6.16-stable review patch. If anyone has any objections, please let me know.
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From: Nikunj A Dadhania nikunj@amd.com
[ Upstream commit 5eb1bcdb6a8c088514019c3a9bda5d565beed1af ]
Currently, when a Secure TSC enabled SNP guest attempts to write to the intercepted GUEST_TSC_FREQ MSR (a read-only MSR), the guest kernel response incorrectly implies a VMM configuration error, when in fact it is the usual VMM configuration to intercept writes to read-only MSRs, unless explicitly documented.
Modify the intercepted TSC MSR #VC handling: * Write to GUEST_TSC_FREQ will generate a #GP instead of terminating the guest * Write to MSR_IA32_TSC will generate a #GP instead of silently ignoring it
However, continue to terminate the guest when reading from intercepted GUEST_TSC_FREQ MSR with Secure TSC enabled, as intercepted reads indicate an improper VMM configuration for Secure TSC enabled SNP guests.
[ bp: simplify comment. ]
Fixes: 38cc6495cdec ("x86/sev: Prevent GUEST_TSC_FREQ MSR interception for Secure TSC enabled guests") Suggested-by: Sean Christopherson seanjc@google.com Signed-off-by: Nikunj A Dadhania nikunj@amd.com Signed-off-by: Borislav Petkov (AMD) bp@alien8.de Reviewed-by: Tom Lendacky thomas.lendacky@amd.com Link: https://lore.kernel.org/20250722074853.22253-1-nikunj@amd.com Signed-off-by: Sasha Levin sashal@kernel.org --- arch/x86/coco/sev/vc-handle.c | 31 ++++++++++++++++--------------- 1 file changed, 16 insertions(+), 15 deletions(-)
diff --git a/arch/x86/coco/sev/vc-handle.c b/arch/x86/coco/sev/vc-handle.c index faf1fce89ed4..c3b4acbde0d8 100644 --- a/arch/x86/coco/sev/vc-handle.c +++ b/arch/x86/coco/sev/vc-handle.c @@ -371,29 +371,30 @@ static enum es_result __vc_handle_msr_caa(struct pt_regs *regs, bool write) * executing with Secure TSC enabled, so special handling is required for * accesses of MSR_IA32_TSC and MSR_AMD64_GUEST_TSC_FREQ. */ -static enum es_result __vc_handle_secure_tsc_msrs(struct pt_regs *regs, bool write) +static enum es_result __vc_handle_secure_tsc_msrs(struct es_em_ctxt *ctxt, bool write) { + struct pt_regs *regs = ctxt->regs; u64 tsc;
/* - * GUEST_TSC_FREQ should not be intercepted when Secure TSC is enabled. - * Terminate the SNP guest when the interception is enabled. + * Writing to MSR_IA32_TSC can cause subsequent reads of the TSC to + * return undefined values, and GUEST_TSC_FREQ is read-only. Generate + * a #GP on all writes. */ - if (regs->cx == MSR_AMD64_GUEST_TSC_FREQ) - return ES_VMM_ERROR; + if (write) { + ctxt->fi.vector = X86_TRAP_GP; + ctxt->fi.error_code = 0; + return ES_EXCEPTION; + }
/* - * 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_ordered(). + * GUEST_TSC_FREQ read should not be intercepted when Secure TSC is + * enabled. Terminate the guest if a read is attempted. */ - if (write) { - WARN_ONCE(1, "TSC MSR writes are verboten!\n"); - return ES_OK; - } + if (regs->cx == MSR_AMD64_GUEST_TSC_FREQ) + return ES_VMM_ERROR;
+ /* Reads of MSR_IA32_TSC should return the current TSC value. */ tsc = rdtsc_ordered(); regs->ax = lower_32_bits(tsc); regs->dx = upper_32_bits(tsc); @@ -416,7 +417,7 @@ static enum es_result vc_handle_msr(struct ghcb *ghcb, struct es_em_ctxt *ctxt) case MSR_IA32_TSC: case MSR_AMD64_GUEST_TSC_FREQ: if (sev_status & MSR_AMD64_SNP_SECURE_TSC) - return __vc_handle_secure_tsc_msrs(regs, write); + return __vc_handle_secure_tsc_msrs(ctxt, write); break; default: break;