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KVM: x86: Partially allow KVM_SET_CPUID{,2} after KVM_RUN

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Commit feb627e8d6f6 ("KVM: x86: Forbid KVM_SET_CPUID{,2} after KVM_RUN")
forbade changing CPUID altogether but unfortunately this is not fully
compatible with existing VMMs. In particular, QEMU reuses vCPU fds for
CPU hotplug after unplug and it calls KVM_SET_CPUID2. Instead of full ban,
check whether the supplied CPUID data is equal to what was previously set.

Reported-by: Igor Mammedov <imammedo@redhat.com>
Fixes: feb627e8d6f6 ("KVM: x86: Forbid KVM_SET_CPUID{,2} after KVM_RUN")
Signed-off-by: Vitaly Kuznetsov <vkuznets@redhat.com>
Message-Id: <20220117150542.2176196-3-vkuznets@redhat.com>
Cc: stable@vger.kernel.org
[Do not call kvm_find_cpuid_entry repeatedly. - Paolo]
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
This commit is contained in:
Vitaly Kuznetsov 2022-01-17 16:05:40 +01:00 committed by Paolo Bonzini
parent ee3a5f9e3d
commit c6617c61e8
2 changed files with 36 additions and 19 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

36
arch/x86/kvm/cpuid.c
View File

@ -119,6 +119,28 @@ static int kvm_check_cpuid(struct kvm_vcpu *vcpu,
return fpu_enable_guest_xfd_features(&vcpu->arch.guest_fpu, xfeatures);
}
/* Check whether the supplied CPUID data is equal to what is already set for the vCPU. */
static int kvm_cpuid_check_equal(struct kvm_vcpu *vcpu, struct kvm_cpuid_entry2 *e2,
int nent)
{
struct kvm_cpuid_entry2 *orig;
int i;
if (nent != vcpu->arch.cpuid_nent)
return -EINVAL;
for (i = 0; i < nent; i++) {
orig = &vcpu->arch.cpuid_entries[i];
if (e2[i].function != orig->function ||
e2[i].index != orig->index ||
e2[i].eax != orig->eax || e2[i].ebx != orig->ebx ||
e2[i].ecx != orig->ecx || e2[i].edx != orig->edx)
return -EINVAL;
}
return 0;
}
static void kvm_update_kvm_cpuid_base(struct kvm_vcpu *vcpu)
{
u32 function;
@ -313,6 +335,20 @@ static int kvm_set_cpuid(struct kvm_vcpu *vcpu, struct kvm_cpuid_entry2 *e2,
__kvm_update_cpuid_runtime(vcpu, e2, nent);
/*
* 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. It would've been better to forbid any
* KVM_SET_CPUID{,2} calls after KVM_RUN altogether but unfortunately
* some VMMs (e.g. QEMU) reuse vCPU fds for CPU hotplug/unplug and do
* KVM_SET_CPUID{,2} again. To support this legacy behavior, check
* whether the supplied CPUID data is equal to what's already set.
*/
if (vcpu->arch.last_vmentry_cpu != -1)
return kvm_cpuid_check_equal(vcpu, e2, nent);
r = kvm_check_cpuid(vcpu, e2, nent);
if (r)
return r;

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

@ -5230,17 +5230,6 @@ 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;
@ -5251,14 +5240,6 @@ 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;