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, 22 insertions, 0 deletions

diff --git a/Documentation/security/keys.txt b/Documentation/security/keys.txt
index 8c183873b2b7..a6a50b359025 100644
--- a/Documentation/security/keys.txt
+++ b/Documentation/security/keys.txt
@@ -999,6 +999,10 @@ payload contents" for more information.
 	struct key *keyring_alloc(const char *description, uid_t uid, gid_t gid,
 				  const struct cred *cred,
 				  key_perm_t perm,
+				  int (*restrict_link)(struct key *,
+						       const struct key_type *,
+						       unsigned long,
+						       const union key_payload *),
 				  unsigned long flags,
 				  struct key *dest);
 
@@ -1010,6 +1014,24 @@ payload contents" for more information.
     KEY_ALLOC_NOT_IN_QUOTA in flags if the keyring shouldn't be accounted
     towards the user's quota).  Error ENOMEM can also be returned.
 
+    If restrict_link not NULL, it should point to a function that will be
+    called each time an attempt is made to link a key into the new keyring.
+    This function is called to check whether a key may be added into the keying
+    or not.  Callers of key_create_or_update() within the kernel can pass
+    KEY_ALLOC_BYPASS_RESTRICTION to suppress the check.  An example of using
+    this is to manage rings of cryptographic keys that are set up when the
+    kernel boots where userspace is also permitted to add keys - provided they
+    can be verified by a key the kernel already has.
+
+    When called, the restriction function will be passed the keyring being
+    added to, the key flags value and the type and payload of the key being
+    added.  Note that when a new key is being created, this is called between
+    payload preparsing and actual key creation.  The function should return 0
+    to allow the link or an error to reject it.
+
+    A convenience function, restrict_link_reject, exists to always return
+    -EPERM to in this case.
+
 
 (*) To check the validity of a key, this function can be called: