CVE-2026-31575

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

mm/userfaultfd: fix hugetlb fault mutex hash calculation

In mfillatomichugetlb(), linearpageindex() is used to calculate the page index for hugetlbfaultmutexhash(). However, linearpageindex() returns the index in PAGESIZE units, while hugetlbfaultmutexhash() expects the index in huge page units. This mismatch means that different addresses within the same huge page can produce different hash values, leading to the use of different mutexes for the same huge page. This can cause races between faulting threads, which can corrupt the reservation map and trigger the BUGON in resvmaprelease().

Fix this by introducing hugetlblinearpageindex(), which returns the page index in huge page granularity, and using it in place of linearpage_index().