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:

quoted

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@google.com
Cc: Jann Horn <jannh@google.com>
Cc: Paolo Bonzini <pbonzini@redhat.com>
Cc: Jason Gunthorpe <jgg@nvidia.com>
Cc: Cornelia Huck <cohuck@redhat.com>
Cc: Peter Xu <peterx@redhat.com>
Cc: Alex Williamson <redacted>
Cc: linux-mm@kvack.org
Cc: linux-arm-kernel@lists.infradead.org
Cc: linux-samsung-soc@vger.kernel.org
Cc: linux-media@vger.kernel.org
Cc: kvm@vger.kernel.org
Signed-off-by: Daniel Vetter <redacted>
---
 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

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