CVE-2025-38084

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

mm/hugetlb: unshare page tables during VMA split, not before

Currently, __split_vma() triggers hugetlb page table unsharing through

vm_ops->may_split(). This happens before the VMA lock and rmap locks are

taken - which is too early, it allows racing VMA-locked page faults in our

process and racing rmap walks from other processes to cause page tables to

be shared again before we actually perform the split.

Fix it by explicitly calling into the hugetlb unshare logic from

__split_vma() in the same place where THP splitting also happens. At that

point, both the VMA and the rmap(s) are write-locked.

An annoying detail is that we can now call into the helper

hugetlb_unshare_pmds() from two different locking contexts:

1. from hugetlb_split(), holding:

- mmap lock (exclusively)

- VMA lock

- file rmap lock (exclusively)

2. hugetlb_unshare_all_pmds(), which I think is designed to be able to

call us with only the mmap lock held (in shared mode), but currently

only runs while holding mmap lock (exclusively) and VMA lock

Backporting note:

This commit fixes a racy protection that was introduced in commit

b30c14cd6102 ("hugetlb: unshare some PMDs when splitting VMAs"); that

commit claimed to fix an issue introduced in 5.13, but it should actually

also go all the way back.

[[email protected]: v2]