Re: [PATCH 1/3] mm: userfaultfd: add new UFFDIO_SIGBUS ioctl

[Peter Xu]

On Fri, May 19, 2023 at 10:32:13AM -0700, Axel Rasmussen wrote:
> On Fri, May 19, 2023 at 9:20 AM Peter Xu <peterx@xxxxxxxxxx> wrote:
> >
> > Hi, Jiaqi,
> >
> > On Fri, May 19, 2023 at 08:04:09AM -0700, Jiaqi Yan wrote:
> > > I don't think CAP_ADMIN is something we can work around: a VMM must be
> > > a good citizen to avoid introducing any vulnerability to the host or
> > > guest.
> > >
> > > On the other hand, "Userfaults allow the implementation of on-demand
> > > paging from userland and more generally they allow userland to take
> > > control of various memory page faults, something otherwise only the
> > > kernel code could do." [3]. I am not familiar with the UFFD internals,
> > > but our use case seems to match what UFFD wants to provide: without
> > > affecting the whole world, give a specific userspace (without
> > > CAP_ADMIN) the ability to handle page faults (indirectly emulate a
> > > HWPOISON page (in my mind I treat it as SetHWPOISON(page) +
> > > TestHWPOISON(page) operation in kernel's PF code)). So is it fair to
> > > say what Axel provided here is "provide !ADMIN somehow"?
> > >
> > > [3]https://docs.kernel.org/admin-guide/mm/userfaultfd.html
> >
> > Userfault keywords on "user", IMHO.  We don't strictly need userfault to
> > resolve anything regarding CAP_ADMIN problems.  MADV_DONTNEED also dosn't
> > need CAP_ADMIN, same to any new madvise() if we want to make it useful for
> > injecting poisoned ptes with !ADMIN and limit it within current->mm.
> >
> > But I think you're right that userfaultfd always tried to avoid having
> > ADMIN and keep everything within its own scope of permissions.
> >
> > So again, totally no objection on make it uffd specific for now if you guys
> > are all happy with it, but just to be clear that it's (to me) mostly for
> > avoiding another WAKE, and afaics that's not really for solving the ADMIN
> > issue here.
> 
> How about this plan:
> 
> Since the concrete use case we have (postcopy live migration) is
> UFFD-specific, let's leave it as a UFFDIO_* operation for now.
> 
> If in the future we come up with a non-UFFD use case, we can add a new
> MADV_* which does this operation at that point. From my perspective
> they could even share most of the same implementation code.
> 
> I don't think it's a big problem keeping the UFFDIO_* version too at
> that point, because it still provides some (perhaps small) value:
> 
> - Combines the operation + waking into one syscall
> - It allows us to support additional UFFD flags which modify / extend
> the operation in UFFD-specific ways, if we want to add those in the
> future
> 
> Seem reasonable?

Ok here.

> 
> If so, I'll send a v2 with documentation updates.

I've reviewed v1 in this case, please have a look, thanks.

-- 
Peter Xu