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

Paolo Bonzini
2021-04-22T06:56:31Z

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