cgroup: add a validation check to cgroup_add_cftyps()

Fengguang reported this bug:

BUG: unable to handle kernel NULL pointer dereference at 0000003c
IP: [<cc90b4ad>] cgroup_cfts_commit+0x27/0x1c1
...
Call Trace:
  [<cc9d1129>] ? kmem_cache_alloc_trace+0x33f/0x3b7
  [<cc90c6fc>] cgroup_add_cftypes+0x8f/0xca
  [<cd78b646>] cgroup_init+0x6a/0x26a
  [<cd764d7d>] start_kernel+0x4d7/0x57a
  [<cd7642ef>] i386_start_kernel+0x92/0x96

This happens in a corner case. If CGROUP_SCHED=y but CFS_BANDWIDTH=n && FAIR_GROUP_SCHED=n && RT_GROUP_SCHED=n, we have:

cpu_files[] = {
	{ }	/* terminate */
}

When we pass cpu_files to cgroup_apply_cftypes(), as cpu_files[0].ss is NULL, we'll access NULL pointer.

The bug was introduced by commit de00ffa56ea3132c6013fc8f07133b8a1014cf53 ("cgroup: make cgroup_subsys->base_cftypes use cgroup_add_cftypes()").

---
 kernel/cgroup.c | 3 +++
 1 file changed, 3 insertions(+)

index eaffc08..465c1d8 100644
@@ -2356,6 +2356,9 @@ int cgroup_add_cftypes(struct cgroup_subsys *ss, struct cftype *cfts)
 {
 	int ret;
 
+	if (!cfts || cfts[0].name[0] == '\0')
+		return 0;
+
 	ret = cgroup_init_cftypes(ss, cfts);
 	if (ret)
 		return ret;

1.8.0.2

Applied to cgroup/for-3.15. Thanks.

tejun