CVE-2026-74364: bpf: Reject exclusive maps as inner maps in map-in-map

In the Linux kernel, the following vulnerability has been resolved:

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

An exclusive map (created with exclproghash) is bound to a single program by hash: checkmapprogcompatibility() refuses to load any program whose digest does not match map->exclprogsha. That check only runs for maps a program references directly, i.e. its usedmaps. A map reached at runtime through a map-of-maps is never in usedmaps, and bpfmapmetaequal() does not consider exclprogsha, 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.