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

[patchbot@xxxxxxx]

commit fd9823faf9df057a69a9a53c2e100691d3f4267c
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 16:56:47 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>
---
 xen/arch/x86/domctl.c                       |  3 ++-
 xen/arch/x86/hvm/vmx/vmx.c                  |  6 ++++++
 xen/arch/x86/msr.c                          | 12 ++++++++++++
 xen/include/public/arch-x86/cpufeatureset.h |  2 +-
 4 files changed, 21 insertions(+), 2 deletions(-)

diff --git a/xen/arch/x86/domctl.c b/xen/arch/x86/domctl.c
index 0423a0cca6..1002659de0 100644
--- a/xen/arch/x86/domctl.c
+++ b/xen/arch/x86/domctl.c
@@ -232,7 +232,8 @@ static int update_domain_cpuid_info(struct domain *d,
          */
         call_policy_changed = (is_hvm_domain(d) &&
                                ((old_7d0 ^ p->feat.raw[0].d) &
-                                cpufeat_mask(X86_FEATURE_IBRSB)));
+                                (cpufeat_mask(X86_FEATURE_IBRSB) |
+                                 cpufeat_mask(X86_FEATURE_L1D_FLUSH))));
         break;
 
     case 0xa:
diff --git a/xen/arch/x86/hvm/vmx/vmx.c b/xen/arch/x86/hvm/vmx/vmx.c
index 8e721dea0f..73f0d526a6 100644
--- a/xen/arch/x86/hvm/vmx/vmx.c
+++ b/xen/arch/x86/hvm/vmx/vmx.c
@@ -569,6 +569,12 @@ static void vmx_cpuid_policy_changed(struct vcpu *v)
         vmx_clear_msr_intercept(v, MSR_PRED_CMD,  VMX_MSR_RW);
     else
         vmx_set_msr_intercept(v, MSR_PRED_CMD,  VMX_MSR_RW);
+
+    /* MSR_FLUSH_CMD is safe to pass through if the guest knows about it. */
+    if ( cp->feat.l1d_flush )
+        vmx_clear_msr_intercept(v, MSR_FLUSH_CMD, VMX_MSR_RW);
+    else
+        vmx_set_msr_intercept(v, MSR_FLUSH_CMD, VMX_MSR_RW);
 }
 
 int vmx_guest_x86_mode(struct vcpu *v)
diff --git a/xen/arch/x86/msr.c b/xen/arch/x86/msr.c
index c2bcaa382a..b675f3ac02 100644
--- a/xen/arch/x86/msr.c
+++ b/xen/arch/x86/msr.c
@@ -123,6 +123,7 @@ int guest_rdmsr(const struct vcpu *v, uint32_t msr, 
uint64_t *val)
     case MSR_AMD_PATCHLOADER:
     case MSR_IA32_UCODE_WRITE:
     case MSR_PRED_CMD:
+    case MSR_FLUSH_CMD:
         /* Write-only */
         goto gp_fault;
 
@@ -221,6 +222,17 @@ int guest_wrmsr(struct vcpu *v, uint32_t msr, uint64_t val)
             wrmsrl(MSR_PRED_CMD, val);
         break;
 
+    case MSR_FLUSH_CMD:
+        if ( !cp->feat.l1d_flush )
+            goto gp_fault; /* MSR available? */
+
+        if ( val & ~FLUSH_CMD_L1D )
+            goto gp_fault; /* Rsvd bit set? */
+
+        if ( v == curr )
+            wrmsrl(MSR_FLUSH_CMD, val);
+        break;
+
     case MSR_INTEL_MISC_FEATURES_ENABLES:
     {
         bool old_cpuid_faulting = msrs->misc_features_enables.cpuid_faulting;
diff --git a/xen/include/public/arch-x86/cpufeatureset.h 
b/xen/include/public/arch-x86/cpufeatureset.h
index 9f4c8246a9..6c82816fd3 100644
--- a/xen/include/public/arch-x86/cpufeatureset.h
+++ b/xen/include/public/arch-x86/cpufeatureset.h
@@ -244,7 +244,7 @@ XEN_CPUFEATURE(AVX512_4VNNIW, 9*32+ 2) /*A  AVX512 Neural 
Network Instructions *
 XEN_CPUFEATURE(AVX512_4FMAPS, 9*32+ 3) /*A  AVX512 Multiply Accumulation 
Single Precision */
 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#master

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