UDP sockets get SOCK_RCU_FREE set when (auto-)bound. This means
sk_is_refcounted(unbound) = true, while sk_is_refcounted(bound) = false.

Because sockmap accepts unbound UDP sockets, a BPF program can increment a
socket's refcount via lookup. If the socket is subsequently bound, the
transition from unbound to bound causes bpf_sk_release() to skip the
decrement of the refcount, causing a memory leak.

unreferenced object 0xffff88810bc2eb40 (size 1984):
  comm "test_progs", pid 2451, jiffies 4295320596
  hex dump (first 32 bytes):
    7f 00 00 01 7f 00 00 01 d2 04 1b b7 04 d2 00 00  ................
    02 00 01 40 00 00 00 00 00 00 00 00 00 00 00 00  ...@............
  backtrace (crc bdee079d):
    kmem_cache_alloc_noprof+0x557/0x660
    sk_prot_alloc+0x69/0x240
    sk_alloc+0x30/0x460
    inet_create+0x2ce/0xf80
    __sock_create+0x25b/0x5c0
    __sys_socket+0x119/0x1d0
    __x64_sys_socket+0x72/0xd0
    do_syscall_64+0xa1/0x5f0
    entry_SYSCALL_64_after_hwframe+0x76/0x7e

Maintain balanced refcounts across sk lookup/release: (re-)set
SOCK_RCU_FREE on proto update to treat the socket (whether bound or
unbound) as not requiring a refcount increment on (a RCU protected) lookup.

Fixes: 0c48eefae712 ("sock_map: Lift socket state restriction for datagram sockets")
Signed-off-by: Michal Luczaj <redacted>
---
Note: this issue is related to commit 67312adc96b5 ("bpf: reject unhashed
sockets in bpf_sk_assign").
---
 net/ipv4/udp_bpf.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/net/ipv4/udp_bpf.c b/net/ipv4/udp_bpf.c
index ad57c4c9eaab..970327b59582 100644
--- a/net/ipv4/udp_bpf.c
+++ b/net/ipv4/udp_bpf.c

@@ -173,6 +173,9 @@ int udp_bpf_update_proto(struct sock *sk, struct sk_psock *psock, bool restore)
 	if (sk->sk_family == AF_INET6)
 		udp_bpf_check_v6_needs_rebuild(psock->sk_proto);
 
+	/* Treat all sockets as non-refcounted, regardless of binding state. */
+	sock_set_flag(sk, SOCK_RCU_FREE);
+
 	sock_replace_proto(sk, &udp_bpf_prots[family]);
 	return 0;
 }

-- 
2.54.0