Re: [PATCH v5 18/21] mm/mmap: Move can_modify_mm() check down the stack

[Jeff Xu]

On Wed, Jul 17, 2024 at 1:07 PM Liam R. Howlett <Liam.Howlett@xxxxxxxxxx> wrote:
>
> From: "Liam R. Howlett" <Liam.Howlett@xxxxxxxxxx>
>
> Without an arch_unmap() call so high in the call stack, the check for
> mseal'ed vmas can be moved lower as well.  This has the benefit of only
> actually checking if things are msealed when there is anything to check.
> That is, we know there is at least one vma that is in the way and needs
> to be checked.
>
> Only call the can_modify_mm() in do_vmi_align_munmap() and the MAP_FIXED
> case of mmap_region().
>
> Signed-off-by: Liam R. Howlett <Liam.Howlett@xxxxxxxxxx>
> Cc: Jeff Xu <jeffxu@xxxxxxxxxxxx>
Reviewed-by: Jeff Xu <jeffxu@xxxxxxxxxxxx>

> ---
>  mm/mmap.c | 24 ++++++++----------------
>  1 file changed, 8 insertions(+), 16 deletions(-)
>
> diff --git a/mm/mmap.c b/mm/mmap.c
> index 117e8240f697..a32f545d3987 100644
> --- a/mm/mmap.c
> +++ b/mm/mmap.c
> @@ -2877,6 +2877,10 @@ do_vmi_align_munmap(struct vma_iterator *vmi, struct vm_area_struct *vma,
>         struct vma_munmap_struct vms;
>         int error;
>
> +       /* Prevent unmapping a sealed VMA. */
> +       if (unlikely(!can_modify_mm(mm, start, end)))
> +               return -EPERM;
> +
It is nice to consolidate the check for do_vmi_align_munmap and
do_vma_munmap to single check.

>         init_vma_munmap(&vms, vmi, vma, start, end, uf, unlock);
>         error = vms_gather_munmap_vmas(&vms, &mas_detach);
>         if (error)
> @@ -2927,13 +2931,6 @@ int do_vmi_munmap(struct vma_iterator *vmi, struct mm_struct *mm,
>         if (end == start)
>                 return -EINVAL;
>
> -       /*
> -        * Prevent unmapping a sealed VMA.
> -        * can_modify_mm assumes we have acquired the lock on MM.
> -        */
> -       if (unlikely(!can_modify_mm(mm, start, end)))
> -               return -EPERM;
> -
>         /* Find the first overlapping VMA */
>         vma = vma_find(vmi, end);
>         if (!vma) {
> @@ -2991,13 +2988,15 @@ unsigned long mmap_region(struct file *file, unsigned long addr,
>         if (!may_expand_vm(mm, vm_flags, pglen - nr_pages))
>                 return -ENOMEM;
>
> -       if (unlikely(!can_modify_mm(mm, addr, end)))
> -               return -EPERM;
>
>         /* Find the first overlapping VMA */
>         vma = vma_find(&vmi, end);
>         init_vma_munmap(&vms, &vmi, vma, addr, end, uf, /* unlock = */ false);
>         if (vma) {
> +               /* Prevent unmapping a sealed VMA. */
> +               if (unlikely(!can_modify_mm(mm, addr, end)))
> +                       return -EPERM;
> +
So the optimization here is : when no vma  found in the given addr
range => no need to call can_modify_mm.

>                 mt_init_flags(&mt_detach, vmi.mas.tree->ma_flags & MT_FLAGS_LOCK_MASK);
>                 mt_on_stack(mt_detach);
>                 mas_init(&mas_detach, &mt_detach, /* addr = */ 0);
> @@ -3370,13 +3369,6 @@ int do_vma_munmap(struct vma_iterator *vmi, struct vm_area_struct *vma,
>  {
>         struct mm_struct *mm = vma->vm_mm;
>
> -       /*
> -        * Prevent unmapping a sealed VMA.
> -        * can_modify_mm assumes we have acquired the lock on MM.
> -        */
> -       if (unlikely(!can_modify_mm(mm, start, end)))
> -               return -EPERM;
> -
>         return do_vmi_align_munmap(vmi, vma, mm, start, end, uf, unlock);
>  }
>
> --
> 2.43.0
>