[PATCH] signal: fix lockup in complete_signal() racing with mt exec

[Oleg Nesterov <oleg@redhat.com>]

Nickolai Zeldovich says:

    kill(2) of a process while a non-leader thread of it is in execve()
    can lock up the machine: complete_signal() loops forever with siglock
    held and interrupts off, and de_thread() then spins on siglock in
    release_task() with tasklist_lock held.

    The curr_target walk uses next_thread(), which wraps from the last
    thread to ->group_leader on the assumption that the leader is first
    on signal->thread_head. de_thread() makes the exec'ing thread, the
    last one, the leader before release_task() unlinks the old leader,
    so a walk that starts at the old leader stays on the new one if
    that thread blocks the signal, and never gets back to curr_target.

Change complete_signal() to use list_next_entry_circular() instead
of next_thread().

Other users of next_thread/while_each_thread/for_other_threads look safe,
so this patch only changes complete_signal() to make a simple fix for
-stable. However, in the longer term we should update next_thread()
or add another helper, but this needs more work; next_thread() should
work even when signal->thread_head is empty.

Fixes: d639cf4abb4d ("change next_thread() to use __next_thread() ?: group_leader")
Reported-by: Nickolai Zeldovich <nickolai@csail.mit.edu>
Link: https://lore.kernel.org/all/CADGQE7EusFrrSoBnhXhdE20tZJrp1EDe=2623mS8vEMY9hn53A@mail.gmail.com/
Originally-by: Nickolai Zeldovich <nickolai@csail.mit.edu>
Signed-off-by: Oleg Nesterov <oleg@redhat.com>
---
 kernel/signal.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/kernel/signal.c b/kernel/signal.c
index 2474507ac92b..bf5518be67a2 100644
--- a/kernel/signal.c
+++ b/kernel/signal.c
@@ -987,7 +987,8 @@ static void complete_signal(int sig, struct task_struct *p, enum pid_type type)
 		 */
 		t = signal->curr_target;
 		while (!wants_signal(sig, t)) {
-			t = next_thread(t);
+			t = list_next_entry_circular(t, &signal->thread_head,
+							thread_node);
 			if (t == signal->curr_target)
 				/*
 				 * No thread needs to be woken.
-- 
2.55.0