[Xen-devel] [PATCH v2] x86: ignore guest microcode loading attempts

["Jan Beulich" <JBeulich@xxxxxxxx>]

The respective MSRs are write-only, and hence attempts by guests to
write to these are - as of 1f1d183d49 ("x86/HVM: don't give the wrong
impression of WRMSR succeeding") no longer ignored. Restore original
behavior for the two affected MSRs.

Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
---
v2: Add comments.
---
While what is being logged for the current osstest failures on the 4.7
and 4.9 branches doesn't fully prove this to be the problem, RCX holding
0x79 and there being a recorded hypervisor level #GP recovery
immediately before the guest triple fault is sufficient indication imo.
What I'm unsure about is whether we want to ignore such writes also for
PV guests. If not, at least the WRMSR change would need to move into
hvm/hvm.c.

--- a/xen/arch/x86/msr.c
+++ b/xen/arch/x86/msr.c
@@ -147,6 +147,8 @@ int guest_rdmsr(const struct vcpu *v, ui
 
     switch ( msr )
     {
+    case MSR_AMD_PATCHLOADER:
+    case MSR_IA32_UCODE_WRITE:
     case MSR_PRED_CMD:
         /* Write-only */
         goto gp_fault;
@@ -200,6 +202,26 @@ int guest_wrmsr(struct vcpu *v, uint32_t
         /* Read-only */
         goto gp_fault;
 
+    case MSR_AMD_PATCHLOADER:
+        /*
+         * See note on MSR_IA32_UCODE_WRITE below, which may or may not apply
+         * to AMD CPUs as well (at least the architectural/CPUID part does).
+         */
+        if ( d->arch.cpuid->x86_vendor != X86_VENDOR_AMD )
+            goto gp_fault;
+        break;
+
+    case MSR_IA32_UCODE_WRITE:
+        /*
+         * Some versions of Windows at least on certain hardware try to load
+         * microcode before setting up an IDT. Therefore we must not inject #GP
+         * for such attempts. Also the MSR is architectural and not qualified
+         * by any CPUID bit.
+         */
+        if ( d->arch.cpuid->x86_vendor != X86_VENDOR_INTEL )
+            goto gp_fault;
+        break;
+
     case MSR_SPEC_CTRL:
         if ( !cp->feat.ibrsb )
             goto gp_fault; /* MSR available? */




_______________________________________________
Xen-devel mailing list
Xen-devel@xxxxxxxxxxxxxxxxxxxx
https://lists.xenproject.org/mailman/listinfo/xen-devel