Re: [PATCH v2] mm: nommu: free unused resources when mremap shrinks the vma

[Lorenzo Stoakes <ljs@kernel.org>]

On Fri, Jul 10, 2026 at 04:30:08PM +0900, Hajime Tazaki wrote:
>
> On Fri, 10 Jul 2026 14:46:48 +0900,
> Hajime Tazaki wrote:
> >
> > When shrinking a VMA via mremap, the bounds are modified directly:
> > mm/nommu.c:do_mremap() {
> >     ...
> >     vma->vm_end = vma->vm_start + new_len;
> >     ...
> > }
> > This shrink the VMA without updating its bounds in the maple tree.
> > If the maple tree (mm->mm_mt) still contains the old bounds, a user
> > process could access the freed portion. The stale maple tree would
> > incorrectly return the shrunk VMA for an address past its new vm_end.
> >
> > This commit fixes this issue by calling vmi_shrink_vma() when shrink
> > happens.  Additionally, if a file-backed map is to be shrunk, it reports
> > -EINVAL like do_munmap() does.
> >
> > The issue is reported by Sashiko review, linked below.
> >
> > Link: https://sashiko.dev/#/patchset/20260702012546.665383-1-thehajime@gmail.com
> > Link: https://sashiko.dev/#/patchset/20260710021028.892645-1-thehajime%40gmail.com
> > Signed-off-by: Hajime Tazaki <thehajime@gmail.com>
>
> I think the original issue reported by Sashiko is addressed, but found
> another existing issue, which might take a bit to look into detail
> (e.g., currently under nommu we cannot map (actually kernel_read())
> /dev/zero, vma_set_anonymous() isn't called for /dev/zero on nommu,
> etc).
>
> So I will address this in a different patch, not in this one.

OK thanks sorry for not getting to these quicker, between jobs atm and my inbox
is an ongoing disaster :)

>
> -- Hajime

Thanks, Lorenzo