bpf: Fix bpf_sk_select_reuseport() memory leak

As pointed out in the original comment, lookup in sockmap can return a TCP
ESTABLISHED socket. Such TCP socket may have had SO_ATTACH_REUSEPORT_EBPF
set before it was ESTABLISHED. In other words, a non-NULL sk_reuseport_cb
does not imply a non-refcounted socket.

Drop sk's reference in both error paths.

unreferenced object 0xffff888101911800 (size 2048):
  comm "test_progs", pid 44109, jiffies 4297131437
  hex dump (first 32 bytes):
    00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00  ................
    80 00 01 00 00 00 00 00 00 00 00 00 00 00 00 00  ................
  backtrace (crc 9336483b):
    __kmalloc_noprof+0x3bf/0x560
    __reuseport_alloc+0x1d/0x40
    reuseport_alloc+0xca/0x150
    reuseport_attach_prog+0x87/0x140
    sk_reuseport_attach_bpf+0xc8/0x100
    sk_setsockopt+0x1181/0x1990
    do_sock_setsockopt+0x12b/0x160
    __sys_setsockopt+0x7b/0xc0
    __x64_sys_setsockopt+0x1b/0x30
    do_syscall_64+0x93/0x180
    entry_SYSCALL_64_after_hwframe+0x76/0x7e

Fixes: 64d8529 ("bpf: Allow bpf_map_lookup_elem for SOCKMAP and SOCKHASH")
Signed-off-by: Michal Luczaj <[email protected]>

net/core/filter.c

@@ -11251,42 +11251,48 @@ BPF_CALL_4(sk_select_reuseport, struct sk_reuseport_kern *, reuse_kern,
 	bool is_sockarray = map->map_type == BPF_MAP_TYPE_REUSEPORT_SOCKARRAY;
 	struct sock_reuseport *reuse;
 	struct sock *selected_sk;
+	int err;
 
 	selected_sk = map->ops->map_lookup_elem(map, key);
 	if (!selected_sk)
 		return -ENOENT;
 
 	reuse = rcu_dereference(selected_sk->sk_reuseport_cb);
 	if (!reuse) {
-		/* Lookup in sock_map can return TCP ESTABLISHED sockets. */
-		if (sk_is_refcounted(selected_sk))
-			sock_put(selected_sk);
-
 		/* reuseport_array has only sk with non NULL sk_reuseport_cb.
 		 * The only (!reuse) case here is - the sk has already been
 		 * unhashed (e.g. by close()), so treat it as -ENOENT.
 		 *
 		 * Other maps (e.g. sock_map) do not provide this guarantee and
 		 * the sk may never be in the reuseport group to begin with.
 		 */
-		return is_sockarray ? -ENOENT : -EINVAL;
+		err = is_sockarray ? -ENOENT : -EINVAL;
+		goto error;
 	}
 
 	if (unlikely(reuse->reuseport_id != reuse_kern->reuseport_id)) {
 		struct sock *sk = reuse_kern->sk;
 
-		if (sk->sk_protocol != selected_sk->sk_protocol)
-			return -EPROTOTYPE;
-		else if (sk->sk_family != selected_sk->sk_family)
-			return -EAFNOSUPPORT;
-
-		/* Catch all. Likely bound to a different sockaddr. */
-		return -EBADFD;
+		if (sk->sk_protocol != selected_sk->sk_protocol) {
+			err = -EPROTOTYPE;
+		} else if (sk->sk_family != selected_sk->sk_family) {
+			err = -EAFNOSUPPORT;
+		} else {
+			/* Catch all. Likely bound to a different sockaddr. */
+			err = -EBADFD;
+		}
+		goto error;
 	}
 
 	reuse_kern->selected_sk = selected_sk;
 
 	return 0;
+error:
+	/* Lookup in sock_map can return TCP ESTABLISHED sockets. */
+	if (sk_is_refcounted(selected_sk))
+		sock_put(selected_sk);
+
+	return err;
 }
 
 static const struct bpf_func_proto sk_select_reuseport_proto = {