Re: [PATCH] mm/userfaultfd: only allocate anon_vma in CoW mappings

[Lance Yang <lance.yang@linux.dev>]

On Wed, Oct 14, 2026 at 01:53:11PM +0200, David Hildenbrand (Arm) wrote:
>find_vma_and_prepare_anon() can end up allocating an anon_vma in VMAs that
>are not CoW mappings, which is wrong.
>
>If we have a MAP_SHARED R/O file mapped, we lack VM_SHARED (but do have
>VM_MAYSHARE) and lack VM_MAYWRITE.
>
>While uffd later rejects such mappings (uffd registration requires
>VM_MAYWRITE, see comment in validate_dst_vma) find_vma_and_prepare_anon()
>still wrongly allocates an anon vma before it validates the vma.
>
>This in turn later confuses copy_page_range()->vma_needs_copy() such
>that we end up calling copy_hugetlb_page_range() with ranges that
>actually do not contain *any* per-pagetable information that would
>require copying, which is the basis for PMD-table sharing in the first
>place.
>
>copy_hugetlb_page_range() should not have to worry about shared
>PMD tables at all: it doesn't make any sense to copy page tables that are
>shared, as they can be fully recreated later. Ending up in the function
>with shareable page tables is the real issue.
>
>If a destination PMD table is shared in copy_hugetlb_page_range(), it
>might get concurrently unshared as we are not holding relevant
>locks, resulting in us using a page table that is no longer mapped and
>possibly freed after calling:
>
>	dst_pte = huge_pte_alloc(dst, dst_vma, addr, sz);
>
>To fix it, do not allocate an anon_vma in non-CoW mappings and add a
>safety guard to copy_hugetlb_page_range(), making sure that we never
>end up messing with shared PMD tables in there.
>
>Before commit 867a43a34ff8a ("userfaultfd: use per-vma locks in userfaultfd
>operations") we would always validate the VMA before allocating an
>anon_vma, and consequently never wrongly allocate an anon_vma.
>
>As uffd_lock_vma() is part of the same lookup machinery and
>also contains a misleading VM_SHARED check, change that one as well
>directly for consistency.
>
>A follow-up cleanup will stop taking the hugetlb_vma_lock_read(src_vma)
>lock in copy_hugetlb_page_range() entirely. Further, the usage of
>VM_SHARED in userfaultfd code should be cleaned up later to focus on
>CoW vs. non-CoW mappings.
>
>Reported-by: Kyle Zeng <kylebot@openai.com>
>Fixes: 867a43a34ff8a ("userfaultfd: use per-vma locks in userfaultfd operations")
>Cc: stable@vger.kernel.org
>Signed-off-by: David Hildenbrand (Arm) <david@kernel.org>
>---

Tested-by: Lance Yang <lance.yang@linux.dev>