In the Linux kernel, the following vulnerability has been resolved:
mm/hugetlb: unshare page tables during VMA split, not before
Currently, _splitvma() triggers hugetlb page table unsharing through vmops->maysplit(). 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 _splitvma() 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 hugetlbunsharepmds() from two different locking contexts:
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.
[jannh@google.com: v2]