KVM: SVM: Probe and load MSR_TSC_AUX regardless of RDTSCP support in host

Probe MSR_TSC_AUX whether or not RDTSCP is supported in the host, and
if probing succeeds, load the guest's MSR_TSC_AUX into hardware prior to
VMRUN.  Because SVM doesn't support interception of RDPID, RDPID cannot
be disallowed in the guest (without resorting to binary translation).
Leaving the host's MSR_TSC_AUX in hardware would leak the host's value to
the guest if RDTSCP is not supported.

Note, there is also a kernel bug that prevents leaking the host's value.
The host kernel initializes MSR_TSC_AUX if and only if RDTSCP is
supported, even though the vDSO usage consumes MSR_TSC_AUX via RDPID.
I.e. if RDTSCP is not supported, there is no host value to leak.  But,
if/when the host kernel bug is fixed, KVM would start leaking MSR_TSC_AUX
in the case where hardware supports RDPID but RDTSCP is unavailable for
whatever reason.

Probing MSR_TSC_AUX will also allow consolidating the probe and define
logic in common x86, and will make it simpler to condition the existence
of MSR_TSX_AUX (from the guest's perspective) on RDTSCP *or* RDPID.

Cc: stable@vger.kernel.org
Signed-off-by: Sean Christopherson <seanjc@google.com>
Message-Id: <20210504171734.1434054-7-seanjc@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

1 files changed, 10 insertions, 8 deletions

diff --git a/arch/x86/kvm/svm/svm.c b/arch/x86/kvm/svm/svm.c
index ebcb5849d69b..13e4dd128177 100644
--- a/arch/x86/kvm/svm/svm.c
+++ b/arch/x86/kvm/svm/svm.c
@@ -212,7 +212,7 @@ DEFINE_PER_CPU(struct svm_cpu_data *, svm_data);
  * RDTSCP and RDPID are not used in the kernel, specifically to allow KVM to
  * defer the restoration of TSC_AUX until the CPU returns to userspace.
  */
-#define TSC_AUX_URET_SLOT	0
+static int tsc_aux_uret_slot __read_mostly = -1;
 
 static const u32 msrpm_ranges[] = {0, 0xc0000000, 0xc0010000};
 
@@ -959,8 +959,10 @@ static __init int svm_hardware_setup(void)
 		kvm_tsc_scaling_ratio_frac_bits = 32;
 	}
 
-	if (boot_cpu_has(X86_FEATURE_RDTSCP))
-		kvm_define_user_return_msr(TSC_AUX_URET_SLOT, MSR_TSC_AUX);
+	if (!kvm_probe_user_return_msr(MSR_TSC_AUX)) {
+		tsc_aux_uret_slot = 0;
+		kvm_define_user_return_msr(tsc_aux_uret_slot, MSR_TSC_AUX);
+	}
 
 	/* Check for pause filtering support */
 	if (!boot_cpu_has(X86_FEATURE_PAUSEFILTER)) {
@@ -1454,8 +1456,8 @@ static void svm_prepare_guest_switch(struct kvm_vcpu *vcpu)
 		}
 	}
 
-	if (static_cpu_has(X86_FEATURE_RDTSCP))
-		kvm_set_user_return_msr(TSC_AUX_URET_SLOT, svm->tsc_aux, -1ull);
+	if (likely(tsc_aux_uret_slot >= 0))
+		kvm_set_user_return_msr(tsc_aux_uret_slot, svm->tsc_aux, -1ull);
 
 	svm->guest_state_loaded = true;
 }
@@ -2664,7 +2666,7 @@ static int svm_get_msr(struct kvm_vcpu *vcpu, struct msr_data *msr_info)
 			msr_info->data |= (u64)svm->sysenter_esp_hi << 32;
 		break;
 	case MSR_TSC_AUX:
-		if (!boot_cpu_has(X86_FEATURE_RDTSCP))
+		if (tsc_aux_uret_slot < 0)
 			return 1;
 		if (!msr_info->host_initiated &&
 		    !guest_cpuid_has(vcpu, X86_FEATURE_RDTSCP))
@@ -2885,7 +2887,7 @@ static int svm_set_msr(struct kvm_vcpu *vcpu, struct msr_data *msr)
 		svm->sysenter_esp_hi = guest_cpuid_is_intel(vcpu) ? (data >> 32) : 0;
 		break;
 	case MSR_TSC_AUX:
-		if (!boot_cpu_has(X86_FEATURE_RDTSCP))
+		if (tsc_aux_uret_slot < 0)
 			return 1;
 
 		if (!msr->host_initiated &&
@@ -2908,7 +2910,7 @@ static int svm_set_msr(struct kvm_vcpu *vcpu, struct msr_data *msr)
 		 * guest via direct_access_msrs, and switch it via user return.
 		 */
 		preempt_disable();
-		r = kvm_set_user_return_msr(TSC_AUX_URET_SLOT, data, -1ull);
+		r = kvm_set_user_return_msr(tsc_aux_uret_slot, data, -1ull);
 		preempt_enable();
 		if (r)
 			return 1;