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 12:59:23 +0200
From:   Heiko Carstens <hca@...ux.ibm.com>
To:     David Hildenbrand <david@...hat.com>,
        Christian Borntraeger <borntraeger@...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>,
        Claudio Imbrenda <imbrenda@...ux.ibm.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

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.