[Xen-devel] [PATCH 2/5] x86/pv: Avoid leaking other guests' MSR_TSC_AUX values into PV context

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

If the CPU pipeline supports RDTSCP or RDPID, a guest can observe the value in
MSR_TSC_AUX, irrespective of whether the relevant CPUID features are
advertised/hidden.

At the moment, paravirt_ctxt_switch_to() only writes to MSR_TSC_AUX if
TSC_MODE_PVRDTSCP mode is enabled, but this is not the default mode.
Therefore, default PV guests can read the value from a previously scheduled
HVM vcpu, or TSC_MODE_PVRDTSCP-enabled PV guest.

Alter the PV path to always write to MSR_TSC_AUX, using 0 in the common case.

To amortise overhead cost, introduce wrmsr_tsc_aux() which performs a lazy
update of the MSR, and use this function consistently across the codebase.

Signed-off-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
---
CC: Jan Beulich <JBeulich@xxxxxxxx>
CC: Jun Nakajima <jun.nakajima@xxxxxxxxx>
CC: Kevin Tian <kevin.tian@xxxxxxxxx>
CC: Boris Ostrovsky <boris.ostrovsky@xxxxxxxxxx>
CC: Suravee Suthikulpanit <suravee.suthikulpanit@xxxxxxx>
CC: Wei Liu <wei.liu2@xxxxxxxxxx>
CC: Roger Pau Monné <roger.pau@xxxxxxxxxx>
CC: Konrad Rzeszutek Wilk <konrad.wilk@xxxxxxxxxx>

N.B. This has been deemed not a security issue, because the MSR doesn't store
any sensitive information.
---
 xen/arch/x86/domain.c      |  6 +++---
 xen/arch/x86/hvm/hvm.c     |  2 +-
 xen/arch/x86/hvm/svm/svm.c |  2 +-
 xen/arch/x86/hvm/vmx/vmx.c |  2 +-
 xen/arch/x86/msr.c         |  2 ++
 xen/include/asm-x86/msr.h  | 16 ++++++++++++++--
 6 files changed, 22 insertions(+), 8 deletions(-)

diff --git a/xen/arch/x86/domain.c b/xen/arch/x86/domain.c
index f93327b..9c3527f 100644
--- a/xen/arch/x86/domain.c
+++ b/xen/arch/x86/domain.c
@@ -1533,9 +1533,9 @@ void paravirt_ctxt_switch_to(struct vcpu *v)
     if ( unlikely(v->arch.debugreg[7] & DR7_ACTIVE_MASK) )
         activate_debugregs(v);
 
-    if ( (v->domain->arch.tsc_mode ==  TSC_MODE_PVRDTSCP) &&
-         boot_cpu_has(X86_FEATURE_RDTSCP) )
-        write_rdtscp_aux(v->domain->arch.incarnation);
+    if ( cpu_has_rdtscp )
+        wrmsr_tsc_aux(v->domain->arch.tsc_mode == TSC_MODE_PVRDTSCP
+                      ? v->domain->arch.incarnation : 0);
 }
 
 /* Update per-VCPU guest runstate shared memory area (if registered). */
diff --git a/xen/arch/x86/hvm/hvm.c b/xen/arch/x86/hvm/hvm.c
index 5d39210..0539551 100644
--- a/xen/arch/x86/hvm/hvm.c
+++ b/xen/arch/x86/hvm/hvm.c
@@ -3579,7 +3579,7 @@ int hvm_msr_write_intercept(unsigned int msr, uint64_t 
msr_content,
         v->arch.hvm_vcpu.msr_tsc_aux = (uint32_t)msr_content;
         if ( cpu_has_rdtscp
              && (v->domain->arch.tsc_mode != TSC_MODE_PVRDTSCP) )
-            wrmsrl(MSR_TSC_AUX, (uint32_t)msr_content);
+            wrmsr_tsc_aux(msr_content);
         break;
 
     case MSR_IA32_APICBASE:
diff --git a/xen/arch/x86/hvm/svm/svm.c b/xen/arch/x86/hvm/svm/svm.c
index 9f58afc..f53f430 100644
--- a/xen/arch/x86/hvm/svm/svm.c
+++ b/xen/arch/x86/hvm/svm/svm.c
@@ -1093,7 +1093,7 @@ static void svm_ctxt_switch_to(struct vcpu *v)
     svm_tsc_ratio_load(v);
 
     if ( cpu_has_rdtscp )
-        wrmsrl(MSR_TSC_AUX, hvm_msr_tsc_aux(v));
+        wrmsr_tsc_aux(hvm_msr_tsc_aux(v));
 }
 
 static void noreturn svm_do_resume(struct vcpu *v)
diff --git a/xen/arch/x86/hvm/vmx/vmx.c b/xen/arch/x86/hvm/vmx/vmx.c
index 5cd689e..31acb0e 100644
--- a/xen/arch/x86/hvm/vmx/vmx.c
+++ b/xen/arch/x86/hvm/vmx/vmx.c
@@ -618,7 +618,7 @@ static void vmx_restore_guest_msrs(struct vcpu *v)
     }
 
     if ( cpu_has_rdtscp )
-        wrmsrl(MSR_TSC_AUX, hvm_msr_tsc_aux(v));
+        wrmsr_tsc_aux(hvm_msr_tsc_aux(v));
 }
 
 void vmx_update_cpu_exec_control(struct vcpu *v)
diff --git a/xen/arch/x86/msr.c b/xen/arch/x86/msr.c
index 7875d9c..7ba9a10 100644
--- a/xen/arch/x86/msr.c
+++ b/xen/arch/x86/msr.c
@@ -24,6 +24,8 @@
 #include <xen/sched.h>
 #include <asm/msr.h>
 
+DEFINE_PER_CPU(uint32_t, tsc_aux);
+
 struct msr_domain_policy __read_mostly hvm_max_msr_domain_policy,
                          __read_mostly  pv_max_msr_domain_policy;
 
diff --git a/xen/include/asm-x86/msr.h b/xen/include/asm-x86/msr.h
index 928f1cc..9475a71 100644
--- a/xen/include/asm-x86/msr.h
+++ b/xen/include/asm-x86/msr.h
@@ -115,8 +115,6 @@ static inline uint64_t rdtsc_ordered(void)
     __write_tsc(val);                                           \
 })
 
-#define write_rdtscp_aux(val) wrmsr(MSR_TSC_AUX, (val), 0)
-
 #define rdpmc(counter,low,high) \
      __asm__ __volatile__("rdpmc" \
                          : "=a" (low), "=d" (high) \
@@ -210,6 +208,20 @@ static inline void write_efer(uint64_t val)
 
 DECLARE_PER_CPU(u32, ler_msr);
 
+DECLARE_PER_CPU(uint32_t, tsc_aux);
+
+/* Lazy update of MSR_TSC_AUX */
+static inline void wrmsr_tsc_aux(uint32_t val)
+{
+    uint32_t *this_tsc_aux = &this_cpu(tsc_aux);
+
+    if ( *this_tsc_aux != val )
+    {
+        wrmsr(MSR_TSC_AUX, val, 0);
+        *this_tsc_aux = val;
+    }
+}
+
 /* MSR policy object for shared per-domain MSRs */
 struct msr_domain_policy
 {
-- 
2.1.4


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