[34-longterm 141/260] KEYS: Fix RCU no-lock warning in keyctl_session_to_parent()

[Paul Gortmaker]

From: David Howells <dhowells@xxxxxxxxxx>

commit 9d1ac65a9698513d00e5608d93fca0c53f536c14 upstream.

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@xxxxxxxxxx>
Signed-off-by: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
Signed-off-by: Paul Gortmaker <paul.gortmaker@xxxxxxxxxxxxx>
---
 security/keys/keyctl.c |    3 +++
 1 files changed, 3 insertions(+), 0 deletions(-)

diff --git a/security/keys/keyctl.c b/security/keys/keyctl.c
index e9c2e7c..d29e022 100644
--- a/security/keys/keyctl.c
+++ b/security/keys/keyctl.c
@@ -1259,6 +1259,7 @@ long keyctl_session_to_parent(void)
 	keyring_r = NULL;
 
 	me = current;
+	rcu_read_lock();
 	write_lock_irq(&tasklist_lock);
 
 	parent = me->real_parent;
@@ -1313,6 +1314,7 @@ 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;
@@ -1321,6 +1323,7 @@ already_same:
 	ret = 0;
 not_permitted:
 	write_unlock_irq(&tasklist_lock);
+	rcu_read_unlock();
 	put_cred(cred);
 	return ret;
 
-- 
1.7.3.3

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