x86/msr: Fix migration compatibility issue with MSR_SPEC_CTRL

This bug existed in early in 2018 between MSR_SPEC_CTRL arriving in microcode,
and SSBD arriving a few months later.  It went unnoticed presumably because
everyone was busy rebooting everything.

The same bug will reappear when adding PSFD support.

Clamp the guest MSR_SPEC_CTRL value to that permitted by CPUID on migrate.
The guest is already playing with reserved bits at this point, and clamping
the value will prevent a migration to a less capable host from failing.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>
(cherry picked from commit 969a57f73f)

xen/arch/x86/hvm/hvm.c

@@ -1370,6 +1370,7 @@ static const uint32_t msrs_to_send[] = {
 
 static int hvm_save_cpu_msrs(struct vcpu *v, hvm_domain_context_t *h)
 {
+    const struct domain *d = v->domain;
     struct hvm_save_descriptor *desc = _p(&h->data[h->cur]);
     struct hvm_msr *ctxt;
     unsigned int i;
@@ -1385,7 +1386,8 @@ static int hvm_save_cpu_msrs(struct vcpu *v, hvm_domain_context_t *h)
     for ( i = 0; i < ARRAY_SIZE(msrs_to_send); ++i )
     {
         uint64_t val;
-        int rc = guest_rdmsr(v, msrs_to_send[i], &val);
+        unsigned int msr = msrs_to_send[i];
+        int rc = guest_rdmsr(v, msr, &val);
 
         /*
          * It is the programmers responsibility to ensure that
@@ -1405,7 +1407,26 @@ static int hvm_save_cpu_msrs(struct vcpu *v, hvm_domain_context_t *h)
         if ( !val )
             continue; /* Skip empty MSRs. */
 
-        ctxt->msr[ctxt->count].index = msrs_to_send[i];
+        /*
+         * Guests are given full access to certain MSRs for performance
+         * reasons.  A consequence is that Xen is unable to enforce that all
+         * bits disallowed by the CPUID policy yield #GP, and an enterprising
+         * guest may be able to set and use a bit it ought to leave alone.
+         *
+         * When migrating from a more capable host to a less capable one, such
+         * bits may be rejected by the destination, and the migration failed.
+         *
+         * Discard such bits here on the source side.  Such bits have reserved
+         * behaviour, and the guest has only itself to blame.
+         */
+        switch ( msr )
+        {
+        case MSR_SPEC_CTRL:
+            val &= msr_spec_ctrl_valid_bits(d->arch.cpuid);
+            break;
+        }
+
+        ctxt->msr[ctxt->count].index = msr;
         ctxt->msr[ctxt->count++].val = val;
     }
 

xen/arch/x86/msr.c

@@ -360,6 +360,24 @@ int guest_rdmsr(struct vcpu *v, uint32_t msr, uint64_t *val)
     return X86EMUL_EXCEPTION;
 }
 
+/*
+ * Caller to confirm that MSR_SPEC_CTRL is available.  Intel and AMD have
+ * separate CPUID features for this functionality, but only set will be
+ * active.
+ */
+uint64_t msr_spec_ctrl_valid_bits(const struct cpuid_policy *cp)
+{
+    bool ssbd = cp->feat.ssbd;
+
+    /*
+     * Note: SPEC_CTRL_STIBP is specified as safe to use (i.e. ignored)
+     * when STIBP isn't enumerated in hardware.
+     */
+    return (SPEC_CTRL_IBRS | SPEC_CTRL_STIBP |
+            (ssbd       ? SPEC_CTRL_SSBD       : 0) |
+            0);
+}
+
 int guest_wrmsr(struct vcpu *v, uint32_t msr, uint64_t val)
 {
     const struct vcpu *curr = current;
@@ -442,18 +460,9 @@ int guest_wrmsr(struct vcpu *v, uint32_t msr, uint64_t val)
         break;
 
     case MSR_SPEC_CTRL:
-        if ( !cp->feat.ibrsb )
-            goto gp_fault; /* MSR available? */
-
-        /*
-         * Note: SPEC_CTRL_STIBP is specified as safe to use (i.e. ignored)
-         * when STIBP isn't enumerated in hardware.
-         */
-        rsvd = ~(SPEC_CTRL_IBRS | SPEC_CTRL_STIBP |
-                 (cp->feat.ssbd ? SPEC_CTRL_SSBD : 0));
-
-        if ( val & rsvd )
-            goto gp_fault; /* Rsvd bit set? */
+        if ( !cp->feat.ibrsb ||
+             (val & ~msr_spec_ctrl_valid_bits(cp)) )
+            goto gp_fault;
         goto set_reg;
 
     case MSR_PRED_CMD:

xen/include/asm-x86/msr.h

@@ -267,6 +267,8 @@ static inline void wrmsr_tsc_aux(uint32_t val)
     }
 }
 
+uint64_t msr_spec_ctrl_valid_bits(const struct cpuid_policy *cp);
+
 extern struct msr_policy     raw_msr_policy,
                             host_msr_policy,
                           pv_max_msr_policy,