Re: [PATCH] KVM: x86: restrict maximal physical address

From: Radim KrÄmÃÅ
Date: Fri Nov 25 2016 - 11:15:05 EST


2016-11-25 16:11+0100, David Hildenbrand:
> Am 25.11.2016 um 15:51 schrieb Radim KrÄmÃÅ:
>> The guest could have configured a maximal physical address that exceeds
>> the host. Prevent that situation as it could easily lead to a bug.
>>
>> Signed-off-by: Radim KrÄmÃÅ <rkrcmar@xxxxxxxxxx>
>> ---
>> arch/x86/kvm/cpuid.c | 8 +++++++-
>> 1 file changed, 7 insertions(+), 1 deletion(-)
>>
>> diff --git a/arch/x86/kvm/cpuid.c b/arch/x86/kvm/cpuid.c
>> index 25f0f15fab1a..aed910e9fbed 100644
>> --- a/arch/x86/kvm/cpuid.c
>> +++ b/arch/x86/kvm/cpuid.c
>> @@ -136,7 +136,13 @@ int kvm_update_cpuid(struct kvm_vcpu *vcpu)
>> ((best->eax & 0xff00) >> 8) != 0)
>> return -EINVAL;
>>
>> - /* Update physical-address width */
>> +
>> + /*
>> + * Update physical-address width.
>> + * Make sure that it does not exceed hardware capabilities.
>> + */
>> + if (cpuid_query_maxphyaddr(vcpu) > boot_cpu_data.x86_phys_bits)
>
> The name maxphyaddr is really misleading. But that is a different story.

Yes, I'll rename it, thanks.

> This check is correct.
>
> However, I wonder if there is any way for user space to query this property?

Do you mean boot_cpu_data.x86_phys_bits?
Userspace can execute CPUID instruction and read the value; QEMU does.

> On s390x, there is a kvm capability to export this information to user
> space. So QEMU can fail (e.g. migration) with a nice error message about
> missing hardware support.
>
> (most probably we still want to block this case, as migration will seem to
> work but than simply fail due to missing hardware support I guess). Maybe
> there is also already a nice check in QEMU that I am not yet aware of :)

This patch is bad. It would break QEMU on all old machines, because
QEMU sets 40 by default.

Heh, QEMU doesn't check at all -- it even allows migration with
"host-phys-bits" feature and will happily change phys-bits when
migrating to another machine.