Re: [PATCH v15 06/13] x86/sev: Prevent GUEST_TSC_FREQ MSR interception for Secure TSC enabled guests

Nikunj A. Dadhania
2025-01-01T08:44:53Z

On 12/24/2024 5:23 PM, Borislav Petkov wrote:
> On Wed, Dec 18, 2024 at 10:50:07AM +0530, Nikunj A. Dadhania wrote:
>> With the condition inside the function, even tough the MSR is not
>> valid in this configuration, I am getting value 0. Is this behavior
>> acceptable ?
>
> The whole untested diff, should DTRT this time:

I have tested the diff and ES_UNSUPPORTED causes unexpected termination of SNP guest(without SecureTSC).

$ sudo wrmsr 0x10 0
KVM: unknown exit reason 24
EAX=00000000 EBX=00000000 ECX=00000000 EDX=00a00f11
ESI=00000000 EDI=00000000 EBP=00000000 ESP=00000000
EIP=0000fff0 EFL=00000002 [-------] CPL=0 II=0 A20=1 SMM=0 HLT=0
...

$ sudo wrmsr 0xc0010134 0
KVM: unknown exit reason 24
EAX=00000000 EBX=00000000 ECX=00000000 EDX=00a00f11
ESI=00000000 EDI=00000000 EBP=00000000 ESP=00000000
...

IMO, the below change appropriately handles all the conditions well and does not affect SNP guests without SecureTSC.

diff --git a/arch/x86/coco/sev/core.c b/arch/x86/coco/sev/core.c
index 84e4e64decf7..a8977c68695b 100644
--- a/arch/x86/coco/sev/core.c
+++ b/arch/x86/coco/sev/core.c
@@ -1428,6 +1428,40 @@ static enum es_result __vc_handle_msr_caa(struct pt_regs *regs, bool write)
return ES_OK;
}

+/*
+ * TSC related accesses should not exit to the hypervisor when a guest is
+ * executing with SecureTSC enabled, so special handling is required for
+ * accesses of MSR_IA32_TSC and MSR_AMD64_GUEST_TSC_FREQ:
+ *
+ * Writes: Writing to MSR_IA32_TSC can cause subsequent reads
+ * of the TSC to return undefined values, so ignore all
+ * writes.
+ * Reads: Reads of MSR_IA32_TSC should return the current TSC
+ * value, use the value returned by RDTSC.
+ */
+static enum es_result __vc_handle_msr_tsc(struct pt_regs *regs, bool write)
+{
+ u64 tsc;
+
+ /*
+ * GUEST_TSC_FREQ should not be intercepted when Secure TSC is
+ * enabled. Terminate the SNP guest when the interception is enabled.
+ */
+ if (regs->cx == MSR_AMD64_GUEST_TSC_FREQ)
+ return ES_VMM_ERROR;
+
+ if (write) {
+ WARN_ONCE(1, "TSC MSR writes are verboten!\n");
+ return ES_OK;
+ }
+
+ tsc = rdtsc_ordered();
+ regs->ax = lower_32_bits(tsc);
+ regs->dx = upper_32_bits(tsc);
+
+ return ES_OK;
+}
+
static enum es_result vc_handle_msr(struct ghcb *ghcb, struct es_em_ctxt *ctxt)
{
struct pt_regs *regs = ctxt->regs;
@@ -1437,8 +1471,16 @@ static enum es_result vc_handle_msr(struct ghcb *ghcb, struct es_em_ctxt *ctxt)
/* Is it a WRMSR? */
write = ctxt->insn.opcode.bytes[1] == 0x30;

- if (regs->cx == MSR_SVSM_CAA)
+ switch (regs->cx) {
+ case MSR_SVSM_CAA:
return __vc_handle_msr_caa(regs, write);
+ case MSR_IA32_TSC:
+ case MSR_AMD64_GUEST_TSC_FREQ:
+ if (sev_status & MSR_AMD64_SNP_SECURE_TSC)
+ return __vc_handle_msr_tsc(regs, write);
+ default:
+ break;
+ }

ghcb_set_rcx(ghcb, regs->cx);
if (write) {