Re: [PATCH v2] KVM: x86/pt: Ignore all unknown Intel PT capabilities

[Sean Christopherson <seanjc@google.com>]

On Mon, Jan 10, 2022, Like Xu wrote:
> From: Like Xu <likexu@tencent.com>
> 
> Some of the new Intel PT capabilities (e.g. SDM Vol3, 32.2.4 Event
> Tracing, it exposes details about the asynchronous events, when they are
> generated, and when their corresponding software event handler completes
> execution) cannot be safely and fully emulated by the KVM, especially
> emulating the simultaneous writing of guest PT packets generated by
> the KVM to the guest PT buffer.
> 
> For KVM, it's better to advertise currently supported features based on
> the "static struct pt_cap_desc" implemented in the host PT driver and
> ignore _all_ unknown features before they have been investigated one by
> one and supported in a safe manner, leaving the rest as system-wide-only
> tracing capabilities.
> 
> Suggested-by: Paolo Bonzini <pbonzini@redhat.com>
> Signed-off-by: Like Xu <likexu@tencent.com>
> ---
> v1 -> v2 Changelog:
> - Be safe and ignore _all_ unknown capabilities. (Paolo)
> 
> Previous:
> https://lore.kernel.org/kvm/20220106085533.84356-1-likexu@tencent.com/
> 
>  arch/x86/kvm/cpuid.c | 2 ++
>  1 file changed, 2 insertions(+)
> 
> diff --git a/arch/x86/kvm/cpuid.c b/arch/x86/kvm/cpuid.c
> index 0b920e12bb6d..439b93359848 100644
> --- a/arch/x86/kvm/cpuid.c
> +++ b/arch/x86/kvm/cpuid.c
> @@ -901,6 +901,8 @@ static inline int __do_cpuid_func(struct kvm_cpuid_array *array, u32 function)
>  			break;
>  		}
>  
> +		/* It's better to be safe and ignore _all_ unknown capabilities. */

No need to justify why unknown capabilities are hidden as that's very much (supposed
to be) standard KVM behavior.

> +		entry->ebx &= GENMASK(5, 0);

Please add a #define somewhere so that this is self-documenting, e.g. see
KVM_SUPPORTED_XCR0.

And why just EBX?  ECX appears to enumerate features too, and EDX is presumably
reserved to enumerate yet more features when EBX/ECX run out of bits.

And is there any possibility of a malicious user/guest using features to cause
problems in the host?  I.e. does KVM need to enforce that the guest can't enable
any unsupported features?

>  		for (i = 1, max_idx = entry->eax; i <= max_idx; ++i) {
>  			if (!do_host_cpuid(array, function, i))
>  				goto out;
> -- 
> 2.33.1
>