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

[Christian Borntraeger <borntraeger@de.ibm.com>]

Am 09.09.21 um 18:22 schrieb David Hildenbrand:
> 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>
> 
> David Hildenbrand (9):
>    s390/gmap: validate VMA in __gmap_zap()
>    s390/gmap: don't unconditionally call pte_unmap_unlock() in
>      __gmap_zap()
>    s390/mm: validate VMA in PGSTE manipulation functions
>    s390/mm: fix VMA and page table handling code in storage key handling
>      functions
>    s390/uv: fully validate the VMA before calling follow_page()
>    s390/pci_mmio: fully validate the VMA before calling follow_pte()
>    s390/mm: no need for pte_alloc_map_lock() if we know the pmd is
>      present
>    s390/mm: optimize set_guest_storage_key()
>    s390/mm: optimize reset_guest_reference_bit()
> 
>   arch/s390/kernel/uv.c    |   2 +-
>   arch/s390/mm/gmap.c      |  11 +++-
>   arch/s390/mm/pgtable.c   | 109 +++++++++++++++++++++++++++------------
>   arch/s390/pci/pci_mmio.c |   4 +-
>   4 files changed, 89 insertions(+), 37 deletions(-)
> 

Thanks applied. Will run some test on those commits, but its already pushed
out to my next tree to give it some coverage.
g