linux-kernel - [30/38] CRED: Fix RCU warning due to previous patch fixing __task

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Date:	Fri, 06 Aug 2010 11:30:51 -0700
From:	Greg KH <gregkh@...e.de>
To:	linux-kernel@...r.kernel.org, stable@...nel.org
Cc:	stable-review@...nel.org, torvalds@...ux-foundation.org,
	akpm@...ux-foundation.org, alan@...rguk.ukuu.org.uk,
	David Howells <dhowells@...hat.com>,
	Oleg Nesterov <oleg@...hat.com>
Subject: [30/38] CRED: Fix RCU warning due to previous patch fixing __task_cred()s checks

2.6.35-stable review patch.  If anyone has any objections, please let us know.

------------------

From: David Howells <dhowells@...hat.com>

commit 694f690d27dadccc8cb9d90532e76593b61fe098 upstream.

Commit 8f92054e7ca1 ("CRED: Fix __task_cred()'s lockdep check and banner
comment") fixed the lockdep checks on __task_cred().  This has shown up
a place in the signalling code where a lock should be held - namely that
check_kill_permission() requires its callers to hold the RCU lock.

Fix group_send_sig_info() to get the RCU read lock around its call to
check_kill_permission().

Without this patch, the following warning can occur:

  ===================================================
  [ INFO: suspicious rcu_dereference_check() usage. ]
  ---------------------------------------------------
  kernel/signal.c:660 invoked rcu_dereference_check() without protection!
  ...

Reported-by: Tetsuo Handa <penguin-kernel@...ove.sakura.ne.jp>
Signed-off-by: David Howells <dhowells@...hat.com>
Acked-by: Oleg Nesterov <oleg@...hat.com>
Signed-off-by: Linus Torvalds <torvalds@...ux-foundation.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@...e.de>

---
 kernel/signal.c |    9 ++++++---
 1 file changed, 6 insertions(+), 3 deletions(-)

--- a/kernel/signal.c
+++ b/kernel/signal.c
@@ -637,7 +637,7 @@ static inline bool si_fromuser(const str
 
 /*
  * Bad permissions for sending the signal
- * - the caller must hold at least the RCU read lock
+ * - the caller must hold the RCU read lock
  */
 static int check_kill_permission(int sig, struct siginfo *info,
 				 struct task_struct *t)
@@ -1127,11 +1127,14 @@ struct sighand_struct *lock_task_sighand
 
 /*
  * send signal info to all the members of a group
- * - the caller must hold the RCU read lock at least
  */
 int group_send_sig_info(int sig, struct siginfo *info, struct task_struct *p)
 {
-	int ret = check_kill_permission(sig, info, p);
+	int ret;
+
+	rcu_read_lock();
+	ret = check_kill_permission(sig, info, p);
+	rcu_read_unlock();
 
 	if (!ret && sig)
 		ret = do_send_sig_info(sig, info, p, true);


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