wait_task_stopped: Check p->exit_state instead of TASK_TRACED (CVE-2007-5500)

patch a3474224 in mainline

The original meaning of the old test (p->state > TASK_STOPPED) was
"not dead", since it was before TASK_TRACED existed and before the
state/exit_state split.  It was a wrong correction in commit
14bf01bb to make this test for
TASK_TRACED instead.  It should have been changed when TASK_TRACED
was introducted and again when exit_state was introduced.
Signed-off-by: Roland McGrath <roland@redhat.com>
Cc: Oleg Nesterov <oleg@tv-sign.ru>
Cc: Alexey Dobriyan <adobriyan@sw.ru>
Cc: Kees Cook <kees@ubuntu.com>
Acked-by: Scott James Remnant <scott@ubuntu.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>

kernel/exit.c

@@ -1336,8 +1336,7 @@ static int wait_task_stopped(struct task_struct *p, int delayed_group_leader,
 		int why = (p->ptrace & PT_PTRACED) ? CLD_TRAPPED : CLD_STOPPED;
 
 		exit_code = p->exit_code;
-		if (unlikely(!exit_code) ||
-		    unlikely(p->state & TASK_TRACED))
+		if (unlikely(!exit_code) || unlikely(p->exit_state))
 			goto bail_ref;
 		return wait_noreap_copyout(p, pid, uid,
 					   why, (exit_code << 8) | 0x7f,