[PATCH] cgroup: Stop task iteration when rebinding subsystem

[Xiu Jianfeng]

We found a refcount UAF bug as follows:

refcount_t: addition on 0; use-after-free.
WARNING: CPU: 1 PID: 342 at lib/refcount.c:25 refcount_warn_saturate+0xa0/0x148
Workqueue: events cpuset_hotplug_workfn
Call trace:
 refcount_warn_saturate+0xa0/0x148
 __refcount_add.constprop.0+0x5c/0x80
 css_task_iter_advance_css_set+0xd8/0x210
 css_task_iter_advance+0xa8/0x120
 css_task_iter_next+0x94/0x158
 update_tasks_root_domain+0x58/0x98
 rebuild_root_domains+0xa0/0x1b0
 rebuild_sched_domains_locked+0x144/0x188
 cpuset_hotplug_workfn+0x138/0x5a0
 process_one_work+0x1e8/0x448
 worker_thread+0x228/0x3e0
 kthread+0xe0/0xf0
 ret_from_fork+0x10/0x20

then a kernel panic will be triggered as below:

Unable to handle kernel paging request at virtual address 00000000c0000010
Call trace:
 cgroup_apply_control_disable+0xa4/0x16c
 rebind_subsystems+0x224/0x590
 cgroup_destroy_root+0x64/0x2e0
 css_free_rwork_fn+0x198/0x2a0
 process_one_work+0x1d4/0x4bc
 worker_thread+0x158/0x410
 kthread+0x108/0x13c
 ret_from_fork+0x10/0x18

The race that cause this bug can be shown as below:

(hotplug cpu)                | (umount cpuset)
mutex_lock(&cpuset_mutex)    | mutex_lock(&cgroup_mutex)
cpuset_hotplug_workfn        |
 rebuild_root_domains        |  rebind_subsystems
  update_tasks_root_domain   |   spin_lock_irq(&css_set_lock)
   css_task_iter_start       |    list_move_tail(&cset->e_cset_node[ss->id]
   while(css_task_iter_next) |                  &dcgrp->e_csets[ss->id]);
   css_task_iter_end         |   spin_unlock_irq(&css_set_lock)
mutex_unlock(&cpuset_mutex)  | mutex_unlock(&cgroup_mutex)

Inside css_task_iter_start/next/end, css_set_lock is hold and then
released, so when iterating task(left side), the css_set may be moved to
another list(right side), then it->cset_head points to the old list head
and it->cset_pos->next points to the head node of new list, which can't
be used as struct css_set.

To fix this issue, introduce CSS_TASK_ITER_STOPPED flag for css_task_iter.
when moving css_set to dcgrp->e_csets[ss->id] in rebind_subsystems(), stop
the task iteration.

Reported-by: Gaosheng Cui <cuigaosheng1@xxxxxxxxxx>
Link: https://www.spinics.net/lists/cgroups/msg37935.html
Signed-off-by: Xiu Jianfeng <xiujianfeng@xxxxxxxxxx>
---
 include/linux/cgroup.h |  1 +
 kernel/cgroup/cgroup.c | 33 ++++++++++++++++++++++++++++++++-
 2 files changed, 33 insertions(+), 1 deletion(-)

diff --git a/include/linux/cgroup.h b/include/linux/cgroup.h
index 567c547cf371..3f1557cb5758 100644
--- a/include/linux/cgroup.h
+++ b/include/linux/cgroup.h
@@ -47,6 +47,7 @@ struct kernel_clone_args;
 
 /* internal flags */
 #define CSS_TASK_ITER_SKIPPED		(1U << 16)
+#define CSS_TASK_ITER_STOPPED		(1U << 17)
 
 /* a css_task_iter should be treated as an opaque object */
 struct css_task_iter {
diff --git a/kernel/cgroup/cgroup.c b/kernel/cgroup/cgroup.c
index 8b8ca2e01019..7b16e8d34fca 100644
--- a/kernel/cgroup/cgroup.c
+++ b/kernel/cgroup/cgroup.c
@@ -240,6 +240,8 @@ static int cgroup_apply_control(struct cgroup *cgrp);
 static void cgroup_finalize_control(struct cgroup *cgrp, int ret);
 static void css_task_iter_skip(struct css_task_iter *it,
 			       struct task_struct *task);
+static void css_task_iter_stop(struct css_task_iter *it,
+			       struct css_set *cset);
 static int cgroup_destroy_locked(struct cgroup *cgrp);
 static struct cgroup_subsys_state *css_create(struct cgroup *cgrp,
 					      struct cgroup_subsys *ss);
@@ -889,6 +891,19 @@ static void css_set_skip_task_iters(struct css_set *cset,
 		css_task_iter_skip(it, task);
 }
 
+/*
+ * @cset is moving to other list, it's not safe to continue the iteration,
+ * because the cset_head of css_task_iter which is from the old list can
+ * not be used as the stop condition of iteration.
+ */
+static void css_set_stop_iters(struct css_set *cset)
+{
+	struct css_task_iter *it, *pos;
+
+	list_for_each_entry_safe(it, pos, &cset->task_iters, iters_node)
+		css_task_iter_stop(it, cset);
+}
+
 /**
  * css_set_move_task - move a task from one css_set to another
  * @task: task being moved
@@ -1861,9 +1876,11 @@ int rebind_subsystems(struct cgroup_root *dst_root, u16 ss_mask)
 		css->cgroup = dcgrp;
 
 		spin_lock_irq(&css_set_lock);
-		hash_for_each(css_set_table, i, cset, hlist)
+		hash_for_each(css_set_table, i, cset, hlist) {
+			css_set_stop_iters(cset);
 			list_move_tail(&cset->e_cset_node[ss->id],
 				       &dcgrp->e_csets[ss->id]);
+		}
 		spin_unlock_irq(&css_set_lock);
 
 		if (ss->css_rstat_flush) {
@@ -4864,6 +4881,15 @@ static void css_task_iter_skip(struct css_task_iter *it,
 	}
 }
 
+static void css_task_iter_stop(struct css_task_iter *it,
+			       struct css_set *cset)
+{
+	lockdep_assert_held(&css_set_lock);
+
+	WARN_ONCE(it->cur_cset != cset, "invalid cur_set for css_task_iter\n");
+	it->flags |= CSS_TASK_ITER_STOPPED;
+}
+
 static void css_task_iter_advance(struct css_task_iter *it)
 {
 	struct task_struct *task;
@@ -4967,6 +4993,11 @@ struct task_struct *css_task_iter_next(struct css_task_iter *it)
 
 	spin_lock_irq(&css_set_lock);
 
+	if (it->flags & CSS_TASK_ITER_STOPPED) {
+		spin_unlock_irq(&css_set_lock);
+		return NULL;
+	}
+
 	/* @it may be half-advanced by skips, finish advancing */
 	if (it->flags & CSS_TASK_ITER_SKIPPED)
 		css_task_iter_advance(it);
-- 
2.17.1