exec: make de_thread() killable

Change de_thread() to use KILLABLE rather than UNINTERRUPTIBLE while
waiting for other threads.  The only complication is that we should
clear ->group_exit_task and ->notify_count before we return, and we
should do this under tasklist_lock.  -EAGAIN is used to match the
initial signal_group_exit() check/return, it doesn't really matter.

This fixes the (unlikely) race with coredump.  de_thread() checks
signal_group_exit() before it starts to kill the subthreads, but this
can't help if another CLONE_VM (but non CLONE_THREAD) task starts the
coredumping after de_thread() unlocks ->siglock.  In this case the
killed sub-thread can block in exit_mm() waiting for coredump_finish(),
execing thread waits for that sub-thead, and the coredumping thread
waits for execing thread.  Deadlock.

Signed-off-by: Oleg Nesterov <oleg@redhat.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 14 insertions, 2 deletions

diff --git a/fs/exec.c b/fs/exec.c
index 9824473a7ec1..19f4fb80cd17 100644
--- a/fs/exec.c
+++ b/fs/exec.c
@@ -878,9 +878,11 @@ static int de_thread(struct task_struct *tsk)
                 sig->notify_count--;
 
         while (sig->notify_count) {
-                __set_current_state(TASK_UNINTERRUPTIBLE);
+                __set_current_state(TASK_KILLABLE);
                 spin_unlock_irq(lock);
                 schedule();
+                if (unlikely(__fatal_signal_pending(tsk)))
+                        goto killed;
                 spin_lock_irq(lock);
         }
         spin_unlock_irq(lock);
@@ -898,9 +900,11 @@ static int de_thread(struct task_struct *tsk)
                         write_lock_irq(&tasklist_lock);
                         if (likely(leader->exit_state))
                                 break;
-                        __set_current_state(TASK_UNINTERRUPTIBLE);
+                        __set_current_state(TASK_KILLABLE);
                         write_unlock_irq(&tasklist_lock);
                         schedule();
+                        if (unlikely(__fatal_signal_pending(tsk)))
+                                goto killed;
                 }
 
                 /*
@@ -994,6 +998,14 @@ no_thread_group:
 
         BUG_ON(!thread_group_leader(tsk));
         return 0;
+
+killed:
+        /* protects against exit_notify() and __exit_signal() */
+        read_lock(&tasklist_lock);
+        sig->group_exit_task = NULL;
+        sig->notify_count = 0;
+        read_unlock(&tasklist_lock);
+        return -EAGAIN;
 }
 
 char *get_task_comm(char *buf, struct task_struct *tsk)