Re: [PATCH v2 0/8] KVM: x86: nVMX GUEST_CR3 bug fix, and then some...

[Reto Buerki]

On 9/27/19 11:45 PM, Sean Christopherson wrote:
> *sigh*
> 
> v2 was shaping up to be a trivial update, until I started working on
> Vitaly's suggestion to add a helper to test for register availability.
> 
> The primary purpose of this series is to fix a CR3 corruption in L2
> reported by Reto Buerki when running with HLT interception disabled in L1.
> On a nested VM-Enter that puts L2 into HLT, KVM never actually enters L2
> and instead mimics HLT interception by canceling the nested run and
> pretending that VM-Enter to L2 completed and then exited on HLT (which
> KVM intercepted).  Because KVM never actually runs L2, KVM skips the
> pending MMU update for L2 and so leaves a stale value in vmcs02.GUEST_CR3.
> If the next wake event for L2 triggers a nested VM-Exit, KVM will refresh
> vmcs12->guest_cr3 from vmcs02.GUEST_CR3 and consume the stale value.
> 
> Fix the issue by unconditionally writing vmcs02.GUEST_CR3 during nested
> VM-Enter instead of deferring the update to vmx_set_cr3(), and skip the
> update of GUEST_CR3 in vmx_set_cr3() when running L2.  I.e. make the
> nested code fully responsible for vmcs02.GUEST_CR3.
> 
> Patch 02/08 is a minor optimization to skip the GUEST_CR3 update if
> vmcs01 is already up-to-date.
> 
> Patches 03 and beyond are Vitaly's fault ;-).
> 
> Patches 03 and 04 are tangentially related cleanup to vmx_set_rflags()
> that was discovered when working through the avail/dirty testing code.
> Ideally they'd be sent as a separate series, but they conflict with the
> avail/dirty helper changes and are themselves minor and straightforward.
> 
> Patches 05 and 06 clean up the register caching code so that there is a
> single enum for all registers which use avail/dirty tracking.  While not
> a true prerequisite for the avail/dirty helpers, the cleanup allows the
> new helpers to take an 'enum kvm_reg' instead of a less helpful 'int reg'.
> 
> Patch 07 is the helpers themselves, as suggested by Vitaly.
> 
> Patch 08 is a truly optional change to ditch decache_cr3() in favor of
> handling CR3 via cache_reg() like any other avail/dirty register.
> 
> 
> Note, I collected the Reviewed-by and Tested-by tags for patches 01 and 02
> even though I inverted the boolean from 'skip_cr3' to 'update_guest_cr3'.
> Please drop the tags if that constitutes a non-trivial functional change.
> 
> v2:
>   - Invert skip_cr3 to update_guest_cr3.  [Liran]
>   - Reword the changelog and comment to be more explicit in detailing
>     how/when KVM will process a nested VM-Enter without runnin L2.  [Liran]
>   - Added Reviewed-by and Tested-by tags.
>   - Add a comment in vmx_set_cr3() to explicitly state that nested
>     VM-Enter is responsible for loading vmcs02.GUEST_CR3.  [Jim]
>   - All of the loveliness in patches 03-08. [Vitaly]
> 
> Sean Christopherson (8):
>   KVM: nVMX: Always write vmcs02.GUEST_CR3 during nested VM-Enter
>   KVM: VMX: Skip GUEST_CR3 VMREAD+VMWRITE if the VMCS is up-to-date
>   KVM: VMX: Consolidate to_vmx() usage in RFLAGS accessors
>   KVM: VMX: Optimize vmx_set_rflags() for unrestricted guest
>   KVM: x86: Add WARNs to detect out-of-bounds register indices
>   KVM: x86: Fold 'enum kvm_ex_reg' definitions into 'enum kvm_reg'
>   KVM: x86: Add helpers to test/mark reg availability and dirtiness
>   KVM: x86: Fold decache_cr3() into cache_reg()
> 
>  arch/x86/include/asm/kvm_host.h |  5 +-
>  arch/x86/kvm/kvm_cache_regs.h   | 67 +++++++++++++++++------
>  arch/x86/kvm/svm.c              |  5 --
>  arch/x86/kvm/vmx/nested.c       | 14 ++++-
>  arch/x86/kvm/vmx/vmx.c          | 94 ++++++++++++++++++---------------
>  arch/x86/kvm/x86.c              | 13 ++---
>  arch/x86/kvm/x86.h              |  6 +--
>  7 files changed, 123 insertions(+), 81 deletions(-)

Series:
Tested-by: Reto Buerki <reet@xxxxxxxxxxx>

Thanks.