linux-kernel - Re: [PATCH v2 0/9] KVM: x86: Fixes for (benign?) truncation bugs

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Message-ID: <61e0dd04-a91a-1946-d694-2ed2fce4abb5@redhat.com>
Date:   Thu, 22 Apr 2021 08:55:24 +0200
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Sean Christopherson <seanjc@...gle.com>
Cc:     Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org, Babu Moger <babu.moger@....com>,
        Joao Martins <joao.m.martins@...cle.com>,
        David Woodhouse <dwmw@...zon.co.uk>
Subject: Re: [PATCH v2 0/9] KVM: x86: Fixes for (benign?) truncation bugs

On 22/04/21 04:21, Sean Christopherson wrote:
> Patches 01 and 02 fix theoretical bugs related to loading CRs through
> the emulator.  The rest of the patches are a bunch of small fixes for
> cases where KVM reads/writes a 64-bit register outside of 64-bit mode.
> 
> I stumbled on this when puzzling over commit 0107973a80ad ("KVM: x86:
> Introduce cr3_lm_rsvd_bits in kvm_vcpu_arch"), which stated that SEV
> guests failed to boot on PCID-enabled hosts.  Why only PCID hosts?
> 
> After much staring, I realized that the initial CR3 load in
> rsm_enter_protected_mode() would skip the MAXPHYADDR check due to the
> vCPU not being in long mode.  But due to the ordering problems with
> PCID, when PCID is enabled in the guest, the second load of CR3 would
> be done with long mode enabled and thus hit the SEV C-bit bug.
> 
> Changing kvm_set_cr3() made me look at the callers, and seeing that
> SVM didn't properly truncate the value made me look at everything else,
> and here we are.
> 
> Note, I strongly suspect the emulator still has bugs.  But, unless the
> guest is deliberately trying to hit these types of bugs, even the ones
> fixed here, they're likely benign.  I figured I was more likely to break
> something than I was to fix something by diving into the emulator, so I
> left it alone.  For now. :-)
> 
> v2: Rebase to kvm/queue, commit 89a22e37c8c2 ("KVM: avoid "deadlock"
>      between install_new_memslots and MMU notifier")
> 
> v1: https://lkml.kernel.org/r/20210213010518.1682691-1-seanjc@google.com
> 
> Sean Christopherson (9):
>    KVM: x86: Remove emulator's broken checks on CR0/CR3/CR4 loads
>    KVM: x86: Check CR3 GPA for validity regardless of vCPU mode
>    KVM: SVM: Truncate GPR value for DR and CR accesses in !64-bit mode
>    KVM: VMX: Truncate GPR value for DR and CR reads in !64-bit mode
>    KVM: nVMX: Truncate bits 63:32 of VMCS field on nested check in
>      !64-bit
>    KVM: nVMX: Truncate base/index GPR value on address calc in !64-bit
>    KVM: x86/xen: Drop RAX[63:32] when processing hypercall
>    KVM: SVM: Use default rAX size for INVLPGA emulation
>    KVM: x86: Rename GPR accessors to make mode-aware variants the
>      defaults
> 
>   arch/x86/kvm/emulate.c        | 68 +----------------------------------
>   arch/x86/kvm/kvm_cache_regs.h | 19 ++++++----
>   arch/x86/kvm/svm/svm.c        | 12 +++++--
>   arch/x86/kvm/vmx/nested.c     | 14 ++++----
>   arch/x86/kvm/vmx/vmx.c        |  6 ++--
>   arch/x86/kvm/x86.c            | 19 ++++++----
>   arch/x86/kvm/x86.h            |  8 ++---
>   7 files changed, 48 insertions(+), 98 deletions(-)
> 

Queued, thanks.

Paolo