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

[Jason Gunthorpe]

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@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(-)

I think this is the right thing to do.

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

Jason