linux-kernel - Re: [PATCH 3/3] mm: unexport follow

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Date:   Wed, 24 Mar 2021 20:17:10 +0100
From:   Daniel Vetter <daniel@...ll.ch>
To:     Jason Gunthorpe <jgg@...dia.com>
Cc:     Daniel Vetter <daniel.vetter@...ll.ch>,
        DRI Development <dri-devel@...ts.freedesktop.org>,
        LKML <linux-kernel@...r.kernel.org>, kvm@...r.kernel.org,
        linux-mm@...ck.org, linux-arm-kernel@...ts.infradead.org,
        linux-samsung-soc@...r.kernel.org, linux-media@...r.kernel.org,
        3pvd@...gle.com, Jann Horn <jannh@...gle.com>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Cornelia Huck <cohuck@...hat.com>,
        Peter Xu <peterx@...hat.com>,
        Alex Williamson <alex.williamson@...hat.com>,
        Daniel Vetter <daniel.vetter@...el.com>
Subject: Re: [PATCH 3/3] mm: unexport follow_pfn

On Wed, Mar 24, 2021 at 09:52:11AM -0300, Jason Gunthorpe wrote:
> On Tue, Mar 16, 2021 at 04:33:03PM +0100, Daniel Vetter wrote:
> > Both kvm (in bd2fae8da794 ("KVM: do not assume PTE is writable after
> > follow_pfn")) and vfio (in 07956b6269d3 ("vfio/type1: Use
> > follow_pte()")) have lost their callsites of follow_pfn(). All the
> > other ones have been switched over to unsafe_follow_pfn because they
> > cannot be fixed without breaking userspace api.
> > 
> > Argueably the vfio code is still racy, but that's kinda a bigger
> > picture. But since it does leak the pte beyond where it drops the pt
> > lock, without anything else like an mmu notifier guaranteeing
> > coherence, the problem is at least clearly visible in the vfio code.
> > So good enough with me.
> > 
> > I've decided to keep the explanation that after dropping the pt lock
> > you must have an mmu notifier if you keep using the pte somehow by
> > adjusting it and moving it into the kerneldoc for the new follow_pte()
> > function.
> > 
> > Cc: 3pvd@...gle.com
> > Cc: Jann Horn <jannh@...gle.com>
> > Cc: Paolo Bonzini <pbonzini@...hat.com>
> > Cc: Jason Gunthorpe <jgg@...dia.com>
> > Cc: Cornelia Huck <cohuck@...hat.com>
> > Cc: Peter Xu <peterx@...hat.com>
> > Cc: Alex Williamson <alex.williamson@...hat.com>
> > Cc: linux-mm@...ck.org
> > Cc: linux-arm-kernel@...ts.infradead.org
> > Cc: linux-samsung-soc@...r.kernel.org
> > Cc: linux-media@...r.kernel.org
> > Cc: kvm@...r.kernel.org
> > Signed-off-by: Daniel Vetter <daniel.vetter@...el.com>
> > ---
> >  include/linux/mm.h |  2 --
> >  mm/memory.c        | 26 +++++---------------------
> >  mm/nommu.c         | 13 +------------
> >  3 files changed, 6 insertions(+), 35 deletions(-)
> 
> I think this is the right thing to do.

Was just about to smash this into the topic branch for testing in
linux-next. Feel like an ack on the series, or at least the two mm
patches?
-Daniel

> 
> Alex is working on fixing VFIO and while kvm is still racy using
> follow pte, I think they are working on it too?
> 
> Jason

-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch