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From: Greg KH on 6 Aug 2010 14:40
2.6.35-stable review patch. If anyone has any objections, please let us know.

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

From: David Howells <dhowells(a)redhat.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(a)i-love.sakura.ne.jp>
Signed-off-by: David Howells <dhowells(a)redhat.com>
Acked-by: Oleg Nesterov <oleg(a)redhat.com>
Signed-off-by: Linus Torvalds <torvalds(a)linux-foundation.org>
Signed-off-by: Greg Kroah-Hartman <gregkh(a)suse.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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