libbpf: Reject non-exclusive metadata maps in the signed loader

The loader verifies map->sha against the metadata hash in its
instructions. map->sha is calculated when BPF_OBJ_GET_INFO_BY_FD is
called on the frozen map.

While the map is frozen, the /signed loader/ must also ensure the map
is exclusive, as, without exclusivity (which a hostile host could just
omit when loading the loader), another BPF program with map access can
mutate the contents afterwards, so the check passes on stale data.

With the extra check as part of the signed loader, it now refuses to
move on with map->sha validation if the host set it up wrongly.

Fixes: fb2b0e2 ("libbpf: Update light skeleton for signing")
Signed-off-by: KP Singh <kpsingh@kernel.org>
Co-developed-by: Daniel Borkmann <daniel@iogearbox.net>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Link: https://lore.kernel.org/r/20260601150248.394863-4-daniel@iogearbox.net
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

Author: sinkap

include/linux/bpf.h

@@ -296,6 +296,7 @@ struct bpf_map_owner {
 
 struct bpf_map {
 	u8 sha[SHA256_DIGEST_SIZE];
+	u32 excl;
 	const struct bpf_map_ops *ops;
 	struct bpf_map *inner_map_meta;
 #ifdef CONFIG_SECURITY

kernel/bpf/syscall.c

@@ -1588,6 +1588,13 @@ static int map_create_alloc(union bpf_attr *attr, bpfptr_t uattr, struct bpf_ver
 			err = -EFAULT;
 			goto free_map;
 		}
+
+		/* See libbpf: emit_signature_match() */
+		BUILD_BUG_ON(offsetof(struct bpf_map, excl) != SHA256_DIGEST_SIZE);
+		BUILD_BUG_ON(!__same_type(map->excl, u32));
+		BUILD_BUG_ON(offsetof(struct bpf_map, sha)  != 0);
+		BUILD_BUG_ON(!__same_type(map->sha, u8[SHA256_DIGEST_SIZE]));
+		map->excl = 1;
 	} else if (attr->excl_prog_hash_size) {
 		bpf_log(log, "Invalid excl_prog_hash_size.\n");
 		err = -EINVAL;

tools/lib/bpf/gen_loader.c

@@ -586,6 +586,23 @@ static void emit_signature_match(struct bpf_gen *gen)
 	__s64 off;
 	int i;
 
+	/*
+	 * Reject if the metadata map is not exclusive. Without exclusivity
+	 * the cached map->sha[] verified above can be stale: another BPF
+	 * program with map access could have mutated the contents between
+	 * BPF_OBJ_GET_INFO_BY_FD and loader execution.
+	 */
+	emit2(gen, BPF_LD_IMM64_RAW_FULL(BPF_REG_1, BPF_PSEUDO_MAP_IDX,
+					 0, 0, 0, 0));
+	emit(gen, BPF_LDX_MEM(BPF_W, BPF_REG_2, BPF_REG_1, SHA256_DIGEST_LENGTH));
+	off = -(gen->insn_cur - gen->insn_start - gen->cleanup_label) / 8 - 2;
+	if (is_simm16(off)) {
+		emit(gen, BPF_MOV64_IMM(BPF_REG_7, -EINVAL));
+		emit(gen, BPF_JMP_IMM(BPF_JNE, BPF_REG_2, 1, off));
+	} else {
+		gen->error = -ERANGE;
+	}
+
 	for (i = 0; i < SHA256_DWORD_SIZE; i++) {
 		emit2(gen, BPF_LD_IMM64_RAW_FULL(BPF_REG_1, BPF_PSEUDO_MAP_IDX,
 						 0, 0, 0, 0));