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 1fc79c9..6e27f6e 100644
--- a/xen/arch/x86/cpu/vpmu_intel.c
+++ b/xen/arch/x86/cpu/vpmu_intel.c
@@ -533,27 +533,7 @@ 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 12e0ee5..8da05f5 100644
--- a/xen/arch/x86/hvm/vmx/vmx.c
+++ b/xen/arch/x86/hvm/vmx/vmx.c
@@ -3081,11 +3081,14 @@ void vmx_vlapic_msr_changed(struct vcpu *v)
 static int vmx_msr_write_intercept(unsigned int msr, uint64_t msr_content)
 {
     struct vcpu *v = current;
+    const struct cpuid_policy *cp = v->domain->arch.cpuid;
 
     HVM_DBG_LOG(DBG_LEVEL_MSR, "ecx=%#x, msr_value=%#"PRIx64, msr, msr_content);
 
     switch ( msr )
     {
+        uint64_t rsvd;
+
     case MSR_IA32_SYSENTER_CS:
         __vmwrite(GUEST_SYSENTER_CS, msr_content);
         break;
@@ -3138,18 +3141,26 @@ static int vmx_msr_write_intercept(unsigned int msr, uint64_t msr_content)
         wrmsrl(MSR_SYSCALL_MASK, msr_content);
         break;
 
-    case MSR_IA32_DEBUGCTLMSR: {
-        uint64_t supported = IA32_DEBUGCTLMSR_LBR | IA32_DEBUGCTLMSR_BTF;
+    case MSR_IA32_DEBUGCTLMSR:
+        rsvd = ~(IA32_DEBUGCTLMSR_LBR | IA32_DEBUGCTLMSR_BTF);
 
-        if ( boot_cpu_has(X86_FEATURE_RTM) )
-            supported |= IA32_DEBUGCTLMSR_RTM;
-        if ( msr_content & ~supported )
+        /* 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);
         }
 
+        if ( cp->feat.rtm )
+            rsvd &= ~IA32_DEBUGCTLMSR_RTM;
+
+        if ( msr_content & rsvd )
+            goto gp_fault;
+
         /*
          * When a guest first enables LBR, arrange to save and restore the LBR
          * MSRs and allow the guest direct access.
@@ -3208,7 +3219,7 @@ static int vmx_msr_write_intercept(unsigned int msr, uint64_t msr_content)
 
         __vmwrite(GUEST_IA32_DEBUGCTL, msr_content);
         break;
-    }
+
     case MSR_IA32_FEATURE_CONTROL:
     case MSR_IA32_VMX_BASIC ... MSR_IA32_VMX_VMFUNC:
         /* None of these MSRs are writeable. */