Re: [PATCH v2 3/3] mm: perform the mapping_map_writable() check after call_mmap()

[Andy Lutomirski <luto@kernel.org>]

On Sun, Apr 30, 2023 at 3:26 PM Lorenzo Stoakes <lstoakes@gmail.com> wrote:
>
> In order for a F_SEAL_WRITE sealed memfd mapping to have an opportunity to
> clear VM_MAYWRITE, we must be able to invoke the appropriate vm_ops->mmap()
> handler to do so. We would otherwise fail the mapping_map_writable() check
> before we had the opportunity to avoid it.

Is there any reason this can't go before patch 3?

If I'm understanding correctly, a comment like the following might
make this a lot more comprehensible:

>
> This patch moves this check after the call_mmap() invocation. Only memfd
> actively denies write access causing a potential failure here (in
> memfd_add_seals()), so there should be no impact on non-memfd cases.
>
> This patch makes the userland-visible change that MAP_SHARED, PROT_READ
> mappings of an F_SEAL_WRITE sealed memfd mapping will now succeed.
>
> Link: https://bugzilla.kernel.org/show_bug.cgi?id=217238
> Signed-off-by: Lorenzo Stoakes <lstoakes@gmail.com>
> ---
>  mm/mmap.c | 12 ++++++------
>  1 file changed, 6 insertions(+), 6 deletions(-)
>
> diff --git a/mm/mmap.c b/mm/mmap.c
> index 646e34e95a37..1608d7f5a293 100644
> --- a/mm/mmap.c
> +++ b/mm/mmap.c
> @@ -2642,17 +2642,17 @@ unsigned long mmap_region(struct file *file, unsigned long addr,
>         vma->vm_pgoff = pgoff;
>
>         if (file) {
> -               if (is_shared_maywrite(vm_flags)) {
> -                       error = mapping_map_writable(file->f_mapping);
> -                       if (error)
> -                               goto free_vma;
> -               }
> -
>                 vma->vm_file = get_file(file);
>                 error = call_mmap(file, vma);
>                 if (error)
>                         goto unmap_and_free_vma;
>

/* vm_ops->mmap() may have changed vma->flags.  Check for writability now. */

> +               if (vma_is_shared_maywrite(vma)) {
> +                       error = mapping_map_writable(file->f_mapping);
> +                       if (error)
> +                               goto close_and_free_vma;
> +               }
> +

Alternatively, if anyone is nervous about the change in ordering here,
there could be a whole new vm_op like adjust_vma_flags() that happens
before any of this.

--Andy