cgroup/cpuset: Don't access cpuset_cgrp_subsys.root in is_in_v2_mode()

Overview

In the Linux kernel, the following vulnerability has been resolved:

cgroup/cpuset: Don't access cpuset_cgrp_subsys.root in is_in_v2_mode()

After seeing the patch [1] to guard is_in_v2_mode() with RCU, it makes me realize that is_in_v2_mode() may be called in a context where a new cgroup filesystem is being rebound with stale cpuset_cgrp_subsys.root pointer. Avoid this potential UaF situation by adding a new cpuset_v2_mode flag which is set when the cpuset_v2_mode mount option is used. This flag is written into only when cpuset_bind() is being called with a stable cpuset_cgrp_subsys.root value. The is_in_v2_mode() helper is modified to read the new cpuset_v2_mode flag instead of accessing cpuset_cgrp_subsys.root directly.

[1] https://lore.kernel.org/lkml/[email protected]

Affected

• Linux >= b8d1b8ee93df8ffbabbeadd65d39853cfad6d698 < 988bf67a13890a37a33201f95947095ca9bec856
• Linux >= b8d1b8ee93df8ffbabbeadd65d39853cfad6d698 < e06b678e3b4e12fdaa0b51a7a5a54abc5dbd6469
• Linux 4.14

Remediation

Refer to the linked advisories for vendor-supplied fixes and affected version ranges.