[PATCH v6 47/51] x86/paravirt: Don't use a PV sched_clock in CoCo guests with trusted TSC

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 <[email protected]>
Reviewed-by: Nikunj A Dadhania <[email protected]>
Signed-off-by: Sean Christopherson <[email protected]>
---
 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 fcf331ffd874..c0e4485484f2 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();
 
-- 
2.55.0.679.g6767b8d81c-goog