On Tue, Jun 3, 2025 at 3:15 PM Tal Zussman <tz2294@xxxxxxxxxxxx> 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@xxxxxxxxxxxx>
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
>
>
