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KVM: x86: Forbid KVM_SET_CPUID{,2} after KVM_RUN

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Commit 63f5a1909f ("KVM: x86: Alert userspace that KVM_SET_CPUID{,2}
after KVM_RUN is broken") officially deprecated KVM_SET_CPUID{,2} ioctls
after first successful KVM_RUN and promissed to make this sequence forbiden
in 5.16. It's time to fulfil the promise.

Signed-off-by: Vitaly Kuznetsov <vkuznets@redhat.com>
Message-Id: <20211122175818.608220-3-vkuznets@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
This commit is contained in:
Vitaly Kuznetsov 2021-11-22 18:58:18 +01:00 committed by Paolo Bonzini
parent 6c1186430a
commit feb627e8d6
2 changed files with 30 additions and 17 deletions
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28
arch/x86/kvm/mmu/mmu.c
View file

@ -5025,6 +5025,14 @@ void kvm_mmu_after_set_cpuid(struct kvm_vcpu *vcpu)
/*
* Invalidate all MMU roles to force them to reinitialize as CPUID
* information is factored into reserved bit calculations.
*
* Correctly handling multiple vCPU models with respect to paging and
* physical address properties) in a single VM would require tracking
* all relevant CPUID information in kvm_mmu_page_role. That is very
* undesirable as it would increase the memory requirements for
* gfn_track (see struct kvm_mmu_page_role comments). For now that
* problem is swept under the rug; KVM's CPUID API is horrific and
* it's all but impossible to solve it without introducing a new API.
*/
vcpu->arch.root_mmu.mmu_role.ext.valid = 0;
vcpu->arch.guest_mmu.mmu_role.ext.valid = 0;
@ -5032,24 +5040,10 @@ void kvm_mmu_after_set_cpuid(struct kvm_vcpu *vcpu)
kvm_mmu_reset_context(vcpu);
/*
* KVM does not correctly handle changing guest CPUID after KVM_RUN, as
* MAXPHYADDR, GBPAGES support, AMD reserved bit behavior, etc.. aren't
* tracked in kvm_mmu_page_role. As a result, KVM may miss guest page
* faults due to reusing SPs/SPTEs. Alert userspace, but otherwise
* sweep the problem under the rug.
*
* KVM's horrific CPUID ABI makes the problem all but impossible to
* solve, as correctly handling multiple vCPU models (with respect to
* paging and physical address properties) in a single VM would require
* tracking all relevant CPUID information in kvm_mmu_page_role. That
* is very undesirable as it would double the memory requirements for
* gfn_track (see struct kvm_mmu_page_role comments), and in practice
* no sane VMM mucks with the core vCPU model on the fly.
* Changing guest CPUID after KVM_RUN is forbidden, see the comment in
* kvm_arch_vcpu_ioctl().
*/
if (vcpu->arch.last_vmentry_cpu != -1) {
pr_warn_ratelimited("KVM: KVM_SET_CPUID{,2} after KVM_RUN may cause guest instability\n");
pr_warn_ratelimited("KVM: KVM_SET_CPUID{,2} will fail after KVM_RUN starting with Linux 5.16\n");
}
KVM_BUG_ON(vcpu->arch.last_vmentry_cpu != -1, vcpu->kvm);
}
void kvm_mmu_reset_context(struct kvm_vcpu *vcpu)

19
arch/x86/kvm/x86.c
View file

@ -5148,6 +5148,17 @@ long kvm_arch_vcpu_ioctl(struct file *filp,
struct kvm_cpuid __user *cpuid_arg = argp;
struct kvm_cpuid cpuid;
/*
* KVM does not correctly handle changing guest CPUID after KVM_RUN, as
* MAXPHYADDR, GBPAGES support, AMD reserved bit behavior, etc.. aren't
* tracked in kvm_mmu_page_role. As a result, KVM may miss guest page
* faults due to reusing SPs/SPTEs. In practice no sane VMM mucks with
* the core vCPU model on the fly, so fail.
*/
r = -EINVAL;
if (vcpu->arch.last_vmentry_cpu != -1)
goto out;
r = -EFAULT;
if (copy_from_user(&cpuid, cpuid_arg, sizeof(cpuid)))
goto out;
@ -5158,6 +5169,14 @@ long kvm_arch_vcpu_ioctl(struct file *filp,
struct kvm_cpuid2 __user *cpuid_arg = argp;
struct kvm_cpuid2 cpuid;
/*
* KVM_SET_CPUID{,2} after KVM_RUN is forbidded, see the comment in
* KVM_SET_CPUID case above.
*/
r = -EINVAL;
if (vcpu->arch.last_vmentry_cpu != -1)
goto out;
r = -EFAULT;
if (copy_from_user(&cpuid, cpuid_arg, sizeof(cpuid)))
goto out;