posix-timers: Fix stack info leak in timer_create()

If userland creates a timer without specifying a sigevent info, we'll
create one ourself, using a stack local variable. Particularly will we
use the timer ID as sival_int. But as sigev_value is a union containing
a pointer and an int, that assignment will only partially initialize
sigev_value on systems where the size of a pointer is bigger than the
size of an int. On such systems we'll copy the uninitialized stack bytes
from the timer_create() call to userland when the timer actually fires
and we're going to deliver the signal.

Initialize sigev_value with 0 to plug the stack info leak.

Found in the PaX patch, written by the PaX Team.

Fixes: 5a9fa73 ("posix-timers: kill ->it_sigev_signo and...")
Signed-off-by: Mathias Krause <[email protected]>
Cc: Oleg Nesterov <[email protected]>
Cc: Brad Spengler <[email protected]>
Cc: PaX Team <[email protected]>
Cc: <[email protected]>	# v2.6.28+
Link: http://lkml.kernel.org/r/[email protected]
Signed-off-by: Thomas Gleixner <[email protected]>

kernel/time/posix-timers.c

@@ -636,6 +636,7 @@ SYSCALL_DEFINE3(timer_create, const clockid_t, which_clock,
 			goto out;
 		}
 	} else {
+		memset(&event.sigev_value, 0, sizeof(event.sigev_value));
 		event.sigev_notify = SIGEV_SIGNAL;
 		event.sigev_signo = SIGALRM;
 		event.sigev_value.sival_int = new_timer->it_id;