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SubjectRe: [PATCH 4.19 093/105] KVM: x86: Skip EFER vs. guest CPUID checks for host-initiated writes
Hi!

> From: Sean Christopherson <sean.j.christopherson@intel.com>
>
> commit 11988499e62b310f3bf6f6d0a807a06d3f9ccc96 upstream.
>
> KVM allows userspace to violate consistency checks related to the
> guest's CPUID model to some degree. Generally speaking, userspace has
> carte blanche when it comes to guest state so long as jamming invalid
> state won't negatively affect the host.
>
> Currently this is seems to be a non-issue as most of the interesting
> EFER checks are missing, e.g. NX and LME, but those will be added
> shortly. Proactively exempt userspace from the CPUID checks so as not
> to break userspace.
>
> Note, the efer_reserved_bits check still applies to userspace writes as
> that mask reflects the host's capabilities, e.g. KVM shouldn't allow a
> guest to run with NX=1 if it has been disabled in the host.

> arch/x86/kvm/x86.c | 37 ++++++++++++++++++++++++-------------
> 1 file changed, 24 insertions(+), 13 deletions(-)
>
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
>
> -static int set_efer(struct kvm_vcpu *vcpu, u64 efer)
> +static int set_efer(struct kvm_vcpu *vcpu, struct msr_data *msr_info)
> {
> u64 old_efer = vcpu->arch.efer;
> + u64 efer = msr_info->data;
>
> - if (!kvm_valid_efer(vcpu, efer))
> - return 1;
> + if (efer & efer_reserved_bits)
> + return false;
>
> - if (is_paging(vcpu)
> - && (vcpu->arch.efer & EFER_LME) != (efer & EFER_LME))
> - return 1;
> + if (!msr_info->host_initiated) {
> + if (!__kvm_valid_efer(vcpu, efer))
> + return 1;

We have "return false" in function returning int. Plus calling
convention here seems to be "nonzero on error" so it should be
returning 1?

> @@ -2356,7 +2367,7 @@ int kvm_set_msr_common(struct kvm_vcpu *
> vcpu->arch.arch_capabilities = data;
> break;
> case MSR_EFER:
> - return set_efer(vcpu, data);
> + return set_efer(vcpu, msr_info);
> case MSR_K7_HWCR:
> data &= ~(u64)0x40; /* ignore flush filter disable */
> data &= ~(u64)0x100; /* ignore ignne emulation enable */
>

--
(english) http://www.livejournal.com/~pavelmachek
(cesky, pictures) http://atrey.karlin.mff.cuni.cz/~pavel/picture/horses/blog.html
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