dm-verity: add dm-verity keyring

Add a dedicated ".dm-verity" keyring for root hash signature
verification, similar to the ".fs-verity" keyring used by fs-verity.

By default the keyring is unused retaining the exact same old behavior.
For systems that provision additional keys only intended for dm-verity
images during boot, the dm_verity.keyring_unsealed=1 kernel parameter
leaves the keyring open.

We want to use this in systemd as a way add keys during boot that are
only used for creating dm-verity devices for later mounting and nothing
else. The discoverable disk image (DDI) spec at [1] heavily relies on
dm-verity and we would like to expand this even more. This will allow us
to do that in a fully backward compatible way.

Once provisioning is complete, userspace restricts and activates it for
dm-verity verification. If userspace fully seals the keyring then it
gains the guarantee that no new keys can be added.

Link: https://uapi-group.org/specifications/specs/discoverable_partitions_specification [1]
Co-developed-by: Aleksa Sarai <cyphar@cyphar.com>
Signed-off-by: Aleksa Sarai <cyphar@cyphar.com>
Signed-off-by: Christian Brauner <brauner@kernel.org>
Signed-off-by: Mikulas Patocka <mpatocka@redhat.com>

Documentation/admin-guide/kernel-parameters.txt

@@ -1370,6 +1370,13 @@ Kernel parameters
 			For details see:
 			Documentation/admin-guide/hw-vuln/reg-file-data-sampling.rst
 
+	dm_verity.keyring_unsealed=
+			[KNL] When set to 1, leave the dm-verity keyring
+			unsealed after initialization so userspace can
+			provision keys. Once the keyring is restricted
+			it becomes active and is searched during signature
+			verification.
+
 	driver_async_probe=  [KNL]
 			List of driver names to be probed asynchronously. *
 			matches with all driver names. If * is specified, the

drivers/md/dm-verity-target.c

@@ -1802,7 +1802,31 @@ static struct target_type verity_target = {
 	.preresume	= verity_preresume,
 #endif /* CONFIG_SECURITY */
 };
-module_dm(verity);
+
+static int __init dm_verity_init(void)
+{
+	int r;
+
+	r = dm_verity_verify_sig_init();
+	if (r)
+		return r;
+
+	r = dm_register_target(&verity_target);
+	if (r) {
+		dm_verity_verify_sig_exit();
+		return r;
+	}
+
+	return 0;
+}
+module_init(dm_verity_init);
+
+static void __exit dm_verity_exit(void)
+{
+	dm_unregister_target(&verity_target);
+	dm_verity_verify_sig_exit();
+}
+module_exit(dm_verity_exit);
 
 /*
  * Check whether a DM target is a verity target.

drivers/md/dm-verity-verify-sig.c

@@ -7,6 +7,7 @@
  */
 #include <linux/device-mapper.h>
 #include <linux/verification.h>
+#include <linux/key.h>
 #include <keys/user-type.h>
 #include <linux/module.h>
 #include "dm-verity.h"
@@ -14,6 +15,12 @@
 
 #define DM_VERITY_VERIFY_ERR(s) DM_VERITY_ROOT_HASH_VERIFICATION " " s
 
+static struct key *dm_verity_keyring;
+
+static bool dm_verity_keyring_unsealed __ro_after_init;
+module_param_named(keyring_unsealed, dm_verity_keyring_unsealed, bool, 0444);
+MODULE_PARM_DESC(keyring_unsealed, "Leave the dm-verity keyring unsealed");
+
 static bool require_signatures;
 module_param(require_signatures, bool, 0444);
 MODULE_PARM_DESC(require_signatures,
@@ -143,6 +150,17 @@ int verity_verify_root_hash(const void *root_hash, size_t root_hash_len,
 					VERIFYING_UNSPECIFIED_SIGNATURE, NULL, NULL);
 #endif
 
+	if (ret != -ENOKEY && ret != -EKEYREJECTED)
+		return ret;
+
+	if (dm_verity_keyring->keys.nr_leaves_on_tree &&
+	    dm_verity_keyring->restrict_link)
+		ret = verify_pkcs7_signature(root_hash, root_hash_len,
+					     sig_data, sig_len,
+					     dm_verity_keyring,
+					     VERIFYING_UNSPECIFIED_SIGNATURE,
+					     NULL, NULL);
+
 	return ret;
 }
 
@@ -152,3 +170,30 @@ void verity_verify_sig_opts_cleanup(struct dm_verity_sig_opts *sig_opts)
 	sig_opts->sig = NULL;
 	sig_opts->sig_size = 0;
 }
+
+int __init dm_verity_verify_sig_init(void)
+{
+	dm_verity_keyring = keyring_alloc(".dm-verity",
+					  GLOBAL_ROOT_UID, GLOBAL_ROOT_GID,
+					  current_cred(),
+					  KEY_POS_SEARCH |
+					  KEY_USR_VIEW | KEY_USR_READ |
+					  KEY_USR_WRITE | KEY_USR_SEARCH |
+					  KEY_USR_SETATTR,
+					  KEY_ALLOC_NOT_IN_QUOTA,
+					  NULL, NULL);
+	if (IS_ERR(dm_verity_keyring))
+		panic("dm-verity can't allocate keyring\n");
+
+	if (!dm_verity_keyring_unsealed &&
+	    keyring_restrict(make_key_ref(dm_verity_keyring, true), NULL, NULL))
+		panic("dm-verity can't seal keyring\n");
+
+	return 0;
+}
+
+void __exit dm_verity_verify_sig_exit(void)
+{
+	key_revoke(dm_verity_keyring);
+	key_put(dm_verity_keyring);
+}

drivers/md/dm-verity-verify-sig.h

@@ -30,6 +30,9 @@ int verity_verify_sig_parse_opt_args(struct dm_arg_set *as, struct dm_verity *v,
 
 void verity_verify_sig_opts_cleanup(struct dm_verity_sig_opts *sig_opts);
 
+int __init dm_verity_verify_sig_init(void);
+void __exit dm_verity_verify_sig_exit(void);
+
 #else
 
 #define DM_VERITY_ROOT_HASH_VERIFICATION_OPTS 0
@@ -56,5 +59,14 @@ static inline void verity_verify_sig_opts_cleanup(struct dm_verity_sig_opts *sig
 {
 }
 
+static inline int dm_verity_verify_sig_init(void)
+{
+	return 0;
+}
+
+static inline void dm_verity_verify_sig_exit(void)
+{
+}
+
 #endif /* CONFIG_DM_VERITY_VERIFY_ROOTHASH_SIG */
 #endif /* DM_VERITY_SIG_VERIFICATION_H */