bpf: Reject exclusive maps for bpf_map_elem iterators

Exclusive maps (aka excl_prog_hash) are meant to be reachable only
from the single program whose hash matches. This is enforced by
check_map_prog_compatibility() when the map is referenced from a
program such as signed BPF loaders.

A bpf_map_elem iterator, however, binds its target map at attach
time in bpf_iter_attach_map() instead of referencing it from the
program, so the exclusivity check is never reached. On top of that,
the iterator exposes the map value as a writable buffer.

Fixes: baefdbd ("bpf: Implement exclusive map creation")
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Link: https://lore.kernel.org/r/20260602133052.423725-2-daniel@iogearbox.net
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

Author: borkmann

kernel/bpf/map_iter.c

@@ -112,6 +112,10 @@ static int bpf_iter_attach_map(struct bpf_prog *prog,
 	map = bpf_map_get_with_uref(linfo->map.map_fd);
 	if (IS_ERR(map))
 		return PTR_ERR(map);
+	if (map->excl_prog_sha) {
+		err = -EPERM;
+		goto put_map;
+	}
 
 	if (map->map_type == BPF_MAP_TYPE_PERCPU_HASH ||
 	    map->map_type == BPF_MAP_TYPE_LRU_PERCPU_HASH ||