Re: [PATCH resend RFC 0/9] s390: fixes, cleanups and optimizations for page table walkers

Claudio Imbrenda
2021-09-28T14:39:00Z

On Tue, 28 Sep 2021 13:06:26 +0200
Christian Borntraeger <borntraeger@de.ibm.com> wrote:

> Am 28.09.21 um 12:59 schrieb Heiko Carstens:
> > On Thu, Sep 09, 2021 at 06:22:39PM +0200, David Hildenbrand wrote:
> >> Resend because I missed ccing people on the actual patches ...
> >>
> >> RFC because the patches are essentially untested and I did not actually
> >> try to trigger any of the things these patches are supposed to fix. It
> >> merely matches my current understanding (and what other code does :) ). I
> >> did compile-test as far as possible.
> >>
> >> After learning more about the wonderful world of page tables and their
> >> interaction with the mmap_sem and VMAs, I spotted some issues in our
> >> page table walkers that allow user space to trigger nasty behavior when
> >> playing dirty tricks with munmap() or mmap() of hugetlb. While some issues
> >> should be hard to trigger, others are fairly easy because we provide
> >> conventient interfaces (e.g., KVM_S390_GET_SKEYS and KVM_S390_SET_SKEYS).
> >>
> >> Future work:
> >> - Don't use get_locked_pte() when it's not required to actually allocate
> >> page tables -- similar to how storage keys are now handled. Examples are
> >> get_pgste() and __gmap_zap.
> >> - Don't use get_locked_pte() and instead let page fault logic allocate page
> >> tables when we actually do need page tables -- also, similar to how
> >> storage keys are now handled. Examples are set_pgste_bits() and
> >> pgste_perform_essa().
> >> - Maybe switch to mm/pagewalk.c to avoid custom page table walkers. For
> >> __gmap_zap() that's very easy.
> >>
> >> Cc: Christian Borntraeger <borntraeger@de.ibm.com>
> >> Cc: Janosch Frank <frankja@linux.ibm.com>
> >> Cc: Cornelia Huck <cohuck@redhat.com>
> >> Cc: Claudio Imbrenda <imbrenda@linux.ibm.com>
> >> Cc: Heiko Carstens <hca@linux.ibm.com>
> >> Cc: Vasily Gorbik <gor@linux.ibm.com>
> >> Cc: Niklas Schnelle <schnelle@linux.ibm.com>
> >> Cc: Gerald Schaefer <gerald.schaefer@linux.ibm.com>
> >> Cc: Ulrich Weigand <Ulrich.Weigand@de.ibm.com>
> >
> > For the whole series:
> > Acked-by: Heiko Carstens <hca@linux.ibm.com>
> >
> > Christian, given that this is mostly about KVM I'd assume this should
> > go via the KVM tree. Patch 6 (pci_mmio) is already upstream.
>
> Right, I think I will queue this even without testing for now.
> Claudio, is patch 7 ok for you with the explanation from David?

yes