_is_refcounted(sk);
    skb->destructor = NULL;
    skb->sk = NULL;
    return sk;
    }
    + *prefetched = false;
    *refcounted = false;
    return NULL;
    }
    diff --git a/include/uapi/linux/bpf.h b/include/uapi/linux/bpf.h
    index a7b5e91dd768..d6fb6f43b0f3 100644
    --- a/include/uapi/linux/bpf.h
    +++ b/include/uapi/linux/bpf.h
    @@ -4158,9 +4158,6 @@ union bpf_attr {
    * **-EOPNOTSUPP** if the operation is not supported, for example
    * a call from outside of TC ingress.
    *
    - * **-ESOCKTNOSUPPORT** if the socket type is not supported
    - * (reuseport).
    - *
    * long bpf_sk_assign(struct bpf_sk_lookup *ctx, struct bpf_sock *sk, u64 flags)
    * Description
    * Helper is overloaded depending on BPF program type. This
    diff --git a/net/core/filter.c b/net/core/filter.c
    index 428df050d021..d4be0a1d754c 100644
    --- a/net/core/filter.c
    +++ b/net/core/filter.c
    @@ -7278,8 +7278,6 @@ BPF_CALL_3(bpf_sk_assign, struct sk_buff *, skb, struct sock *, sk, u64, flags)
    return -EOPNOTSUPP;
    if (unlikely(dev_net(skb->dev) != sock_net(sk)))
    return -ENETUNREACH;
    - if (unlikely(sk_fullsock(sk) && sk->sk_reuseport))
    - return -ESOCKTNOSUPPORT;
    if (sk_is_refcounted(sk) &&
    unlikely(!refcount_inc_not_zero(&sk->sk_refcnt)))
    return -ENOENT;
    diff --git a/net/ipv4/udp.c b/net/ipv4/udp.c
    index eb79268f216d..b256f1f73b4d 100644
    --- a/net/ipv4/udp.c
    +++ b/net/ipv4/udp.c
    @@ -2388,7 +2388,11 @@ int __udp4_lib_rcv(struct sk_buff *skb, struct udp_table *udptable,
    if (udp4_csum_init(skb, uh, proto))
    goto csum_error;

    - sk = skb_steal_sock(skb, &refcounted);
    + sk = inet_steal_sock(net, skb, sizeof(struct udphdr), saddr, uh->source, daddr, uh->dest,
    + &refcounted, udp_ehashfn);
    + if (IS_ERR(sk))
    + goto no_sk;
    +
    if (sk) {
    struct dst_entry *dst = skb_dst(skb);
    int ret;
    @@ -2409,7 +2413,7 @@ int __udp4_lib_rcv(struct sk_buff *skb, struct udp_table *udptable,
    sk = __udp4_lib_lookup_skb(skb, uh->source, uh->dest, udptable);
    if (sk)
    return udp_unicast_rcv_skb(sk, skb, uh);
    -
    +no_sk:
    if (!xfrm4_policy_check(NULL, XFRM_POLICY_IN, skb))
    goto drop;
    nf_reset_ct(skb);
    diff --git a/net/ipv6/udp.c b/net/ipv6/udp.c
    index 8a6d94cabee0..2d4c05bc322a 100644
    --- a/net/ipv6/udp.c
    +++ b/net/ipv6/udp.c
    @@ -923,9 +923,9 @@ int __udp6_lib_rcv(struct sk_buff *skb, struct udp_table *udptable,
    enum skb_drop_reason reason = SKB_DROP_REASON_NOT_SPECIFIED;
    const struct in6_addr *saddr, *daddr;
    struct net *net = dev_net(skb->dev);
    + bool refcounted;
    struct udphdr *uh;
    struct sock *sk;
    - bool refcounted;
    u32 ulen = 0;

    if (!pskb_may_pull(skb, sizeof(struct udphdr)))
    @@ -962,7 +962,11 @@ int __udp6_lib_rcv(struct sk_buff *skb, struct udp_table *udptable,
    goto csum_error;

    /* Check if the socket is already available, e.g. due to early demux */
    - sk = skb_steal_sock(skb, &refcounted);
    + sk = inet6_steal_sock(net, skb, sizeof(struct udphdr), saddr, uh->source, daddr, uh->dest,
    + &refcounted, udp6_ehashfn);
    + if (IS_ERR(sk))
    + goto no_sk;
    +
    if (sk) {
    struct dst_entry *dst = skb_dst(skb);
    int ret;
    @@ -996,7 +1000,7 @@ int __udp6_lib_rcv(struct sk_buff *skb, struct udp_table *udptable,
    goto report_csum_error;
    return udp6_unicast_rcv_skb(sk, skb, uh);
    }
    -
    +no_sk:
    reason = SKB_DROP_REASON_NO_SOCKET;

    if (!uh->check)
    diff --git a/tools/include/uapi/linux/bpf.h b/tools/include/uapi/linux/bpf.h
    index a7b5e91dd768..d6fb6f43b0f3 100644
    --- a/tools/include/uapi/linux/bpf.h
    +++ b/tools/include/uapi/linux/bpf.h
    @@ -4158,9 +4158,6 @@ union bpf_attr {
    * **-EOPNOTSUPP** if the operation is not supported, for example
    * a call from outside of TC ingress.
    *
    - * **-ESOCKTNOSUPPORT** if the socket type is not supported
    - * (reuseport).
    - *
    * long bpf_sk_assign(struct bpf_sk_lookup *ctx, struct bpf_sock *sk, u64 flags)
    * Description
    * Helper is overloaded depending on BPF program type. This
    --
    2.40.1

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