KEYS: Add a 'trusted' flag and a 'trusted only' flag

Add KEY_FLAG_TRUSTED to indicate that a key either comes from a trusted source
or had a cryptographic signature chain that led back to a trusted key the
kernel already possessed.

Add KEY_FLAGS_TRUSTED_ONLY to indicate that a keyring will only accept links to
keys marked with KEY_FLAGS_TRUSTED.
Signed-off-by: David Howells <dhowells@redhat.com>
Reviewed-by: Kees Cook <keescook@chromium.org>

include/linux/key-type.h

@@ -45,6 +45,7 @@ struct key_preparsed_payload {
 	const void	*data;		/* Raw data */
 	size_t		datalen;	/* Raw datalen */
 	size_t		quotalen;	/* Quota length for proposed payload */
+	bool		trusted;	/* True if key is trusted */
 };
 
 typedef int (*request_key_actor_t)(struct key_construction *key,

include/linux/key.h

@@ -168,6 +168,8 @@ struct key {
 #define KEY_FLAG_NEGATIVE	5	/* set if key is negative */
 #define KEY_FLAG_ROOT_CAN_CLEAR	6	/* set if key can be cleared by root without permission */
 #define KEY_FLAG_INVALIDATED	7	/* set if key has been invalidated */
+#define KEY_FLAG_TRUSTED	8	/* set if key is trusted */
+#define KEY_FLAG_TRUSTED_ONLY	9	/* set if keyring only accepts links to trusted keys */
 
 	/* the key type and key description string
 	 * - the desc is used to match a key against search criteria
@@ -218,6 +220,7 @@ extern struct key *key_alloc(struct key_type *type,
 #define KEY_ALLOC_IN_QUOTA	0x0000	/* add to quota, reject if would overrun */
 #define KEY_ALLOC_QUOTA_OVERRUN	0x0001	/* add to quota, permit even if overrun */
 #define KEY_ALLOC_NOT_IN_QUOTA	0x0002	/* not in quota */
+#define KEY_ALLOC_TRUSTED	0x0004	/* Key should be flagged as trusted */
 
 extern void key_revoke(struct key *key);
 extern void key_invalidate(struct key *key);

kernel/system_keyring.c

@@ -40,6 +40,7 @@ static __init int system_trusted_keyring_init(void)
 	if (IS_ERR(system_trusted_keyring))
 		panic("Can't allocate system trusted keyring\n");
 
+	set_bit(KEY_FLAG_TRUSTED_ONLY, &system_trusted_keyring->flags);
 	return 0;
 }
 
@@ -82,7 +83,8 @@ static __init int load_system_certificate_list(void)
 					   plen,
 					   (KEY_POS_ALL & ~KEY_POS_SETATTR) |
 					   KEY_USR_VIEW,
-					   KEY_ALLOC_NOT_IN_QUOTA);
+					   KEY_ALLOC_NOT_IN_QUOTA |
+					   KEY_ALLOC_TRUSTED);
 		if (IS_ERR(key)) {
 			pr_err("Problem loading in-kernel X.509 certificate (%ld)\n",
 			       PTR_ERR(key));

security/keys/key.c

@@ -300,6 +300,8 @@ struct key *key_alloc(struct key_type *type, const char *desc,
 
 	if (!(flags & KEY_ALLOC_NOT_IN_QUOTA))
 		key->flags |= 1 << KEY_FLAG_IN_QUOTA;
+	if (flags & KEY_ALLOC_TRUSTED)
+		key->flags |= 1 << KEY_FLAG_TRUSTED;
 
 	memset(&key->type_data, 0, sizeof(key->type_data));
 
@@ -813,6 +815,7 @@ key_ref_t key_create_or_update(key_ref_t keyring_ref,
 	prep.data = payload;
 	prep.datalen = plen;
 	prep.quotalen = index_key.type->def_datalen;
+	prep.trusted = flags & KEY_ALLOC_TRUSTED;
 	if (index_key.type->preparse) {
 		ret = index_key.type->preparse(&prep);
 		if (ret < 0) {
@@ -827,6 +830,11 @@ key_ref_t key_create_or_update(key_ref_t keyring_ref,
 	}
 	index_key.desc_len = strlen(index_key.description);
 
+	key_ref = ERR_PTR(-EPERM);
+	if (!prep.trusted && test_bit(KEY_FLAG_TRUSTED_ONLY, &keyring->flags))
+		goto error_free_prep;
+	flags |= prep.trusted ? KEY_ALLOC_TRUSTED : 0;
+
 	ret = __key_link_begin(keyring, &index_key, &edit);
 	if (ret < 0) {
 		key_ref = ERR_PTR(ret);

security/keys/keyring.c

@@ -1183,6 +1183,10 @@ int key_link(struct key *keyring, struct key *key)
 	key_check(keyring);
 	key_check(key);
 
+	if (test_bit(KEY_FLAG_TRUSTED_ONLY, &keyring->flags) &&
+	    !test_bit(KEY_FLAG_TRUSTED, &key->flags))
+		return -EPERM;
+
 	ret = __key_link_begin(keyring, &key->index_key, &edit);
 	if (ret == 0) {
 		kdebug("begun {%d,%d}", keyring->serial, atomic_read(&keyring->usage));