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

[David Hildenbrand <david@redhat.com>]

On 14.09.21 18:50, Claudio Imbrenda wrote:
> On Thu,  9 Sep 2021 18:22:39 +0200
> David Hildenbrand <david@redhat.com> 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
> 
> this is an interesting series, and the code makes sense, but I would
> really like to see some regression tests, and maybe even some
> selftests to trigger (at least some of) the issues.

Yep, it most certainly needs regression testing before picking any of 
this. selftests would be great, but I won't find time for it in the 
foreseeable future.

> 
> the follow-up question is: how did we manage to go on so long without
> noticing these issues? :D

Excellent question - I guess we simply weren't aware of the dos and 
don'ts when dealing with process page tables :)

> 
>> 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(-)
>>
>>
>> base-commit: 7d2a07b769330c34b4deabeed939325c77a7ec2f
> 


-- 
Thanks,

David / dhildenb