bpf: Acquire map uref in .init_seq_private for sock{map,hash} iterator

sock_map_iter_attach_target() acquires a map uref, and the uref may be
released before or in the middle of iterating map elements. For example,
the uref could be released in sock_map_iter_detach_target() as part of
bpf_link_release(), or could be released in bpf_map_put_with_uref() as
part of bpf_map_release().

Fixing it by acquiring an extra map uref in .init_seq_private and
releasing it in .fini_seq_private.

Fixes: 0365351524d7 ("net: Allow iterating sockmap and sockhash")
Signed-off-by: Hou Tao <[e-mail]>
Acked-by: Yonghong Song <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Alexei Starovoitov <[e-mail]>

net/core/sock_map.c

···
 {
 	struct sock_map_seq_info *info = priv_data;
 
+	bpf_map_inc_with_uref(aux->map);
 	info->map = aux->map;
 	return 0;
+}
+
+static void sock_map_fini_seq_private(void *priv_data)
+{
+	struct sock_map_seq_info *info = priv_data;
+
+	bpf_map_put_with_uref(info->map);
 }
 
 static const struct bpf_iter_seq_info sock_map_iter_seq_info = {
 	.seq_ops		= &sock_map_seq_ops,
 	.init_seq_private	= sock_map_init_seq_private,
+	.fini_seq_private	= sock_map_fini_seq_private,
 	.seq_priv_size		= sizeof(struct sock_map_seq_info),
 };
 
···
 };
 
 static int sock_hash_init_seq_private(void *priv_data,
-				     struct bpf_iter_aux_info *aux)
+				      struct bpf_iter_aux_info *aux)
 {
 	struct sock_hash_seq_info *info = priv_data;
 
+	bpf_map_inc_with_uref(aux->map);
 	info->map = aux->map;
 	info->htab = container_of(aux->map, struct bpf_shtab, map);
 	return 0;
 }
 
+static void sock_hash_fini_seq_private(void *priv_data)
+{
+	struct sock_hash_seq_info *info = priv_data;
+
+	bpf_map_put_with_uref(info->map);
+}
+
 static const struct bpf_iter_seq_info sock_hash_iter_seq_info = {
 	.seq_ops		= &sock_hash_seq_ops,
 	.init_seq_private	= sock_hash_init_seq_private,
+	.fini_seq_private	= sock_hash_fini_seq_private,
 	.seq_priv_size		= sizeof(struct sock_hash_seq_info),
 };