linux-kernel - Re: [PATCH v5.5 03/30] KVM: Require total number of memslot pages to fit in an unsigned long

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Message-ID: <7bda9e32-3fbe-b5cf-f9e1-30b5c0b13f37@oracle.com>
Date:   Tue, 9 Nov 2021 01:38:24 +0100
From:   "Maciej S. Szmigiero" <maciej.szmigiero@...cle.com>
To:     Sean Christopherson <seanjc@...gle.com>
Cc:     James Morse <james.morse@....com>,
        Alexandru Elisei <alexandru.elisei@....com>,
        Suzuki K Poulose <suzuki.poulose@....com>,
        Atish Patra <atish.patra@....com>,
        David Hildenbrand <david@...hat.com>,
        Cornelia Huck <cohuck@...hat.com>,
        Claudio Imbrenda <imbrenda@...ux.ibm.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>,
        linux-arm-kernel@...ts.infradead.org, kvmarm@...ts.cs.columbia.edu,
        linux-mips@...r.kernel.org, kvm@...r.kernel.org,
        kvm-ppc@...r.kernel.org, kvm-riscv@...ts.infradead.org,
        linux-riscv@...ts.infradead.org, linux-kernel@...r.kernel.org,
        Ben Gardon <bgardon@...gle.com>, Marc Zyngier <maz@...nel.org>,
        Huacai Chen <chenhuacai@...nel.org>,
        Aleksandar Markovic <aleksandar.qemu.devel@...il.com>,
        Paul Mackerras <paulus@...abs.org>,
        Anup Patel <anup.patel@....com>,
        Paul Walmsley <paul.walmsley@...ive.com>,
        Palmer Dabbelt <palmer@...belt.com>,
        Albert Ou <aou@...s.berkeley.edu>,
        Christian Borntraeger <borntraeger@...ibm.com>,
        Janosch Frank <frankja@...ux.ibm.com>,
        Paolo Bonzini <pbonzini@...hat.com>
Subject: Re: [PATCH v5.5 03/30] KVM: Require total number of memslot pages to
 fit in an unsigned long

On 04.11.2021 01:25, Sean Christopherson wrote:
> Explicitly disallow creating more memslot pages than can fit in an
> unsigned long, KVM doesn't correctly handle a total number of memslot
> pages that doesn't fit in an unsigned long and remedying that would be a
> waste of time.
> 
> For a 64-bit kernel, this is a nop as memslots are not allowed to overlap
> in the gfn address space.
> 
> With a 32-bit kernel, userspace can at most address 3gb of virtual memory,
> whereas wrapping the total number of pages would require 4tb+ of guest
> physical memory.  Even with x86's second address space for SMM, userspace
> would need to alias all of guest memory more than one _thousand_ times.
> And on older x86 hardware with MAXPHYADDR < 43, the guest couldn't
> actually access any of those aliases even if userspace lied about
> guest.MAXPHYADDR.
> 
> On 390 and arm64, this is a nop as they don't support 32-bit hosts.
> 
> On x86, practically speaking this is simply acknowledging reality as the
> existing kvm_mmu_calculate_default_mmu_pages() assumes the total number
> of pages fits in an "unsigned long".
> 
> On PPC, this is likely a nop as every flavor of PPC KVM assumes gfns (and
> gpas!) fit in unsigned long.  arch/powerpc/kvm/book3s_32_mmu_host.c goes
> a step further and fails the build if CONFIG_PTE_64BIT=y, which
> presumably means that it does't support 64-bit physical addresses.
> 
> On MIPS, this is also likely a nop as the core MMU helpers assume gpas
> fit in unsigned long, e.g. see kvm_mips_##name##_pte.
> 
> And finally, RISC-V is a "don't care" as it doesn't exist in any release,
> i.e. there is no established ABI to break.
> 
> Signed-off-by: Sean Christopherson <seanjc@...gle.com>

Reviewed-by: Maciej S. Szmigiero <maciej.szmigiero@...cle.com>