Re: [PATCH 3/3] mm: unexport follow_pfn

[Daniel Vetter]

On Thu, Apr 08, 2021 at 01:40:59PM +0200, Paolo Bonzini wrote:
> On 08/04/21 12:05, Daniel Vetter wrote:
> > On Mon, Mar 29, 2021 at 10:31:01AM -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
> > > 
> > > vfio and kvm
> > 
> > Hm I thought kvm is non-racy due to the mmu notifier catch races?
> 
> No, but the plan is indeed to have some struct for each page that uses
> follow_pfn and update it from the MMU notifiers.

Thanks for clarifying, I've fixed the commit message to mention both vfio
and kvm as Jason suggested. I didn't know that the follow_pte usage in kvm
still has some gaps wrt what's invalidated with mmu notifiers.

Thanks, Daniel

> 
> Paolo
> 
> > > 
> > > > 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@xxxxxxxxxx
> > > > Cc: Jann Horn <jannh@xxxxxxxxxx>
> > > > Cc: Paolo Bonzini <pbonzini@xxxxxxxxxx>
> > > > Cc: Jason Gunthorpe <jgg@xxxxxxxxxx>
> > > > Cc: Cornelia Huck <cohuck@xxxxxxxxxx>
> > > > Cc: Peter Xu <peterx@xxxxxxxxxx>
> > > > Cc: Alex Williamson <alex.williamson@xxxxxxxxxx>
> > > > Cc: linux-mm@xxxxxxxxx
> > > > Cc: linux-arm-kernel@xxxxxxxxxxxxxxxxxxx
> > > > Cc: linux-samsung-soc@xxxxxxxxxxxxxxx
> > > > Cc: linux-media@xxxxxxxxxxxxxxx
> > > > Cc: kvm@xxxxxxxxxxxxxxx
> > > > Signed-off-by: Daniel Vetter <daniel.vetter@xxxxxxxxx>
> > > > ---
> > > >   include/linux/mm.h |  2 --
> > > >   mm/memory.c        | 26 +++++---------------------
> > > >   mm/nommu.c         | 13 +------------
> > > >   3 files changed, 6 insertions(+), 35 deletions(-)
> > > 
> > > Reviewed-by: Jason Gunthorpe <jgg@xxxxxxxxxx>
> > 
> > Thanks for your r-b tags, I'll add them.
> > -Daniel
> > 
> > > 
> > > Jason
> > 
> 
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-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch