[PATCH] keys: Discard key spinlock and use RCU for key payload

The attached patch changes the key implementation in a number of ways:

 (1) It removes the spinlock from the key structure.

 (2) The key flags are now accessed using atomic bitops instead of
     write-locking the key spinlock and using C bitwise operators.

     The three instantiation flags are dealt with with the construction
     semaphore held during the request_key/instantiate/negate sequence, thus
     rendering the spinlock superfluous.

     The key flags are also now bit numbers not bit masks.

 (3) The key payload is now accessed using RCU. This permits the recursive
     keyring search algorithm to be simplified greatly since no locks need be
     taken other than the usual RCU preemption disablement. Searching now does
     not require any locks or semaphores to be held; merely that the starting
     keyring be pinned.

 (4) The keyring payload now includes an RCU head so that it can be disposed
     of by call_rcu(). This requires that the payload be copied on unlink to
     prevent introducing races in copy-down vs search-up.

 (5) The user key payload is now a structure with the data following it. It
     includes an RCU head like the keyring payload and for the same reason. It
     also contains a data length because the data length in the key may be
     changed on another CPU whilst an RCU protected read is in progress on the
     payload. This would then see the supposed RCU payload and the on-key data
     length getting out of sync.

     I'm tempted to drop the key's datalen entirely, except that it's used in
     conjunction with quota management and so is a little tricky to get rid
     of.

 (6) Update the keys documentation.

Signed-Off-By: David Howells <dhowells@redhat.com>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>

1 files changed, 7 insertions, 16 deletions

diff --git a/security/keys/keyctl.c b/security/keys/keyctl.c
index dc0011b3fac9..cedb7326de29 100644
--- a/security/keys/keyctl.c
+++ b/security/keys/keyctl.c
@@ -728,7 +728,6 @@ long keyctl_chown_key(key_serial_t id, uid_t uid, gid_t gid)
         /* make the changes with the locks held to prevent chown/chown races */
         ret = -EACCES;
         down_write(&key->sem);
-        write_lock(&key->lock);
 
         if (!capable(CAP_SYS_ADMIN)) {
                 /* only the sysadmin can chown a key to some other UID */
@@ -755,7 +754,6 @@ long keyctl_chown_key(key_serial_t id, uid_t uid, gid_t gid)
         ret = 0;
 
  no_access:
-        write_unlock(&key->lock);
         up_write(&key->sem);
         key_put(key);
  error:
@@ -784,26 +782,19 @@ long keyctl_setperm_key(key_serial_t id, key_perm_t perm)
                 goto error;
         }
 
-        /* make the changes with the locks held to prevent chown/chmod
-         * races */
+        /* make the changes with the locks held to prevent chown/chmod races */
         ret = -EACCES;
         down_write(&key->sem);
-        write_lock(&key->lock);
 
-        /* if we're not the sysadmin, we can only chmod a key that we
-         * own */
-        if (!capable(CAP_SYS_ADMIN) && key->uid != current->fsuid)
-                goto no_access;
-
-        /* changing the permissions mask */
-        key->perm = perm;
-        ret = 0;
+        /* if we're not the sysadmin, we can only change a key that we own */
+        if (capable(CAP_SYS_ADMIN) || key->uid == current->fsuid) {
+                key->perm = perm;
+                ret = 0;
+        }
 
- no_access:
-        write_unlock(&key->lock);
         up_write(&key->sem);
         key_put(key);
- error:
+error:
         return ret;
 
 } /* end keyctl_setperm_key() */