keys: Provide request_key_rcu()

Provide a request_key_rcu() function that can be used to request a key
under RCU conditions.  It can only search and check permissions; it cannot
allocate a new key, upcall or wait for an upcall to complete.  It may
return a partially constructed key.

Signed-off-by: David Howells <dhowells@redhat.com>

2 files changed, 19 insertions, 0 deletions

diff --git a/Documentation/security/keys/core.rst b/Documentation/security/keys/core.rst
index 82dd457ff78d..003f1452a5b7 100644
--- a/Documentation/security/keys/core.rst
+++ b/Documentation/security/keys/core.rst
@@ -1147,6 +1147,16 @@ payload contents" for more information.
     case error ERESTARTSYS will be returned.
 
 
+ *  To search for a key under RCU conditions, call::
+
+        struct key *request_key_rcu(const struct key_type *type,
+                                    const char *description);
+
+    which is similar to request_key() except that it does not check for keys
+    that are under construction and it will not call out to userspace to
+    construct a key if it can't find a match.
+
+
  *  When it is no longer required, the key should be released using::
 
         void key_put(struct key *key);
diff --git a/Documentation/security/keys/request-key.rst b/Documentation/security/keys/request-key.rst
index 07af991463b5..7caedc4d29f1 100644
--- a/Documentation/security/keys/request-key.rst
+++ b/Documentation/security/keys/request-key.rst
@@ -36,6 +36,11 @@ or::
                                                    size_t callout_len,
                                                    void *aux);
 
+or::
+
+        struct key *request_key_rcu(const struct key_type *type,
+                                    const char *description);
+
 Or by userspace invoking the request_key system call::
 
         key_serial_t request_key(const char *type,
@@ -57,6 +62,10 @@ The two async in-kernel calls may return keys that are still in the process of
 being constructed.  The two non-async ones will wait for construction to
 complete first.
 
+The request_key_rcu() call is like the in-kernel request_key() call, except
+that it doesn't check for keys that are under construction and doesn't attempt
+to construct missing keys.
+
 The userspace interface links the key to a keyring associated with the process
 to prevent the key from going away, and returns the serial number of the key to
 the caller.