There is a potential race between cgroup_exit() and the
migration path. This race happens because cgroup_exit path
reads the css_set and does cg_list empty check outside of
css_set lock. This can potentially race with the migrate path
trying to move the tasks to a different css_set. For instance,
below is the interleaved sequence of events, where race is
observed:
cpuset_hotplug_workfn()
cgroup_transfer_tasks()
cgroup_migrate()
cgroup_migrate_execute()
css_set_move_task()
list_del_init(&task->cg_list);
<TASK EXIT>
cgroup_exit()
cset = task_css_set(tsk);
if (!list_empty(&tsk->cg_list))
<TASK NOT DISSOCIATED FROM ITS CSS_SET>
list_add_tail(&task->cg_list, use_mg_tasks
In above sequence, as cgroup_exit() read the cg_list for
the task as empty, it didn't disassociate it from its
current css_set, and was moved to new css_set instance
css_set_move_task() called from cpuset_hotplug_workfn()
path. This eventually can result in use after free scenarios,
while accessing the same task_struct again, like in following
sequence:
kernfs_seq_start()
cgroup_seqfile_start()
cgroup_pidlist_start()
css_task_iter_next()
__put_task_struct()
<NULL pointer dereference>
Fix this problem, by moving the css_set and cg_list fetch in
cgroup_exit() inside css_set lock.
Signed-off-by: Neeraj Upadhyay <neer...@codeaurora.org>
---
Hi,
We observed this issue for cgroup code corresponding to stable
v4.4.85 snapshot 3144d81 ("cgroup, kthread: close race window where
new kthreads can be migrated to non-root cgroups"). Can you please
tell us, if there are any patches in latest code, which
fixes these issue?
kernel/cgroup/cgroup.c | 20 ++++++++++++--------
1 file changed, 12 insertions(+), 8 deletions(-)
diff --git a/kernel/cgroup/cgroup.c b/kernel/cgroup/cgroup.c
index df2e0f1..f746b70 100644
--- a/kernel/cgroup/cgroup.c
+++ b/kernel/cgroup/cgroup.c
@@ -692,10 +692,10 @@ static void css_set_move_task(struct task_struct *task,
if (to_cset) {
/*
- * We are synchronized through cgroup_threadgroup_rwsem
- * against PF_EXITING setting such that we can't race
- * against cgroup_exit() changing the css_set to
- * init_css_set and dropping the old one.
+ * We are synchronized through css_set_lock against
+ * PF_EXITING setting such that we can't race against
+ * cgroup_exit() disassociating the task from the
+ * css_set.
*/
WARN_ON_ONCE(task->flags & PF_EXITING);
@@ -4934,20 +4934,24 @@ void cgroup_exit(struct task_struct *tsk)
int i;
/*
- * Unlink from @tsk from its css_set. As migration path can't race
- * with us, we can check css_set and cg_list without synchronization.
+ * Avoid potential race with the migrate path.
+ */
+ spin_lock_irq(&css_set_lock);
+
+ /*
+ * Unlink from @tsk from its css_set.
*/
cset = task_css_set(tsk);
if (!list_empty(&tsk->cg_list)) {
- spin_lock_irq(&css_set_lock);
css_set_move_task(tsk, cset, NULL, false);
cset->nr_tasks--;
- spin_unlock_irq(&css_set_lock);
} else {
get_css_set(cset);
}
+ spin_unlock_irq(&css_set_lock);
+
/* see cgroup_post_fork() for details */
do_each_subsys_mask(ss, i, have_exit_callback) {
ss->exit(tsk);
--
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