5.15-stable review patch. If anyone has any objections, please let me know.
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From: Lorenz Bauer lmb@isovalent.com
[ Upstream commit 67312adc96b5a585970d03b62412847afe2c6b01 ]
The semantics for bpf_sk_assign are as follows:
sk = some_lookup_func() bpf_sk_assign(skb, sk) bpf_sk_release(sk)
That is, the sk is not consumed by bpf_sk_assign. The function therefore needs to make sure that sk lives long enough to be consumed from __inet_lookup_skb. The path through the stack for a TCPv4 packet is roughly:
netif_receive_skb_core: takes RCU read lock __netif_receive_skb_core: sch_handle_ingress: tcf_classify: bpf_sk_assign() deliver_ptype_list_skb: deliver_skb: ip_packet_type->func == ip_rcv: ip_rcv_core: ip_rcv_finish_core: dst_input: ip_local_deliver: ip_local_deliver_finish: ip_protocol_deliver_rcu: tcp_v4_rcv: __inet_lookup_skb: skb_steal_sock
The existing helper takes advantage of the fact that everything happens in the same RCU critical section: for sockets with SOCK_RCU_FREE set bpf_sk_assign never takes a reference. skb_steal_sock then checks SOCK_RCU_FREE again and does sock_put if necessary.
This approach assumes that SOCK_RCU_FREE is never set on a sk between bpf_sk_assign and skb_steal_sock, but this invariant is violated by unhashed UDP sockets. A new UDP socket is created in TCP_CLOSE state but without SOCK_RCU_FREE set. That flag is only added in udp_lib_get_port() which happens when a socket is bound.
When bpf_sk_assign was added it wasn't possible to access unhashed UDP sockets from BPF, so this wasn't a problem. This changed in commit 0c48eefae712 ("sock_map: Lift socket state restriction for datagram sockets"), but the helper wasn't adjusted accordingly. The following sequence of events will therefore lead to a refcount leak:
1. Add socket(AF_INET, SOCK_DGRAM) to a sockmap. 2. Pull socket out of sockmap and bpf_sk_assign it. Since SOCK_RCU_FREE is not set we increment the refcount. 3. bind() or connect() the socket, setting SOCK_RCU_FREE. 4. skb_steal_sock will now set refcounted = false due to SOCK_RCU_FREE. 5. tcp_v4_rcv() skips sock_put().
Fix the problem by rejecting unhashed sockets in bpf_sk_assign(). This matches the behaviour of __inet_lookup_skb which is ultimately the goal of bpf_sk_assign().
Fixes: cf7fbe660f2d ("bpf: Add socket assign support") Cc: Joe Stringer joe@cilium.io Signed-off-by: Lorenz Bauer lmb@isovalent.com Reviewed-by: Kuniyuki Iwashima kuniyu@amazon.com Link: https://lore.kernel.org/r/20230720-so-reuseport-v6-2-7021b683cdae@isovalent.... Signed-off-by: Martin KaFai Lau martin.lau@kernel.org Signed-off-by: Sasha Levin sashal@kernel.org --- net/core/filter.c | 2 ++ 1 file changed, 2 insertions(+)
diff --git a/net/core/filter.c b/net/core/filter.c index 458756334c4f9..76432aa3b717c 100644 --- a/net/core/filter.c +++ b/net/core/filter.c @@ -6959,6 +6959,8 @@ BPF_CALL_3(bpf_sk_assign, struct sk_buff *, skb, struct sock *, sk, u64, flags) return -ENETUNREACH; if (unlikely(sk_fullsock(sk) && sk->sk_reuseport)) return -ESOCKTNOSUPPORT; + if (sk_unhashed(sk)) + return -EOPNOTSUPP; if (sk_is_refcounted(sk) && unlikely(!refcount_inc_not_zero(&sk->sk_refcnt))) return -ENOENT;