CVE-2024-36000: mm/hugetlb: fix missing hugetlb_lock for resv uncharge

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

mm/hugetlb: fix missing hugetlblock for resv uncharge

There is a recent report on UFFDIOCOPY over hugetlb:

https://lore.kernel.org/all/000000000000ee06de0616177560@google.com/

350: lockdepassertheld(&hugetlblock);

Should be an issue in hugetlb but triggered in an userfault context, where it goes into the unlikely path where two threads modifying the resv map together. Mike has a fix in that path for resv uncharge but it looks like the locking criteria was overlooked: hugetlbcgroupunchargefoliorsvd() will update the cgroup pointer, so it requires to be called with the lock held.

— NVD

Linux Kernel is vulnerable to a denial of service, caused by an error related to missing hugetlblock for resv uncharge. By sending a specially crafted request, a local authenticated attacker could exploit this vulnerability to cause a denial of service.

— IBM