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

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Date:   Fri, 12 Feb 2021 17:05:09 -0800
From:   Sean Christopherson <seanjc@...gle.com>
To:     Paolo Bonzini <pbonzini@...hat.com>
Cc:     Sean Christopherson <seanjc@...gle.com>,
        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: [PATCH 0/9] KVM: x86: Fixes for (benign?) truncation bugs

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. :-)

P.S. A few of the segmentation tests in kvm-unit-tests fail with
     unrestricted guest disabled, but those failure go back to at least
     v5.9.  I'll bisect 'em next week.

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        | 11 ++++--
 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, 47 insertions(+), 98 deletions(-)

-- 
2.30.0.478.g8a0d178c01-goog