+ mm-vma-write-lock-source-vma-before-duplicating-it-in-__split_vma.patch added to mm-unstable branch

[Andrew Morton <akpm@linux-foundation.org>]

The patch titled
     Subject: mm/vma: write lock source VMA before duplicating it in __split_vma()
has been added to the -mm mm-unstable branch.  Its filename is
     mm-vma-write-lock-source-vma-before-duplicating-it-in-__split_vma.patch

This patch will shortly appear at
     https://git.kernel.org/pub/scm/linux/kernel/git/akpm/25-new.git/tree/patches/mm-vma-write-lock-source-vma-before-duplicating-it-in-__split_vma.patch

This patch will later appear in the mm-unstable branch at
    git://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm

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------------------------------------------------------
From: "David Hildenbrand (Arm)" <david@kernel.org>
Subject: mm/vma: write lock source VMA before duplicating it in __split_vma()
Date: Fri, 23 Oct 2026 14:47:28 +0100

__split_vma() duplicates the source VMA before taking its VMA write lock,
and until the VMA is write-locked, a concurrent thread might set
VMA_MAYBE_GUARD_BIT, resulting in the duplication missing the flag.

If this happens, then fork may fail to duplicate guard markers and, if a
file-backed mapping, THP collapse may also discard the markers.

Resolve this by taking the VMA write lock before vm_area_dup() and, since
all callers of vm_area_dup() now hold the VMA write lock, assert that this
is the case there, and add a comment to explain why it's needed.

[ljs@kernel.org: add VMA write assert in vm_area_dup(), commit message touchup]
Link: https://lore.kernel.org/20261023-fix-vma-atomic-v1-2-8ff4fae1fc85@kernel.org
Fixes: 2ab7f1bbafc9 ("mm/madvise: allow guard page install/remove under VMA lock")
Signed-off-by: David Hildenbrand (Arm) <david@kernel.org>
Signed-off-by: Lorenzo Stoakes (ARM) <ljs@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Reviewed-by: Lorenzo Stoakes (ARM) <ljs@kernel.org>
Cc: Liam R. Howlett <liam@infradead.org>
Cc: Vlastimil Babka <vbabka@kernel.org>
Cc: Mike Rapoport <rppt@kernel.org>
Cc: Suren Baghdasaryan <surenb@google.com>
Cc: Michal Hocko <mhocko@suse.com>
Cc: Jann Horn <jannh@google.com>
Cc: Pedro Falcato <pfalcato@suse.de>
Cc: Davidlohr Bueso <dave@stgolabs.net>
Cc: SJ Park <sj@kernel.org>
Cc: Kees Cook <kees@kernel.org>
Cc: Colin Cross <ccross@google.com>
Cc: <stable@vger.kernel.org>
---

 mm/vma.c      |    2 +-
 mm/vma_init.c |    3 +++
 2 files changed, 4 insertions(+), 1 deletion(-)

--- a/mm/vma.c~mm-vma-write-lock-source-vma-before-duplicating-it-in-__split_vma
+++ a/mm/vma.c
@@ -538,6 +538,7 @@ __split_vma(struct vma_iterator *vmi, st
 			return err;
 	}
 
+	vma_start_write(vma);
 	new = vm_area_dup(vma);
 	if (!new)
 		return -ENOMEM;
@@ -568,7 +569,6 @@ __split_vma(struct vma_iterator *vmi, st
 	if (new->vm_ops && new->vm_ops->open)
 		new->vm_ops->open(new);
 
-	vma_start_write(vma);
 	vma_start_write(new);
 
 	init_vma_prep(&vp, vma);
--- a/mm/vma_init.c~mm-vma-write-lock-source-vma-before-duplicating-it-in-__split_vma
+++ a/mm/vma_init.c
@@ -127,6 +127,9 @@ struct vm_area_struct *vm_area_dup(struc
 {
 	struct vm_area_struct *new = kmem_cache_alloc(vm_area_cachep, GFP_KERNEL);
 
+	/* Must hold a write lock to stabilise atomic VMA flags. */
+	vma_assert_write_locked(orig);
+
 	if (!new)
 		return NULL;
 
_

Patches currently in -mm which might be from david@kernel.org are

mm-fix-address-wraparound-when-scanning-large-folio-ptes.patch
mm-hugetlb-fix-use-after-free-in-hugetlb_mfill_atomic_pte.patch
mm-hugetlb-tolerate-uffd-bits-set-without-uffd-for-prot_none-ptes.patch
mm-hugetlb-tolerate-uffd_wp-markers-outside-uffd-wp-vmas.patch
mm-mremap-fix-calling-fixup_hugetlb_reservations-on-the-wrong-vma.patch
arm64-mm-move-__check_safe_pte_update-fix.patch
mm-sparse-move-mem_section-init-to-sparse_extreme_init.patch
mm-sparse-refactor-sparse_sections_init.patch
mm-sparse-move-initialization-of-section-metadata-to-sparse_metadata_init.patch
mm-sparse-rename-and-cleanup-sparse_init_nid.patch
mm-sparse-cleanup-sparse_init_one_section.patch
mm-sparse-rename-__highest_present_section_nr-to-__highest_used_section_nr.patch
mm-sparse-remove-pfn_in_present_section.patch
mm-sparse-move-__highest_used_section_nr-handling.patch
scripts-gdb-mmpy-remove-fallbacks-for-section_has_mem_map-and-section_is_early.patch
mm-sparse-remove-section_marked_present.patch
mm-sparse-remove-flags-parameter-from-sparse_init_one_section.patch
fs-proc-page-clarify-comment-in-get_max_dump_pfn.patch
mm-memory_hotplug-drop-config_have_arch_pfn_valid-handling-from-pfn_to_online_page.patch
vmcoreinfo-export-section_has_mem_map.patch
mm-disallow-ioremaping-ordinary-ram-in-generic_access_phys.patch
mm-userfaultfd-only-allocate-anon_vma-in-cow-mappings.patch
mm-hugetlb-disable-pmd-sharing-for-uffd-missing-mode.patch
mm-vma-write-lock-source-vma-before-duplicating-it-in-__split_vma.patch
mm-madvise-preserve-vma_softdirty_bit-when-changing-anonymous-names.patch