bpf: Reject exclusive maps as inner maps in map-in-map

An exclusive map (created with excl_prog_hash) is bound to a single
program by hash: check_map_prog_compatibility() refuses to load any
program whose digest does not match map->excl_prog_sha. That check
only runs for maps a program references directly, i.e. its used_maps.
A map reached at runtime through a map-of-maps is never in used_maps,
and bpf_map_meta_equal() does not consider excl_prog_sha, so an
exclusive map can be inserted into a non-exclusive outer map and
then looked up and mutated by an unrelated program, bypassing the
exclusivity guarantee.

For the signed loader this defeats the metadata map exclusivity check
added in the signed loader: the cached map->sha[] is validated against
the signed hash while another program on a hostile host rewrites the
frozen map's contents through the outer map.

Fixes: baefdbd ("bpf: Implement exclusive map creation")
Reported-by: sashiko <sashiko@sashiko.dev>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Link: https://lore.kernel.org/r/20260601150248.394863-2-daniel@iogearbox.net
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

Author: borkmann

kernel/bpf/map_in_map.c

@@ -20,7 +20,8 @@ struct bpf_map *bpf_map_meta_alloc(int inner_map_ufd)
 	/* Does not support >1 level map-in-map */
 	if (inner_map->inner_map_meta)
 		return ERR_PTR(-EINVAL);
-
+	if (inner_map->excl_prog_sha)
+		return ERR_PTR(-ENOTSUPP);
 	if (!inner_map->ops->map_meta_equal)
 		return ERR_PTR(-ENOTSUPP);
 
@@ -101,6 +102,8 @@ void *bpf_map_fd_get_ptr(struct bpf_map *map,
 	inner_map = __bpf_map_get(f);
 	if (IS_ERR(inner_map))
 		return inner_map;
+	if (inner_map->excl_prog_sha)
+		return ERR_PTR(-ENOTSUPP);
 
 	inner_map_meta = map->inner_map_meta;
 	if (inner_map_meta->ops->map_meta_equal(inner_map_meta, inner_map))