The patch titled Subject: mm: lock VMA in dup_anon_vma() before setting ->anon_vma has been added to the -mm mm-hotfixes-unstable branch. Its filename is mm-lock-vma-in-dup_anon_vma-before-setting-anon_vma.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: Jann Horn jannh@google.com Subject: mm: lock VMA in dup_anon_vma() before setting ->anon_vma Date: Fri, 21 Jul 2023 05:46:43 +0200

When VMAs are merged, dup_anon_vma() is called with `dst` pointing to the VMA that is being expanded to cover the area previously occupied by another VMA. This currently happens while `dst` is not write-locked.

This means that, in the `src->anon_vma && !dst->anon_vma` case, as soon as the assignment `dst->anon_vma = src->anon_vma` has happened, concurrent page faults can happen on `dst` under the per-VMA lock. This is already icky in itself, since such page faults can now install pages into `dst` that are attached to an `anon_vma` that is not yet tied back to the `anon_vma` with an `anon_vma_chain`. But if `anon_vma_clone()` fails due to an out-of-memory error, things get much worse: `anon_vma_clone()` then reverts `dst->anon_vma` back to NULL, and `dst` remains completely unconnected to the `anon_vma`, even though we can have pages in the area covered by `dst` that point to the `anon_vma`.

This means the `anon_vma` of such pages can be freed while the pages are still mapped into userspace, which leads to UAF when a helper like folio_lock_anon_vma_read() tries to look up the anon_vma of such a page.

This theoretically is a security bug, but I believe it is really hard to actually trigger as an unprivileged user because it requires that you can make an order-0 GFP_KERNEL allocation fail, and the page allocator tries pretty hard to prevent that.

I think doing the vma_start_write() call inside dup_anon_vma() is the most straightforward fix for now.

For a kernel-assisted reproducer, see the notes section of the patch mail.

Link: https://lkml.kernel.org/r/20230721034643.616851-1-jannh@google.com Fixes: 5e31275cc997 ("mm: add per-VMA lock and helper functions to control it") Signed-off-by: Jann Horn jannh@google.com Reviewed-by: Suren Baghdasaryan surenb@google.com Cc: stable@vger.kernel.org Signed-off-by: Andrew Morton akpm@linux-foundation.org ---

mm/mmap.c | 1 + 1 file changed, 1 insertion(+)

--- a/mm/mmap.c~mm-lock-vma-in-dup_anon_vma-before-setting-anon_vma +++ a/mm/mmap.c @@ -615,6 +615,7 @@ static inline int dup_anon_vma(struct vm * anon pages imported. */ if (src->anon_vma && !dst->anon_vma) { + vma_start_write(dst); dst->anon_vma = src->anon_vma; return anon_vma_clone(dst, src); } _

Patches currently in -mm which might be from jannh@google.com are

mm-fix-memory-ordering-for-mm_lock_seq-and-vm_lock_seq.patch mm-lock-vma-in-dup_anon_vma-before-setting-anon_vma.patch