Subject: x86: avoid leaking PKRU and BND* between vCPU-s
From: Jan Beulich <[email-masked]>

PKRU is explicitly "XSAVE-managed but not XSAVE-enabled", so guests
might access the register (via {RD,WR}PKRU) without setting XCR0.PKRU.
Force context switching as well as migrating the register as soon as
CR4.PKE is being set the first time.

For MPX (BND<n>, BNDCFGU, and BNDSTATUS) the situation is less clear,
and the SDM has not entirely consistent information for that case.
While experimentally the instructions don't change register state as
long as the two XCR0 bits aren't both 1, be on the safe side and enable
both if BNDCFGS.EN is being set the first time.

This is XSA-220.

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

--- a/xen/arch/x86/hvm/hvm.c
+++ b/xen/arch/x86/hvm/hvm.c
@@ -2452,6 +2452,27 @@ int hvm_set_cr4(unsigned long value, boo
             paging_update_paging_modes(v);
     }
 
+    /*
+     * {RD,WR}PKRU are not gated on XCR0.PKRU and hence an oddly behaving
+     * guest may enable the feature in CR4 without enabling it in XCR0. We
+     * need to context switch / migrate PKRU nevertheless.
+     */
+    if ( (value & X86_CR4_PKE) && !(v->arch.xcr0_accum & XSTATE_PKRU) )
+    {
+        int rc = handle_xsetbv(XCR_XFEATURE_ENABLED_MASK,
+                               get_xcr0() | XSTATE_PKRU);
+
+        if ( rc )
+        {
+            HVM_DBG_LOG(DBG_LEVEL_1, "Failed to force XCR0.PKRU: %d", rc);
+            goto gpf;
+        }
+
+        if ( handle_xsetbv(XCR_XFEATURE_ENABLED_MASK,
+                           get_xcr0() & ~XSTATE_PKRU) )
+            /* nothing, best effort only */;
+    }
+
     return X86EMUL_OKAY;
 
  gpf:
--- a/xen/arch/x86/hvm/vmx/vmx.c
+++ b/xen/arch/x86/hvm/vmx/vmx.c
@@ -31,6 +31,7 @@
 #include <asm/regs.h>
 #include <asm/cpufeature.h>
 #include <asm/processor.h>
+#include <asm/xstate.h>
 #include <asm/guest_access.h>
 #include <asm/debugreg.h>
 #include <asm/msr.h>
@@ -783,6 +784,45 @@ static int vmx_load_vmcs_ctxt(struct vcp
     return 0;
 }
 
+static bool_t vmx_set_guest_bndcfgs(struct vcpu *v, u64 val)
+{
+    if ( !cpu_has_mpx || !cpu_has_vmx_mpx ||
+         !is_canonical_address(val) ||
+         (val & IA32_BNDCFGS_RESERVED) )
+        return 0;
+
+    /*
+     * While MPX instructions are supposed to be gated on XCR0.BND*, let's
+     * nevertheless force the relevant XCR0 bits on when the feature is being
+     * enabled in BNDCFGS.
+     */
+    if ( (val & IA32_BNDCFGS_ENABLE) &&
+         !(v->arch.xcr0_accum & (XSTATE_BNDREGS | XSTATE_BNDCSR)) )
+    {
+        uint64_t xcr0 = get_xcr0();
+        int rc;
+
+        if ( v != current )
+            return 0;
+
+        rc = handle_xsetbv(XCR_XFEATURE_ENABLED_MASK,
+                           xcr0 | XSTATE_BNDREGS | XSTATE_BNDCSR);
+
+        if ( rc )
+        {
+            HVM_DBG_LOG(DBG_LEVEL_1, "Failed to force XCR0.BND*: %d", rc);
+            return 0;
+        }
+
+        if ( handle_xsetbv(XCR_XFEATURE_ENABLED_MASK, xcr0) )
+            /* nothing, best effort only */;
+    }
+
+    __vmwrite(GUEST_BNDCFGS, val);
+
+    return 1;
+}
+
 static unsigned int __init vmx_init_msr(void)
 {
     return (cpu_has_mpx && cpu_has_vmx_mpx) +
@@ -822,11 +862,8 @@ static int vmx_load_msr(struct vcpu *v,
         switch ( ctxt->msr[i].index )
         {
         case MSR_IA32_BNDCFGS:
-            if ( cpu_has_mpx && cpu_has_vmx_mpx &&
-                 is_canonical_address(ctxt->msr[i].val) &&
-                 !(ctxt->msr[i].val & IA32_BNDCFGS_RESERVED) )
-                __vmwrite(GUEST_BNDCFGS, ctxt->msr[i].val);
-            else if ( ctxt->msr[i].val )
+            if ( !vmx_set_guest_bndcfgs(v, ctxt->msr[i].val) &&
+                 ctxt->msr[i].val )
                 err = -ENXIO;
             break;
         case MSR_IA32_XSS:
@@ -2878,11 +2915,8 @@ static int vmx_msr_write_intercept(unsig
         break;
     }
     case MSR_IA32_BNDCFGS:
-        if ( !cpu_has_mpx || !cpu_has_vmx_mpx ||
-             !is_canonical_address(msr_content) ||
-             (msr_content & IA32_BNDCFGS_RESERVED) )
+        if ( !vmx_set_guest_bndcfgs(v, msr_content) )
             goto gp_fault;
-        __vmwrite(GUEST_BNDCFGS, msr_content);
         break;
     case IA32_FEATURE_CONTROL_MSR:
     case MSR_IA32_VMX_BASIC...MSR_IA32_VMX_TRUE_ENTRY_CTLS: