bpf: Add hash chain signature support for arbitrary maps

This patch introduces hash chain support for signature verification of
arbitrary bpf map objects which was described here:
https://lore.kernel.org/linux-security-module/[email protected]/

The UAPI is extended to allow for in-kernel checking of maps passed in
via the fd_array. A hash chain is constructed from the maps, in order
specified by the signature_maps field. The hash chain is terminated
with the hash of the program itself.

Tested-by: [email protected]
Signed-off-by: Blaise Boscaccy <[email protected]>

Author: Blaise Boscaccy

include/uapi/linux/bpf.h

@@ -1621,6 +1621,12 @@ union bpf_attr {
 		 * verification.
 		 */
 		__s32		keyring_id;
+		/* Pointer to a buffer containing the maps used in the signature
+		 * hash chain of the BPF program.
+		 */
+		__aligned_u64   signature_maps;
+		/* Size of the signature maps buffer. */
+		__u32		signature_maps_size;
 	};
 
 	struct { /* anonymous struct used by BPF_OBJ_* commands */

kernel/bpf/syscall.c

@@ -2802,14 +2802,35 @@ static bool is_perfmon_prog_type(enum bpf_prog_type prog_type)
 	}
 }
 
+static inline int bpf_map_get_hash(int map_fd, void *buffer)
+{
+	struct bpf_map *map;
+
+	CLASS(fd, f)(map_fd);
+	map = __bpf_map_get(f);
+	if (IS_ERR(map))
+		return PTR_ERR(map);
+
+	if (!map->ops->map_get_hash)
+		return -EINVAL;
+
+	return map->ops->map_get_hash(map, SHA256_DIGEST_SIZE, buffer);
+}
+
 static int bpf_prog_verify_signature(struct bpf_prog *prog, union bpf_attr *attr,
 				     bool is_kernel)
 {
 	bpfptr_t usig = make_bpfptr(attr->signature, is_kernel);
-	struct bpf_dynptr_kern sig_ptr, insns_ptr;
+	bpfptr_t umaps;
+	struct bpf_dynptr_kern sig_ptr, insns_ptr, hash_ptr;
 	struct bpf_key *key = NULL;
 	void *sig;
+	int *maps;
+	int map_fd;
 	int err = 0;
+	u64 buffer[SHA256_DIGEST_SIZE * 2 / sizeof(u64)];
+	u64 hash[SHA256_DIGEST_SIZE / sizeof(u64)];
+	int n;
 
 	if (system_keyring_id_check(attr->keyring_id) == 0)
 		key = bpf_lookup_system_key(attr->keyring_id);
@@ -2830,16 +2851,60 @@ static int bpf_prog_verify_signature(struct bpf_prog *prog, union bpf_attr *attr
 	bpf_dynptr_init(&insns_ptr, prog->insnsi, BPF_DYNPTR_TYPE_LOCAL, 0,
 			prog->len * sizeof(struct bpf_insn));
 
-	err = bpf_verify_pkcs7_signature((struct bpf_dynptr *)&insns_ptr,
-					 (struct bpf_dynptr *)&sig_ptr, key);
+	if (!attr->signature_maps_size) {
+		err = bpf_verify_pkcs7_signature((struct bpf_dynptr *)&insns_ptr,
+						 (struct bpf_dynptr *)&sig_ptr, key);
+	} else {
+		bpf_dynptr_init(&hash_ptr, hash, BPF_DYNPTR_TYPE_LOCAL, 0,
+				sizeof(hash));
+		umaps = make_bpfptr(attr->signature_maps, is_kernel);
+		maps = kvmemdup_bpfptr(umaps, attr->signature_maps_size * sizeof(*maps));
+		if (IS_ERR(maps)) {
+			err = PTR_ERR(maps);
+			goto out;
+		}
+		/* Process the map array in reverse order to generate a hash chain
+		 * h(n) = sha256(h(n + 1), sha256(map(n)))
+		 * h(n_len) = sha256(map(n_len))
+		 */
+		for (n = attr->signature_maps_size - 1; n >= 0; n--) {
+			err = copy_from_bpfptr_offset(&map_fd,
+						      make_bpfptr(attr->fd_array, is_kernel),
+						      maps[n] * sizeof(map_fd),
+						      sizeof(map_fd));
+			if (err)
+				goto free_maps;
+
+			if (n == attr->signature_maps_size - 1)
+				err = bpf_map_get_hash(map_fd, hash);
+			else {
+				memcpy(buffer, hash, sizeof(hash));
+				err = bpf_map_get_hash(map_fd, buffer + ARRAY_SIZE(hash));
+				sha256((u8 *)buffer, sizeof(buffer), (u8 *)&hash);
+			}
+			if (err)
+				goto free_maps;
+		}
+		/* Calculate final hash with program instructions
+		 * f_hash = sha256(sha256(prog), h(0))
+		 */
+		sha256((u8 *)prog->insnsi, prog->len * sizeof(struct bpf_insn), (u8 *)&buffer);
+		memcpy(buffer + ARRAY_SIZE(hash), hash, sizeof(hash));
+		sha256((u8 *)buffer, sizeof(buffer), (u8 *)&hash);
+		err = bpf_verify_pkcs7_signature((struct bpf_dynptr *)&hash_ptr,
+						 (struct bpf_dynptr *)&sig_ptr, key);
 
+free_maps:
+		kvfree(maps);
+	}
+out:
 	bpf_key_put(key);
 	kvfree(sig);
 	return err;
 }
 
 /* last field in 'union bpf_attr' used by this command */
-#define BPF_PROG_LOAD_LAST_FIELD keyring_id
+#define BPF_PROG_LOAD_LAST_FIELD signature_maps_size
 
 static int bpf_prog_load(union bpf_attr *attr, bpfptr_t uattr, u32 uattr_size)
 {

tools/include/uapi/linux/bpf.h

@@ -1621,6 +1621,12 @@ union bpf_attr {
 		 * verification.
 		 */
 		__s32		keyring_id;
+		/* Pointer to a buffer containing the maps used in the signature
+		 * hash chain of the BPF program.
+		 */
+		__aligned_u64   signature_maps;
+		/* Size of the signature maps buffer. */
+		__u32		signature_maps_size;
 	};
 
 	struct { /* anonymous struct used by BPF_OBJ_* commands */