CVE-2025-38084
mm/hugetlb: unshare page tables during VMA split, not before
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mm/hugetlb: unshare page tables during VMA split, not before
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. [jannh@google.com: v2]
Reserved 2025-04-16 | Published 2025-06-28 | Updated 2025-06-28 | Assigner Linuxgit.kernel.org/...c/e8847d18cd9fff1edbb45e963d9141273c3b539c
git.kernel.org/...c/366298f2b04d2bf1f2f2b7078405bdf9df9bd5d0
git.kernel.org/...c/2511ac64bc1617ca716d3ba8464e481a647c1902
git.kernel.org/...c/af6cfcd0efb7f051af221c418ec8b37a10211947
git.kernel.org/...c/9cf5b2a3b72c23fb7b84736d5d19ee6ea718762b
git.kernel.org/...c/8a21d5584826f4880f45bbf8f72375f4e6c0ff2a
git.kernel.org/...c/081056dc00a27bccb55ccc3c6f230a3d5fd3f7e0
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