On 7/2/2026 1:02 AM, 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.
>
> Reviewed-by: David Woodhouse <dwmw@amazon.co.uk>
> Signed-off-by: Sean Christopherson <seanjc@google.com>
Reviewed-by: Nikunj A Dadhania <nikunj@amd.com>
> ---
> arch/x86/kernel/tsc.c | 9 +++++++++
> 1 file changed, 9 insertions(+)
>
> diff --git a/arch/x86/kernel/tsc.c b/arch/x86/kernel/tsc.c
> index 012321fed5e5..a146fc7b5e74 100644
> --- a/arch/x86/kernel/tsc.c
> +++ b/arch/x86/kernel/tsc.c
> @@ -283,6 +283,15 @@ bool using_native_sched_clock(void)
> 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;
> +
> if (!stable)
> clear_sched_clock_stable();
>