The patch titled Subject: mm/vma: enforce VMA fork limit on unfaulted,faulted mremap merge too has been added to the -mm mm-hotfixes-unstable branch. Its filename is mm-vma-enforce-vma-fork-limit-on-unfaultedfaulted-mremap-merge-too.patch

This patch will shortly appear at https://git.kernel.org/pub/scm/linux/kernel/git/akpm/25-new.git/tree/patches...

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

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------------------------------------------------------ From: Lorenzo Stoakes lorenzo.stoakes@oracle.com Subject: mm/vma: enforce VMA fork limit on unfaulted,faulted mremap merge too Date: Mon, 5 Jan 2026 20:11:49 +0000

The is_mergeable_anon_vma() function uses vmg->middle as the source VMA. However when merging a new VMA, this field is NULL.

In all cases except mremap(), the new VMA will either be newly established and thus lack an anon_vma, or will be an expansion of an existing VMA thus we do not care about whether VMA is CoW'd or not.

In the case of an mremap(), we can end up in a situation where we can accidentally allow an unfaulted/faulted merge with a VMA that has been forked, violating the general rule that we do not permit this for reasons of anon_vma lock scalability.

Now we have the ability to be aware of the fact we are copying a VMA and also know which VMA that is, we can explicitly check for this, so do so.

This is pertinent since commit 879bca0a2c4f ("mm/vma: fix incorrectly disallowed anonymous VMA merges"), as this patch permits unfaulted/faulted merges that were previously disallowed running afoul of this issue.

While we are here, vma_had_uncowed_parents() is a confusing name, so make it simple and rename it to vma_is_fork_child().

Link: https://lkml.kernel.org/r/6e2b9b3024ae1220961c8b81d74296d4720eaf2b.176763827... Fixes: 879bca0a2c4f ("mm/vma: fix incorrectly disallowed anonymous VMA merges") Signed-off-by: Lorenzo Stoakes lorenzo.stoakes@oracle.com Cc: David Hildenbrand (Red Hat) david@kernel.org Cc: Jann Horn jannh@google.com Cc: Jeongjun Park aha310510@gmail.com Cc: Liam Howlett liam.howlett@oracle.com Cc: Pedro Falcato pfalcato@suse.de Cc: Rik van Riel riel@surriel.com Cc: Vlastimil Babka vbabka@suse.cz Cc: Yeoreum Yun yeoreum.yun@arm.com Cc: stable@vger.kernel.org Signed-off-by: Andrew Morton akpm@linux-foundation.org ---

mm/vma.c | 27 +++++++++++++++------------ 1 file changed, 15 insertions(+), 12 deletions(-)

--- a/mm/vma.c~mm-vma-enforce-vma-fork-limit-on-unfaultedfaulted-mremap-merge-too +++ a/mm/vma.c @@ -67,18 +67,13 @@ struct mmap_state { .state = VMA_MERGE_START, \ }

-/* - * If, at any point, the VMA had unCoW'd mappings from parents, it will maintain - * more than one anon_vma_chain connecting it to more than one anon_vma. A merge - * would mean a wider range of folios sharing the root anon_vma lock, and thus - * potential lock contention, we do not wish to encourage merging such that this - * scales to a problem. - */ -static bool vma_had_uncowed_parents(struct vm_area_struct *vma) +/* Was this VMA ever forked from a parent, i.e. maybe contains CoW mappings? */ +static bool vma_is_fork_child(struct vm_area_struct *vma) { /* * The list_is_singular() test is to avoid merging VMA cloned from - * parents. This can improve scalability caused by anon_vma lock. + * parents. This can improve scalability caused by the anon_vma root + * lock. */ return vma && vma->anon_vma && !list_is_singular(&vma->anon_vma_chain); } @@ -115,11 +110,19 @@ static bool is_mergeable_anon_vma(struct VM_WARN_ON(src && src_anon != src->anon_vma);

/* Case 1 - we will dup_anon_vma() from src into tgt. */ - if (!tgt_anon && src_anon) - return !vma_had_uncowed_parents(src); + if (!tgt_anon && src_anon) { + struct vm_area_struct *copied_from = vmg->copied_from; + + if (vma_is_fork_child(src)) + return false; + if (vma_is_fork_child(copied_from)) + return false; + + return true; + } /* Case 2 - we will simply use tgt's anon_vma. */ if (tgt_anon && !src_anon) - return !vma_had_uncowed_parents(tgt); + return !vma_is_fork_child(tgt); /* Case 3 - the anon_vma's are already shared. */ return src_anon == tgt_anon; } _

Patches currently in -mm which might be from lorenzo.stoakes@oracle.com are

mm-vma-fix-anon_vma-uaf-on-mremap-faulted-unfaulted-merge.patch tools-testing-selftests-add-tests-for-tgt-src-mremap-merges.patch mm-vma-enforce-vma-fork-limit-on-unfaultedfaulted-mremap-merge-too.patch tools-testing-selftests-add-forked-un-faulted-vma-merge-tests.patch