Subject: x86/vtx: Fix the checking for unknown/invalid MSR_DEBUGCTL bits
From: Andrew Cooper <[email-masked]>

The VPMU_MODE_OFF early-exit in vpmu_do_wrmsr() introduced by c/s
11fe998e56 bypasses all reserved bit checking in the general case.  As a
result, a guest can enable BTS when it shouldn't be permitted to, and
lock up the entire host.

With vPMU active (not a security supported configuration, but useful for
debugging), the reserved bit checking in broken, caused by the original
BTS changeset 1a8aa75ed.

From a correctness standpoint, it is not possible to have two different
pieces of code responsible for different parts of value checking, if
there isn't an accumulation of bits which have been checked.  A
practical upshot of this is that a guest can set any value it
wishes (usually resulting in a vmentry failure for bad guest state).

Therefore, fix this by implementing all the reserved bit checking in the
main MSR_DEBUGCTL block, and removing all handling of DEBUGCTL from the
vPMU MSR logic.

This is XSA-269

Signed-off-by: Andrew Cooper <[email-masked]>
Reviewed-by: Jan Beulich <[email-masked]>

diff --git a/xen/arch/x86/cpu/vpmu_intel.c b/xen/arch/x86/cpu/vpmu_intel.c
index 7f9add1..6276c10 100644
--- a/xen/arch/x86/cpu/vpmu_intel.c
+++ b/xen/arch/x86/cpu/vpmu_intel.c
@@ -68,10 +68,6 @@
 #define MSR_PMC_ALIAS_MASK       (~(MSR_IA32_PERFCTR0 ^ MSR_IA32_A_PERFCTR0))
 static bool_t __read_mostly full_width_write;
 
-/* Intel-specific VPMU features */
-#define VPMU_CPU_HAS_DS                     0x100 /* Has Debug Store */
-#define VPMU_CPU_HAS_BTS                    0x200 /* Has Branch Trace Store */
-
 /*
  * MSR_CORE_PERF_FIXED_CTR_CTRL contains the configuration of all fixed
  * counters. 4 bits for every counter.
@@ -563,27 +559,7 @@ static int core2_vpmu_do_wrmsr(unsigned int msr, uint64_t msr_content,
     uint64_t *enabled_cntrs;
 
     if ( !core2_vpmu_msr_common_check(msr, &type, &index) )
-    {
-        /* Special handling for BTS */
-        if ( msr == MSR_IA32_DEBUGCTLMSR )
-        {
-            supported |= IA32_DEBUGCTLMSR_TR | IA32_DEBUGCTLMSR_BTS |
-                         IA32_DEBUGCTLMSR_BTINT;
-
-            if ( cpu_has(&current_cpu_data, X86_FEATURE_DSCPL) )
-                supported |= IA32_DEBUGCTLMSR_BTS_OFF_OS |
-                             IA32_DEBUGCTLMSR_BTS_OFF_USR;
-            if ( !(msr_content & ~supported) &&
-                 vpmu_is_set(vpmu, VPMU_CPU_HAS_BTS) )
-                return 0;
-            if ( (msr_content & supported) &&
-                 !vpmu_is_set(vpmu, VPMU_CPU_HAS_BTS) )
-                printk(XENLOG_G_WARNING
-                       "%pv: Debug Store unsupported on this CPU\n",
-                       current);
-        }
         return -EINVAL;
-    }
 
     ASSERT(!supported);
 
diff --git a/xen/arch/x86/hvm/vmx/vmx.c b/xen/arch/x86/hvm/vmx/vmx.c
index 955643c..d07d941 100644
--- a/xen/arch/x86/hvm/vmx/vmx.c
+++ b/xen/arch/x86/hvm/vmx/vmx.c
@@ -2976,6 +2976,8 @@ static int vmx_msr_write_intercept(unsigned int msr, uint64_t msr_content)
 
     switch ( msr )
     {
+        uint64_t rsvd;
+
     case MSR_IA32_SYSENTER_CS:
         __vmwrite(GUEST_SYSENTER_CS, msr_content);
         break;
@@ -2990,17 +2992,29 @@ static int vmx_msr_write_intercept(unsigned int msr, uint64_t msr_content)
         __vmwrite(GUEST_SYSENTER_EIP, msr_content);
         break;
     case MSR_IA32_DEBUGCTLMSR: {
+        uint32_t ebx, ecx = 0;
         int i, rc = 0;
-        uint64_t supported = IA32_DEBUGCTLMSR_LBR | IA32_DEBUGCTLMSR_BTF;
 
-        if ( boot_cpu_has(X86_FEATURE_RTM) )
-            supported |= IA32_DEBUGCTLMSR_RTM;
-        if ( msr_content & ~supported )
+        rsvd = ~(IA32_DEBUGCTLMSR_LBR | IA32_DEBUGCTLMSR_BTF);
+
+        /* TODO: Wire vPMU settings properly through the CPUID policy */
+        if ( vpmu_is_set(vcpu_vpmu(v), VPMU_CPU_HAS_BTS) )
         {
-            /* Perhaps some other bits are supported in vpmu. */
-            if ( vpmu_do_wrmsr(msr, msr_content, supported) )
-                break;
+            rsvd &= ~(IA32_DEBUGCTLMSR_TR | IA32_DEBUGCTLMSR_BTS |
+                      IA32_DEBUGCTLMSR_BTINT);
+
+            if ( cpu_has(&current_cpu_data, X86_FEATURE_DSCPL) )
+                rsvd &= ~(IA32_DEBUGCTLMSR_BTS_OFF_OS |
+                          IA32_DEBUGCTLMSR_BTS_OFF_USR);
         }
+
+        hvm_cpuid(7, NULL, &ebx, &ecx, NULL);
+        if ( ebx & cpufeat_mask(X86_FEATURE_RTM) )
+            rsvd &= ~IA32_DEBUGCTLMSR_RTM;
+
+        if ( msr_content & rsvd )
+            goto gp_fault;
+
         if ( msr_content & IA32_DEBUGCTLMSR_LBR )
         {
             const struct lbr_info *lbr = last_branch_msr_get();
diff --git a/xen/include/asm-x86/vpmu.h b/xen/include/asm-x86/vpmu.h
index ed9ec07..75b1973 100644
--- a/xen/include/asm-x86/vpmu.h
+++ b/xen/include/asm-x86/vpmu.h
@@ -77,6 +77,10 @@ struct vpmu_struct {
 /* PV(H) guests: VPMU registers are accessed by guest from shared page */
 #define VPMU_CACHED                         0x40
 
+/* Intel-specific VPMU features */
+#define VPMU_CPU_HAS_DS                     0x100 /* Has Debug Store */
+#define VPMU_CPU_HAS_BTS                    0x200 /* Has Branch Trace Store */
+
 static inline void vpmu_set(struct vpmu_struct *vpmu, const u32 mask)
 {
     vpmu->flags |= mask;