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@google.com>
---
 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;
+