exec: do not abuse ->cred_guard_mutex in threadgroup_lock()
threadgroup_lock() takes signal->cred_guard_mutex to ensure that
thread_group_leader() is stable. This doesn't look nice, the scope of
this lock in do_execve() is huge.
And as Dave pointed out this can lead to deadlock, we have the
following dependencies:
do_execve: cred_guard_mutex -> i_mutex
cgroup_mount: i_mutex -> cgroup_mutex
attach_task_by_pid: cgroup_mutex -> cred_guard_mutex
Change de_thread() to take threadgroup_change_begin() around the
switch-the-leader code and change threadgroup_lock() to avoid
->cred_guard_mutex.
Note that de_thread() can't sleep with ->group_rwsem held, this can
obviously deadlock with the exiting leader if the writer is active, so it
does threadgroup_change_end() before schedule().
Reported-by: Dave Jones <davej@redhat.com>
Acked-by: Tejun Heo <tj@kernel.org>
Acked-by: Li Zefan <lizefan@huawei.com>
Signed-off-by: Oleg Nesterov <oleg@redhat.com>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
diff --git a/fs/exec.c b/fs/exec.c
index 260f89f..963f510 100644
--- a/fs/exec.c
+++ b/fs/exec.c
@@ -898,11 +898,13 @@
sig->notify_count = -1; /* for exit_notify() */
for (;;) {
+ threadgroup_change_begin(tsk);
write_lock_irq(&tasklist_lock);
if (likely(leader->exit_state))
break;
__set_current_state(TASK_KILLABLE);
write_unlock_irq(&tasklist_lock);
+ threadgroup_change_end(tsk);
schedule();
if (unlikely(__fatal_signal_pending(tsk)))
goto killed;
@@ -960,6 +962,7 @@
if (unlikely(leader->ptrace))
__wake_up_parent(leader, leader->parent);
write_unlock_irq(&tasklist_lock);
+ threadgroup_change_end(tsk);
release_task(leader);
}
