[Xen-changelog] [xen stable-4.8] x86/msr: Virtualise MSR_FLUSH_CMD for guests

[patchbot@xxxxxxx]

commit 1f56fba4865d9acbd075df88ec67ee6358f11b63
Author:     Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
AuthorDate: Fri Apr 13 15:34:01 2018 +0000
Commit:     Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
CommitDate: Tue Aug 14 17:27:27 2018 +0100

    x86/msr: Virtualise MSR_FLUSH_CMD for guests
    
    Guests (outside of the nested virt case, which isn't supported yet) don't 
need
    L1D_FLUSH for their L1TF mitigations, but offering/emulating MSR_FLUSH_CMD 
is
    easy and doesn't pose an issue for Xen.
    
    The MSR is offered to HVM guests only.  PV guests attempting to use it would
    trap for emulation, and the L1D cache would fill long before the return to
    guest context.  As such, PV guests can't make any use of the L1D_FLUSH
    functionality.
    
    This is part of XSA-273 / CVE-2018-3646.
    
    Signed-off-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
    Reviewed-by: Jan Beulich <jbeulich@xxxxxxxx>
    (cherry picked from commit fd9823faf9df057a69a9a53c2e100691d3f4267c)
---
 xen/arch/x86/hvm/hvm.c                      | 12 ++++++++++++
 xen/arch/x86/hvm/vmx/vmx.c                  |  6 ++++++
 xen/arch/x86/traps.c                        |  5 +++++
 xen/include/public/arch-x86/cpufeatureset.h |  2 +-
 4 files changed, 24 insertions(+), 1 deletion(-)

diff --git a/xen/arch/x86/hvm/hvm.c b/xen/arch/x86/hvm/hvm.c
index 56823bd616..3e74616185 100644
--- a/xen/arch/x86/hvm/hvm.c
+++ b/xen/arch/x86/hvm/hvm.c
@@ -3933,6 +3933,7 @@ int hvm_msr_read_intercept(unsigned int msr, uint64_t 
*msr_content)
     case MSR_AMD_PATCHLOADER:
     case MSR_IA32_UCODE_WRITE:
     case MSR_PRED_CMD:
+    case MSR_FLUSH_CMD:
         /* Write-only */
         goto gp_fault;
 
@@ -4164,6 +4165,17 @@ int hvm_msr_write_intercept(unsigned int msr, uint64_t 
msr_content,
         wrmsrl(MSR_PRED_CMD, msr_content);
         break;
 
+    case MSR_FLUSH_CMD:
+        hvm_cpuid(7, NULL, NULL, NULL, &edx);
+        if ( !(edx & cpufeat_mask(X86_FEATURE_L1D_FLUSH)) )
+            goto gp_fault; /* MSR available? */
+
+        if ( msr_content & ~FLUSH_CMD_L1D )
+            goto gp_fault; /* Rsvd bit set? */
+
+        wrmsrl(MSR_FLUSH_CMD, msr_content);
+        break;
+
     case MSR_ARCH_CAPABILITIES:
         /* Read-only */
         goto gp_fault;
diff --git a/xen/arch/x86/hvm/vmx/vmx.c b/xen/arch/x86/hvm/vmx/vmx.c
index 2cac5b7eb0..37f6211b15 100644
--- a/xen/arch/x86/hvm/vmx/vmx.c
+++ b/xen/arch/x86/hvm/vmx/vmx.c
@@ -575,6 +575,12 @@ static void vmx_cpuid_policy_changed(struct vcpu *v)
         vmx_disable_intercept_for_msr(v, MSR_PRED_CMD, MSR_TYPE_R | 
MSR_TYPE_W);
     else
         vmx_enable_intercept_for_msr(v, MSR_PRED_CMD, MSR_TYPE_R | MSR_TYPE_W);
+
+    /* MSR_FLUSH_CMD is safe to pass through if the guest knows about it. */
+    if ( _7d0 & cpufeat_mask(X86_FEATURE_L1D_FLUSH) )
+        vmx_disable_intercept_for_msr(v, MSR_FLUSH_CMD, MSR_TYPE_R | 
MSR_TYPE_W);
+    else
+        vmx_enable_intercept_for_msr(v, MSR_FLUSH_CMD, MSR_TYPE_R | 
MSR_TYPE_W);
 }
 
 static int vmx_guest_x86_mode(struct vcpu *v)
diff --git a/xen/arch/x86/traps.c b/xen/arch/x86/traps.c
index 027ebc1d9b..1a22895907 100644
--- a/xen/arch/x86/traps.c
+++ b/xen/arch/x86/traps.c
@@ -2507,6 +2507,7 @@ static int priv_op_read_msr(unsigned int reg, uint64_t 
*val,
         return X86EMUL_OKAY;
 
     case MSR_PRED_CMD:
+    case MSR_FLUSH_CMD:
         /* Write-only */
         break;
 
@@ -2768,6 +2769,10 @@ static int priv_op_write_msr(unsigned int reg, uint64_t 
val,
         wrmsrl(MSR_PRED_CMD, val);
         return X86EMUL_OKAY;
 
+    case MSR_FLUSH_CMD:
+        /* Not available to PV guests. */
+        break;
+
     case MSR_INTEL_MISC_FEATURES_ENABLES:
         if ( !boot_cpu_has(X86_FEATURE_MSR_MISC_FEATURES) ||
              (val & ~MSR_MISC_FEATURES_CPUID_FAULTING) )
diff --git a/xen/include/public/arch-x86/cpufeatureset.h 
b/xen/include/public/arch-x86/cpufeatureset.h
index f5e99f56cc..5eedd075dd 100644
--- a/xen/include/public/arch-x86/cpufeatureset.h
+++ b/xen/include/public/arch-x86/cpufeatureset.h
@@ -238,7 +238,7 @@ XEN_CPUFEATURE(IBPB,          8*32+12) /*A  IBPB support 
only (no IBRS, used by
 /* Intel-defined CPU features, CPUID level 0x00000007:0.edx, word 9 */
 XEN_CPUFEATURE(IBRSB,         9*32+26) /*A  IBRS and IBPB support (used by 
Intel) */
 XEN_CPUFEATURE(STIBP,         9*32+27) /*A! STIBP */
-XEN_CPUFEATURE(L1D_FLUSH,     9*32+28) /*   MSR_FLUSH_CMD and L1D flush. */
+XEN_CPUFEATURE(L1D_FLUSH,     9*32+28) /*S  MSR_FLUSH_CMD and L1D flush. */
 XEN_CPUFEATURE(ARCH_CAPS,     9*32+29) /*   IA32_ARCH_CAPABILITIES MSR */
 XEN_CPUFEATURE(SSBD,          9*32+31) /*A  MSR_SPEC_CTRL.SSBD available */
 
--
generated by git-patchbot for /home/xen/git/xen.git#stable-4.8

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