[PATCH] Keys: Use RCU to manage session keyring pointer

The attached patch uses RCU to manage the session keyring pointer in struct
signal_struct.  This means that searching need not disable interrupts and get
a the sighand spinlock to access this pointer.  Furthermore, by judicious use
of rcu_read_(un)lock(), this patch also avoids the need to take and put
refcounts on the session keyring itself, thus saving on even more atomic ops.

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

2 files changed, 24 insertions, 25 deletions

diff --git a/security/keys/process_keys.c b/security/keys/process_keys.c
index 059c350cac46..972e30172687 100644
--- a/security/keys/process_keys.c
+++ b/security/keys/process_keys.c
@@ -1,6 +1,6 @@
 /* process_keys.c: management of a process's keyrings
  *
- * Copyright (C) 2004 Red Hat, Inc. All Rights Reserved.
+ * Copyright (C) 2004-5 Red Hat, Inc. All Rights Reserved.
  * Written by David Howells (dhowells@redhat.com)
  *
  * This program is free software; you can redistribute it and/or
@@ -181,7 +181,7 @@ static int install_process_keyring(struct task_struct *tsk)
                         goto error;
                 }
 
-                /* attach or swap keyrings */
+                /* attach keyring */
                 spin_lock_irqsave(&tsk->sighand->siglock, flags);
                 if (!tsk->signal->process_keyring) {
                         tsk->signal->process_keyring = keyring;
@@ -227,12 +227,14 @@ static int install_session_keyring(struct task_struct *tsk,
 
         /* install the keyring */
         spin_lock_irqsave(&tsk->sighand->siglock, flags);
-        old = tsk->signal->session_keyring;
-        tsk->signal->session_keyring = keyring;
+        old = rcu_dereference(tsk->signal->session_keyring);
+        rcu_assign_pointer(tsk->signal->session_keyring, keyring);
         spin_unlock_irqrestore(&tsk->sighand->siglock, flags);
 
         ret = 0;
 
+        /* we're using RCU on the pointer */
+        synchronize_kernel();
         key_put(old);
  error:
         return ret;
@@ -245,8 +247,6 @@ static int install_session_keyring(struct task_struct *tsk,
  */
 int copy_thread_group_keys(struct task_struct *tsk)
 {
-        unsigned long flags;
-
         key_check(current->thread_group->session_keyring);
         key_check(current->thread_group->process_keyring);
 
@@ -254,10 +254,10 @@ int copy_thread_group_keys(struct task_struct *tsk)
         tsk->signal->process_keyring = NULL;
 
         /* same session keyring */
-        spin_lock_irqsave(&current->sighand->siglock, flags);
+        rcu_read_lock();
         tsk->signal->session_keyring =
-                key_get(current->signal->session_keyring);
-        spin_unlock_irqrestore(&current->sighand->siglock, flags);
+                key_get(rcu_dereference(current->signal->session_keyring));
+        rcu_read_unlock();
 
         return 0;
 
@@ -381,8 +381,7 @@ struct key *search_process_keyrings_aux(struct key_type *type,
                                         key_match_func_t match)
 {
         struct task_struct *tsk = current;
-        unsigned long flags;
-        struct key *key, *ret, *err, *tmp;
+        struct key *key, *ret, *err;
 
         /* we want to return -EAGAIN or -ENOKEY if any of the keyrings were
          * searchable, but we failed to find a key or we found a negative key;
@@ -436,17 +435,18 @@ struct key *search_process_keyrings_aux(struct key_type *type,
         }
 
         /* search the session keyring last */
-        spin_lock_irqsave(&tsk->sighand->siglock, flags);
-
-        tmp = tsk->signal->session_keyring;
-        if (!tmp)
-                tmp = tsk->user->session_keyring;
-        atomic_inc(&tmp->usage);
-
-        spin_unlock_irqrestore(&tsk->sighand->siglock, flags);
+        if (tsk->signal->session_keyring) {
+                rcu_read_lock();
+                key = keyring_search_aux(
+                        rcu_dereference(tsk->signal->session_keyring),
+                        type, description, match);
+                rcu_read_unlock();
+        }
+        else {
+                key = keyring_search_aux(tsk->user->session_keyring,
+                                         type, description, match);
+        }
 
-        key = keyring_search_aux(tmp, type, description, match);
-        key_put(tmp);
         if (!IS_ERR(key))
                 goto found;
 
diff --git a/security/keys/request_key.c b/security/keys/request_key.c
index 1919540f047d..54aa7b70e63b 100644
--- a/security/keys/request_key.c
+++ b/security/keys/request_key.c
@@ -175,13 +175,12 @@ static struct key *__request_key_construction(struct key_type *type,
         key->expiry = now.tv_sec + key_negative_timeout;
 
         if (current->signal->session_keyring) {
-                unsigned long flags;
                 struct key *keyring;
 
-                spin_lock_irqsave(&current->sighand->siglock, flags);
-                keyring = current->signal->session_keyring;
+                rcu_read_lock();
+                keyring = rcu_dereference(current->signal->session_keyring);
                 atomic_inc(&keyring->usage);
-                spin_unlock_irqrestore(&current->sighand->siglock, flags);
+                rcu_read_unlock();
 
                 key_link(keyring, key);
                 key_put(keyring);