KEYS: Add a facility to restrict new links into a keyring

Add a facility whereby proposed new links to be added to a keyring can be
vetted, permitting them to be rejected if necessary.  This can be used to
block public keys from which the signature cannot be verified or for which
the signature verification fails.  It could also be used to provide
blacklisting.

This affects operations like add_key(), KEYCTL_LINK and KEYCTL_INSTANTIATE.

To this end:

 (1) A function pointer is added to the key struct that, if set, points to
     the vetting function.  This is called as:

	int (*restrict_link)(struct key *keyring,
			     const struct key_type *key_type,
			     unsigned long key_flags,
			     const union key_payload *key_payload),

     where 'keyring' will be the keyring being added to, key_type and
     key_payload will describe the key being added and key_flags[*] can be
     AND'ed with KEY_FLAG_TRUSTED.

     [*] This parameter will be removed in a later patch when
     	 KEY_FLAG_TRUSTED is removed.

     The function should return 0 to allow the link to take place or an
     error (typically -ENOKEY, -ENOPKG or -EKEYREJECTED) to reject the
     link.

     The pointer should not be set directly, but rather should be set
     through keyring_alloc().

     Note that if called during add_key(), preparse is called before this
     method, but a key isn't actually allocated until after this function
     is called.

 (2) KEY_ALLOC_BYPASS_RESTRICTION is added.  This can be passed to
     key_create_or_update() or key_instantiate_and_link() to bypass the
     restriction check.

 (3) KEY_FLAG_TRUSTED_ONLY is removed.  The entire contents of a keyring
     with this restriction emplaced can be considered 'trustworthy' by
     virtue of being in the keyring when that keyring is consulted.

 (4) key_alloc() and keyring_alloc() take an extra argument that will be
     used to set restrict_link in the new key.  This ensures that the
     pointer is set before the key is published, thus preventing a window
     of unrestrictedness.  Normally this argument will be NULL.

 (5) As a temporary affair, keyring_restrict_trusted_only() is added.  It
     should be passed to keyring_alloc() as the extra argument instead of
     setting KEY_FLAG_TRUSTED_ONLY on a keyring.  This will be replaced in
     a later patch with functions that look in the appropriate places for
     authoritative keys.

Signed-off-by: David Howells <dhowells@redhat.com>
Reviewed-by: Mimi Zohar <zohar@linux.vnet.ibm.com>

1 files changed, 66 insertions, 7 deletions

diff --git a/security/keys/keyring.c b/security/keys/keyring.c
index f931ccfeefb0..d2d1f3378008 100644
--- a/security/keys/keyring.c
+++ b/security/keys/keyring.c
@@ -491,13 +491,18 @@ static long keyring_read(const struct key *keyring,
  */
 struct key *keyring_alloc(const char *description, kuid_t uid, kgid_t gid,
 			  const struct cred *cred, key_perm_t perm,
-			  unsigned long flags, struct key *dest)
+			  unsigned long flags,
+			  int (*restrict_link)(struct key *,
+					       const struct key_type *,
+					       unsigned long,
+					       const union key_payload *),
+			  struct key *dest)
 {
 	struct key *keyring;
 	int ret;
 
 	keyring = key_alloc(&key_type_keyring, description,
-			    uid, gid, cred, perm, flags);
+			    uid, gid, cred, perm, flags, restrict_link);
 	if (!IS_ERR(keyring)) {
 		ret = key_instantiate_and_link(keyring, NULL, 0, dest, NULL);
 		if (ret < 0) {
@@ -510,6 +515,51 @@ struct key *keyring_alloc(const char *description, kuid_t uid, kgid_t gid,
 }
 EXPORT_SYMBOL(keyring_alloc);
 
+/**
+ * keyring_restrict_trusted_only - Restrict additions to a keyring to trusted keys only
+ * @keyring: The keyring being added to.
+ * @type: The type of key being added.
+ * @flags: The key flags.
+ * @payload: The payload of the key intended to be added.
+ *
+ * Reject the addition of any links to a keyring that point to keys that aren't
+ * marked as being trusted.  It can be overridden by passing
+ * KEY_ALLOC_BYPASS_RESTRICTION to key_instantiate_and_link() when adding a key
+ * to a keyring.
+ *
+ * This is meant to be passed as the restrict_link parameter to
+ * keyring_alloc().
+ */
+int keyring_restrict_trusted_only(struct key *keyring,
+				  const struct key_type *type,
+				  unsigned long flags,
+				  const union key_payload *payload)
+{
+	return flags & KEY_FLAG_TRUSTED ? 0 : -EPERM;
+}
+
+/**
+ * restrict_link_reject - Give -EPERM to restrict link
+ * @keyring: The keyring being added to.
+ * @type: The type of key being added.
+ * @flags: The key flags.
+ * @payload: The payload of the key intended to be added.
+ *
+ * Reject the addition of any links to a keyring.  It can be overridden by
+ * passing KEY_ALLOC_BYPASS_RESTRICTION to key_instantiate_and_link() when
+ * adding a key to a keyring.
+ *
+ * This is meant to be passed as the restrict_link parameter to
+ * keyring_alloc().
+ */
+int restrict_link_reject(struct key *keyring,
+			 const struct key_type *type,
+			 unsigned long flags,
+			 const union key_payload *payload)
+{
+	return -EPERM;
+}
+
 /*
  * By default, we keys found by getting an exact match on their descriptions.
  */
@@ -1191,6 +1241,17 @@ void __key_link_end(struct key *keyring,
 	up_write(&keyring->sem);
 }
 
+/*
+ * Check addition of keys to restricted keyrings.
+ */
+static int __key_link_check_restriction(struct key *keyring, struct key *key)
+{
+	if (!keyring->restrict_link)
+		return 0;
+	return keyring->restrict_link(keyring,
+				      key->type, key->flags, &key->payload);
+}
+
 /**
  * key_link - Link a key to a keyring
  * @keyring: The keyring to make the link in.
@@ -1221,14 +1282,12 @@ 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));
-		ret = __key_link_check_live_key(keyring, key);
+		ret = __key_link_check_restriction(keyring, key);
+		if (ret == 0)
+			ret = __key_link_check_live_key(keyring, key);
 		if (ret == 0)
 			__key_link(key, &edit);
 		__key_link_end(keyring, &key->index_key, edit);