linux-kernel - Re: [PATCH v1] mm/gup: disallow FOLL_FORCE|FOLL

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Date:   Tue, 22 Nov 2022 13:59:25 -0400
From:   Jason Gunthorpe <jgg@...dia.com>
To:     Mike Kravetz <mike.kravetz@...cle.com>
Cc:     David Hildenbrand <david@...hat.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        Peter Xu <peterx@...hat.com>,
        John Hubbard <jhubbard@...dia.com>,
        syzbot+f0b97304ef90f0d0b1dc@...kaller.appspotmail.com
Subject: Re: [PATCH v1] mm/gup: disallow FOLL_FORCE|FOLL_WRITE on hugetlb
 mappings

On Tue, Nov 22, 2022 at 09:41:07AM -0800, Mike Kravetz wrote:
> On 11/22/22 10:05, David Hildenbrand wrote:
> > On 21.11.22 22:33, Andrew Morton wrote:
> > > On Mon, 21 Nov 2022 09:05:43 +0100 David Hildenbrand <david@...hat.com> wrote:
> > > 
> > > > > > MikeK do you have test cases?
> > > > > 
> > > > > Sorry, I do not have any test cases.
> > > > > 
> > > > > I can ask one of our product groups about their usage.  But, that would
> > > > > certainly not be a comprehensive view.
> > > > 
> > > > With
> > > > 
> > > > https://lkml.kernel.org/r/20221116102659.70287-1-david@redhat.com
> > > > 
> > > > on it's way, the RDMA concern should be gone, hopefully.
> > > > 
> > > > @Andrew, can you queue this one? Thanks.
> > > 
> > > This is all a little tricky.
> > > 
> > > It's not good that 6.0 and earlier permit unprivileged userspace to
> > > trigger a WARN.  But we cannot backport this fix into earlier kernels
> > > because it requires the series "mm/gup: remove FOLL_FORCE usage from
> > > drivers (reliable R/O long-term pinning)".
> > > 
> > > Is it possible to come up with a fix for 6.1 and earlier which won't
> > > break RDMA?
> > 
> > Let's recap:
> 
> Thanks!
> 
> > 
> > (1) Nobody so far reported a RDMA regression, it was all pure
> >     speculation. The only report we saw was via ptrace when fuzzing
> >     syscalls.
> > 
> > (2) To trigger it, one would need a hugetlb MAP_PRIVATE mappings without
> >     PROT_WRITE. For example:
> > 
> >       mmap(0, SIZE, PROT_READ,
> >            MAP_PRIVATE|MAP_ANON|MAP_HUGETLB|MAP_HUGE_2MB, -1, 0)
> >     or
> >       mmap(0, SIZE, PROT_READ, MAP_PRIVATE, hugetlbfd, 0)
> > 
> >     While that's certainly valid, it's not the common use case with
> >     hugetlb pages.
> 
> FWIW, I did check with our product teams and they do not knowingly make use
> of private mappings without write.  Of course, that is only a small and
> limited sample size.

Yeah, if it is only this case I'm comfortable as well

Jason