|From the Red Hat bugzilla , :
1) There is an issue when memcg unregisters events that were attached to
the same eventfd:
- On the first call mem_cgroup_usage_unregister_event() removes all
events attached to a given eventfd, and if there were no events left,
thresholds->primary would become NULL;
- Since there were several events registered, cgroups core will call
mem_cgroup_usage_unregister_event() again, but now kernel will oops,
as the function doesn't expect that threshold->primary may be NULL.
2) In some cases it may happen that pmd_none_or_clear_bad() is called
with the mmap_sem hold in read mode. In those cases the huge page
faults can allocate hugepmds under pmd_none_or_clear_bad() and that
can trigger a false positive from pmd_bad() that will not like to see
a pmd materializing as trans huge.
A privileged user in the KVM guest can use this flaw to crash the host.
An unprivileged local user could use this flaw to crash the system.