KEYS: Fix RCU no-lock warning in keyctl_session_to_parent()

There's an protected access to the parent process's credentials in the middle
of keyctl_session_to_parent().  This results in the following RCU warning:

  ===================================================
  [ INFO: suspicious rcu_dereference_check() usage. ]
  ---------------------------------------------------
  security/keys/keyctl.c:1291 invoked rcu_dereference_check() without protection!

  other info that might help us debug this:

  rcu_scheduler_active = 1, debug_locks = 0
  1 lock held by keyctl-session-/2137:
   #0:  (tasklist_lock){.+.+..}, at: [<ffffffff811ae2ec>] keyctl_session_to_parent+0x60/0x236

  stack backtrace:
  Pid: 2137, comm: keyctl-session- Not tainted 2.6.36-rc2-cachefs+ #1
  Call Trace:
   [<ffffffff8105606a>] lockdep_rcu_dereference+0xaa/0xb3
   [<ffffffff811ae379>] keyctl_session_to_parent+0xed/0x236
   [<ffffffff811af77e>] sys_keyctl+0xb4/0xb6
   [<ffffffff81001eab>] system_call_fastpath+0x16/0x1b

The code should take the RCU read lock to make sure the parents credentials
don't go away, even though it's holding a spinlock and has IRQ disabled.

Signed-off-by: David Howells <dhowells@redhat.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 3 insertions, 0 deletions

diff --git a/security/keys/keyctl.c b/security/keys/keyctl.c
index b2b0998d6abd..3868c6700b58 100644
--- a/security/keys/keyctl.c
+++ b/security/keys/keyctl.c
@@ -1272,6 +1272,7 @@ long keyctl_session_to_parent(void)
         keyring_r = NULL;
 
         me = current;
+        rcu_read_lock();
         write_lock_irq(&tasklist_lock);
 
         parent = me->real_parent;
@@ -1319,6 +1320,7 @@ long keyctl_session_to_parent(void)
         set_ti_thread_flag(task_thread_info(parent), TIF_NOTIFY_RESUME);
 
         write_unlock_irq(&tasklist_lock);
+        rcu_read_unlock();
         if (oldcred)
                 put_cred(oldcred);
         return 0;
@@ -1327,6 +1329,7 @@ already_same:
         ret = 0;
 not_permitted:
         write_unlock_irq(&tasklist_lock);
+        rcu_read_unlock();
         put_cred(cred);
         return ret;