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

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Date:   Tue, 28 Sep 2021 13:06:26 +0200
From:   Christian Borntraeger <borntraeger@...ibm.com>
To:     Heiko Carstens <hca@...ux.ibm.com>,
        David Hildenbrand <david@...hat.com>,
        Claudio Imbrenda <imbrenda@...ux.ibm.com>
Cc:     linux-kernel@...r.kernel.org, linux-s390@...r.kernel.org,
        kvm@...r.kernel.org, linux-mm@...ck.org,
        Janosch Frank <frankja@...ux.ibm.com>,
        Cornelia Huck <cohuck@...hat.com>,
        Vasily Gorbik <gor@...ux.ibm.com>,
        Niklas Schnelle <schnelle@...ux.ibm.com>,
        Gerald Schaefer <gerald.schaefer@...ux.ibm.com>,
        Ulrich Weigand <Ulrich.Weigand@...ibm.com>
Subject: Re: [PATCH resend RFC 0/9] s390: fixes, cleanups and optimizations
 for page table walkers


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@...ibm.com>
>> Cc: Janosch Frank <frankja@...ux.ibm.com>
>> Cc: Cornelia Huck <cohuck@...hat.com>
>> Cc: Claudio Imbrenda <imbrenda@...ux.ibm.com>
>> Cc: Heiko Carstens <hca@...ux.ibm.com>
>> Cc: Vasily Gorbik <gor@...ux.ibm.com>
>> Cc: Niklas Schnelle <schnelle@...ux.ibm.com>
>> Cc: Gerald Schaefer <gerald.schaefer@...ux.ibm.com>
>> Cc: Ulrich Weigand <Ulrich.Weigand@...ibm.com>
> 
> For the whole series:
> Acked-by: Heiko Carstens <hca@...ux.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?