Re: [PATCH 2/3] userfaultfd: prevent unregistering VMAs through a different userfaultfd

[James Houghton <jthoughton@google.com>]

On Tue, Jun 3, 2025 at 3:15 PM Tal Zussman <tz2294@columbia.edu> wrote:
>
> Currently, a VMA registered with a uffd can be unregistered through a
> different uffd asssociated with the same mm_struct.
>
> Change this behavior to be stricter by requiring VMAs to be unregistered
> through the same uffd they were registered with.
>
> While at it, correct the comment for the no userfaultfd case. This seems
> to be a copy-paste artifact from the analagous userfaultfd_register()
> check.
>
> Fixes: 86039bd3b4e6 ("userfaultfd: add new syscall to provide memory externalization")
> Signed-off-by: Tal Zussman <tz2294@columbia.edu>

Thanks, Tal! I like this patch, but I can't really meaningfully
comment on if it's worth it to change the UAPI.

> ---
>  fs/userfaultfd.c | 15 +++++++++++++--
>  1 file changed, 13 insertions(+), 2 deletions(-)
>
> diff --git a/fs/userfaultfd.c b/fs/userfaultfd.c
> index 22f4bf956ba1..9289e30b24c4 100644
> --- a/fs/userfaultfd.c
> +++ b/fs/userfaultfd.c
> @@ -1477,6 +1477,16 @@ static int userfaultfd_unregister(struct userfaultfd_ctx *ctx,
>                 if (!vma_can_userfault(cur, cur->vm_flags, wp_async))
>                         goto out_unlock;
>
> +               /*
> +                * Check that this vma isn't already owned by a different
> +                * userfaultfd. This provides for more strict behavior by
> +                * preventing a VMA registered with a userfaultfd from being
> +                * unregistered through a different userfaultfd.
> +                */
> +               if (cur->vm_userfaultfd_ctx.ctx &&
> +                   cur->vm_userfaultfd_ctx.ctx != ctx)
> +                       goto out_unlock;
> +

Very minor nitpick: I think this check should go above the
!vma_can_userfault() check above, as `wp_async` was derived from
`ctx`, not `cur->vm_userfaultfd_ctx.ctx`.

>                 found = true;
>         } for_each_vma_range(vmi, cur, end);

I don't really like this for_each_vma_range() for loop, but I guess it
is meaningful to the user: invalid unregistration attempts will fail
quickly instead of potentially making some progress. So unfortunately,
without a good reason, I suppose we can't get rid of it. :(

>         BUG_ON(!found);
> @@ -1491,10 +1501,11 @@ static int userfaultfd_unregister(struct userfaultfd_ctx *ctx,
>                 cond_resched();
>
>                 BUG_ON(!vma_can_userfault(vma, vma->vm_flags, wp_async));
> +               BUG_ON(vma->vm_userfaultfd_ctx.ctx &&
> +                      vma->vm_userfaultfd_ctx.ctx != ctx);

IMO, this new BUG_ON should either be
(1) moved and should not be a BUG_ON. See the WARN_ON_ONCE() below,
OR
(2) removed.

Perhaps the older BUG_ON() should be removed/changed too.

>
>                 /*
> -                * Nothing to do: this vma is already registered into this
> -                * userfaultfd and with the right tracking mode too.
> +                * Nothing to do: this vma is not registered with userfaultfd.
>                  */
>                 if (!vma->vm_userfaultfd_ctx.ctx)
>                         goto skip;

if (WARN_ON_ONCE(vmx->vm_userfaultfd_ctx.ctx != ctx)) {
    ret = -EINVAL;
    break;
}

where the WARN_ON_ONCE() indicates that the VMA should have been
filtered out earlier. The WARN_ON_ONCE() isn't even really necessary.


>
> --
> 2.39.5
>
>