Re: [PATCH 0/4] KVM: x86: Add checks on host-reserved cr4 bits

From: Paolo Bonzini
Date: Wed Jan 15 2020 - 13:05:25 EST


On 10/12/19 23:44, Sean Christopherson wrote:
> KVM currently doesn't incorporate the platform's capabilities in its cr4
> reserved bit checks and instead checks only whether KVM is aware of the
> feature in general and whether or not userspace has advertised the feature
> to the guest.
>
> Lack of checking allows userspace/guest to set unsupported bits in cr4.
> For the most part, setting unsupported bits will simply cause VM-Enter to
> fail. The one existing exception is OSXSAVE, which is conditioned on host
> support as checking only guest_cpuid_has() would result in KVM attempting
> XSAVE, leading to faults and WARNs.
>
> 57-bit virtual addressing has introduced another case where setting an
> unsupported bit (cr4.LA57) can induce a fault in the host. In the LA57
> case, userspace can set the guest's cr4.LA57 by advertising LA57 support
> via CPUID and abuse the bogus cr4.LA57 to effectively bypass KVM's
> non-canonical address check, ultimately causing a #GP when VMX writes
> the guest's bogus address to MSR_KERNEL_GS_BASE during VM-Enter.
>
> Given that the best case scenario is a failed VM-Enter, there's no sane
> reason to allow setting unsupported bits in cr4. Fix the LA57 bug by not
> allowing userspace or the guest to set cr4 bits that are not supported
> by the platform.
>
> Sean Christopherson (4):
> KVM: x86: Don't let userspace set host-reserved cr4 bits
> KVM: x86: Ensure all logical CPUs have consistent reserved cr4 bits
> KVM: x86: Drop special XSAVE handling from guest_cpuid_has()
> KVM: x86: Add macro to ensure reserved cr4 bits checks stay in sync
>
> arch/x86/kvm/cpuid.h | 4 ---
> arch/x86/kvm/svm.c | 1 +
> arch/x86/kvm/vmx/vmx.c | 1 +
> arch/x86/kvm/x86.c | 65 +++++++++++++++++++++++++++++-------------
> 4 files changed, 47 insertions(+), 24 deletions(-)
>

Queued, thanks.

Paolo