cgroup: Fix use after free of cgrp (cgrp->css_sets)

Running a 3.4 kernel + Fedora-18 (systemd) userland on my Allwinner A10
(arm cortex a8), I'm seeing repeated, reproducable list_del list corruption
errors when build with CONFIG_DEBUG_LIST, and the backtrace always shows
free_css_set_work as the function making the problematic list_del call.

I've tracked this doen to a use after free of the cgrp struct, specifically
of the cgrp->css_sets list_head, which gets cleared by free_css_set_work.

Since free_css_set_work runs form a workqueue, it is possible for it to not be
done with clearing the list when the cgrp gets free-ed. To avoid this the code
adding the links increases cgrp->count, and the freeing code running from the
workqueue decreases cgrp->count *after* doing list_del, and then if the count
goes to 0 calls cgroup_wakeup_rmdir_waiter().

However cgroup_rmdir() is missing a check for cgrp->count != 0, causing it
to still continue with the rmdir (which leads to the free-ing of the cgrp),
before free_css_set_work is done. Sometimes the free-ed memory is re-used
before free_css_set_work gets around to unlinking link->cgrp_link_list,
triggering the list_del list corruption messages.

This patch fixes this by properly checking for cgrp->count != 0 and waiting
for the cgroup_rmdir_waitq in that case.

Change-Id: I9dbc02a0a75d5dffa1b65d67456e00139dea57c3
Signed-off-by: Hans de Goede <hdegoede@redhat.com>

1 files changed, 5 insertions, 1 deletions

diff --git a/kernel/cgroup.c b/kernel/cgroup.c
index ca8bfff..a4d2e51 100644
--- a/kernel/cgroup.c
+++ b/kernel/cgroup.c
@@ -270,7 +270,7 @@ static void check_for_release(struct cgroup *cgrp);
 
 /*
  * A queue for waiters to do rmdir() cgroup. A tasks will sleep when
- * cgroup->count == 0 && list_empty(&cgroup->children) && subsys has some
+ * list_empty(&cgroup->children) && subsys has some
  * reference to css->refcnt. In general, this refcnt is expected to goes down
  * to zero, soon.
  *
@@ -3938,6 +3938,10 @@ static int cgroup_clear_css_refs(struct cgroup *cgrp)
 	struct cgroup_subsys *ss;
 	unsigned long flags;
 	bool failed = false;
+
+	if (atomic_read(&cgrp->count) != 0)
+		return false;
+
 	local_irq_save(flags);
 	for_each_subsys(cgrp->root, ss) {
 		struct cgroup_subsys_state *css = cgrp->subsys[ss->subsys_id];