coredump: ensure that SIGKILL always kills the dumping thread

prepare_signal() blesses SIGKILL sent to the dumping process but this
signal can be "lost" anyway.  The problems is, complete_signal() sees
SIGNAL_GROUP_EXIT and skips the "kill them all" logic.  And even if the
dumping process is single-threaded (so the target is always "correct"),
the group-wide SIGKILL is not recorded in task->pending and thus
__fatal_signal_pending() won't be true.  A multi-threaded case has even
more problems.

And even ignoring all technical details, SIGNAL_GROUP_EXIT doesn't look
right to me.  This coredumping process is not exiting yet, it can do a lot
of work dumping the core.

With this patch the dumping process doesn't have SIGNAL_GROUP_EXIT, we set
signal->group_exit_task instead.  This makes signal_group_exit() true and
thus this should equally close the races with exit/exec/stop but allows to
kill the dumping thread reliably.

Notes:
	- It is not clear what should we do with ->group_exit_code
	  if the dumper was killed, see the next change.

	- we need more (hopefully straightforward) changes to ensure
	  that SIGKILL actually interrupts the coredump. Basically we
	  need to check __fatal_signal_pending() in dump_write() and
	  dump_seek().
Signed-off-by: Oleg Nesterov <oleg@redhat.com>
Tested-by: Mandeep Singh Baines <msb@chromium.org>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Neil Horman <nhorman@redhat.com>
Cc: "Rafael J. Wysocki" <rjw@sisk.pl>
Cc: Roland McGrath <roland@hack.frob.com>
Cc: Tejun Heo <tj@kernel.org>
Cc: Al Viro <viro@zeniv.linux.org.uk>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

fs/coredump.c

@@ -263,7 +263,6 @@ static int zap_process(struct task_struct *start, int exit_code)
 	struct task_struct *t;
 	int nr = 0;
 
-	start->signal->flags = SIGNAL_GROUP_EXIT;
 	start->signal->group_exit_code = exit_code;
 	start->signal->group_stop_count = 0;
 
@@ -291,8 +290,9 @@ static int zap_threads(struct task_struct *tsk, struct mm_struct *mm,
 	if (!signal_group_exit(tsk->signal)) {
 		mm->core_state = core_state;
 		nr = zap_process(tsk, exit_code);
+		tsk->signal->group_exit_task = tsk;
 		/* ignore all signals except SIGKILL, see prepare_signal() */
-		tsk->signal->flags |= SIGNAL_GROUP_COREDUMP;
+		tsk->signal->flags = SIGNAL_GROUP_COREDUMP;
 		clear_tsk_thread_flag(tsk, TIF_SIGPENDING);
 	}
 	spin_unlock_irq(&tsk->sighand->siglock);
@@ -343,6 +343,7 @@ static int zap_threads(struct task_struct *tsk, struct mm_struct *mm,
 				if (unlikely(p->mm == mm)) {
 					lock_task_sighand(p, &flags);
 					nr += zap_process(p, exit_code);
+					p->signal->flags = SIGNAL_GROUP_EXIT;
 					unlock_task_sighand(p, &flags);
 				}
 				break;
@@ -394,6 +395,11 @@ static void coredump_finish(struct mm_struct *mm)
 	struct core_thread *curr, *next;
 	struct task_struct *task;
 
+	spin_lock_irq(&current->sighand->siglock);
+	current->signal->group_exit_task = NULL;
+	current->signal->flags = SIGNAL_GROUP_EXIT;
+	spin_unlock_irq(&current->sighand->siglock);
+
 	next = mm->core_state->dumper.next;
 	while ((curr = next) != NULL) {
 		next = curr->next;