Re: [PATCH bpf-next v3 6/7] bpf, net: Support SO_REUSEPORT sockets with bpf_sk_assign

From: Lorenz Bauer lmb@isovalent.com Date: Mon, 26 Jun 2023 16:09:03 +0100

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+qycmpQzKupquhkxbvWK4OFyDuuLMBNROnfWMZxU...

---

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;
  

• }

As *refcounted should be false here (TCP_LISTEN and UDP sk have SOCK_RCU_FREE and other sk does not reach here), I prefer adding WARN_ON_ONCE() to catch a future bug:

WARN_ON_ONCE(*refcounted); sock_put(sk);

• 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