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

[Peter Xu <peterx@redhat.com>]

On Wed, Jun 04, 2025 at 03:23:38PM +0200, David Hildenbrand wrote:
> On 04.06.25 00:14, Tal Zussman 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.
> 
> I consider it a BUG that should be fixed. Hoping Peter can share his
> opinion.

Agree it smells like unintentional, it's just that the man page indeed
didn't mention what would happen if the userfaultfd isn't the one got
registered but only requesting them to be "compatible".

DESCRIPTION
       Unregister a memory address range from userfaultfd.  The pages in
       the range must be “compatible” (see UFFDIO_REGISTER(2const)).

So it sounds still possible if we have existing userapp creating multiple
userfaultfds (for example, for scalability reasons on using multiple
queues) to manage its own mm address space, one uffd in charge of a portion
of VMAs, then it can randomly take one userfaultfd to do unregistrations.
Such might break.

> 
> > 
> > Fixes: 86039bd3b4e6 ("userfaultfd: add new syscall to provide memory externalization")
> > Signed-off-by: Tal Zussman <tz2294@columbia.edu>
> > ---
> >   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;
> 
> So we allow !cur->vm_userfaultfd_ctx.ctx to allow unregistering when there
> was nothing registered.
> 
> A bit weird to set "found = true" in that case. Maybe it's fine, just
> raising it ...

This part should be ok, as found is defined as:

	/*
	 * Search for not compatible vmas.
	 */
	found = false;

So it's still compatible VMA even if not registered.

It's just that I'm not yet sure how this change benefits the kernel
(besides the API can look slightly cleaner).  There seems to still have a
low risk of breaking userapps.  It could be a matter of whether there can
be any real security concerns.

If not, maybe we don't need to risk such a change for almost nothing (I
almost never think "API cleaness" a goal when it's put together with
compatilibities).

Thanks,

-- 
Peter Xu