Re: [PATCH v2 30/38] x86/paravirt: Don't use a PV sched_clock in CoCo guests with trusted TSC

[Kirill A. Shutemov]

On Wed, Feb 26, 2025 at 06:18:46PM -0800, Sean Christopherson wrote:
> Silently ignore attempts to switch to a paravirt sched_clock when running
> as a CoCo guest with trusted TSC.  In hand-wavy theory, a misbehaving
> hypervisor could attack the guest by manipulating the PV clock to affect
> guest scheduling in some weird and/or predictable way.  More importantly,
> reading TSC on such platforms is faster than any PV clock, and sched_clock
> is all about speed.
> 
> Signed-off-by: Sean Christopherson <seanjc@xxxxxxxxxx>
> ---
>  arch/x86/kernel/paravirt.c | 9 +++++++++
>  1 file changed, 9 insertions(+)
> 
> diff --git a/arch/x86/kernel/paravirt.c b/arch/x86/kernel/paravirt.c
> index a3a1359cfc26..c538c608d9fb 100644
> --- a/arch/x86/kernel/paravirt.c
> +++ b/arch/x86/kernel/paravirt.c
> @@ -89,6 +89,15 @@ DEFINE_STATIC_CALL(pv_sched_clock, native_sched_clock);
>  int __init __paravirt_set_sched_clock(u64 (*func)(void), bool stable,
>  				      void (*save)(void), void (*restore)(void))
>  {
> +	/*
> +	 * Don't replace TSC with a PV clock when running as a CoCo guest and
> +	 * the TSC is secure/trusted; PV clocks are emulated by the hypervisor,
> +	 * which isn't in the guest's TCB.
> +	 */
> +	if (cc_platform_has(CC_ATTR_GUEST_SNP_SECURE_TSC) ||
> +	    boot_cpu_has(X86_FEATURE_TDX_GUEST))
> +		return -EPERM;
> +

Looks like a call for generic CC_ATTR_GUEST_SECURE_TSC that would be true
for TDX and SEV with CC_ATTR_GUEST_SNP_SECURE_TSC.

>  	if (!stable)
>  		clear_sched_clock_stable();
>  
> -- 
> 2.48.1.711.g2feabab25a-goog
> 

-- 
  Kiryl Shutsemau / Kirill A. Shutemov