[11/16] x86/paravirt: Don't use a PV sched_clock in CoCo guests with trusted TSC

Commit Message

Sean Christopherson Feb. 1, 2025, 2:17 a.m. UTC

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 <[e-mail]>
---
 arch/x86/kernel/paravirt.c | 9 +++++++++
 1 file changed, 9 insertions(+)

Patch

diff --git a/arch/x86/kernel/paravirt.c b/arch/x86/kernel/paravirt.c
index 55c819673a9d..980440d34997 100644
--- a/arch/x86/kernel/paravirt.c
+++ b/arch/x86/kernel/paravirt.c
@@ -88,6 +88,15 @@  DEFINE_STATIC_CALL(pv_sched_clock, native_sched_clock);
 
 void __paravirt_set_sched_clock(u64 (*func)(void), bool stable)
 {
+	/*
+	 * 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;
+
 	if (!stable)
 		clear_sched_clock_stable();