x86/hvm: always do SMAP check when updating runstate_guest(v)

In the current implementation, we honor the guest's CPL and AC
to determain whether do the SMAP check or not for runstate_guest(v).
However, this doesn't work. The VMCS feild is invalid when we try
to get geust's SS by hvm_get_segment_register(), since the
right VMCS has not beed loaded for the current VCPU.

In this patch, we always do the SMAP check when updating
runstate_guest(v) for the guest when SMAP is enabled by it.

Reported-by: Sander Eikelenboom <linux@eikelenboom.it>
Signed-off-by: Feng Wu <feng.wu@intel.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>
Tested-by: Sander Eikelenboom <linux@eikelenboom.it>

xen/arch/x86/domain.c

@@ -180,6 +180,14 @@ void dump_pageframe_info(struct domain *d)
     spin_unlock(&d->page_alloc_lock);
 }
 
+smap_check_policy_t smap_policy_change(struct vcpu *v,
+    smap_check_policy_t new_policy)
+{
+    smap_check_policy_t old_policy = v->arch.smap_check_policy;
+    v->arch.smap_check_policy = new_policy;
+    return old_policy;
+}
+
 /*
  * The hole may be at or above the 44-bit boundary, so we need to determine
  * the total bit count until reaching 32 significant (not squashed out) bits
@@ -1349,22 +1357,31 @@ static void paravirt_ctxt_switch_to(struct vcpu *v)
 }
 
 /* Update per-VCPU guest runstate shared memory area (if registered). */
-bool_t update_runstate_area(const struct vcpu *v)
+bool_t update_runstate_area(struct vcpu *v)
 {
+    bool_t rc;
+    smap_check_policy_t smap_policy;
+
     if ( guest_handle_is_null(runstate_guest(v)) )
         return 1;
 
+    smap_policy = smap_policy_change(v, SMAP_CHECK_ENABLED);
+
     if ( has_32bit_shinfo(v->domain) )
     {
         struct compat_vcpu_runstate_info info;
 
         XLAT_vcpu_runstate_info(&info, &v->runstate);
         __copy_to_guest(v->runstate_guest.compat, &info, 1);
-        return 1;
+        rc = 1;
     }
+    else
+        rc = __copy_to_guest(runstate_guest(v), &v->runstate, 1) !=
+             sizeof(v->runstate);
 
-    return __copy_to_guest(runstate_guest(v), &v->runstate, 1) !=
-           sizeof(v->runstate);
+    smap_policy_change(v, smap_policy);
+
+    return rc;
 }
 
 static void _update_runstate_area(struct vcpu *v)

xen/arch/x86/mm/guest_walk.c

@@ -164,25 +164,38 @@ guest_walk_tables(struct vcpu *v, struct p2m_domain *p2m,
         struct segment_register seg;
         const struct cpu_user_regs *regs = guest_cpu_user_regs();
 
-        hvm_get_segment_register(v, x86_seg_ss, &seg);
-
         /* SMEP: kernel-mode instruction fetches from user-mode mappings
          * should fault.  Unlike NX or invalid bits, we're looking for _all_
          * entries in the walk to have _PAGE_USER set, so we need to do the
          * whole walk as if it were a user-mode one and then invert the answer. */
         smep =  hvm_smep_enabled(v) && (pfec & PFEC_insn_fetch);
 
-        /*
-         * SMAP: kernel-mode data accesses from user-mode mappings should fault
-         * A fault is considered as a SMAP violation if the following
-         * conditions come true:
-         *   - X86_CR4_SMAP is set in CR4
-         *   - A user page is accessed
-         *   - CPL = 3 or X86_EFLAGS_AC is clear
-         *   - Page fault in kernel mode
-         */
-        smap = hvm_smap_enabled(v) &&
-               ((seg.attr.fields.dpl == 3) || !(regs->eflags & X86_EFLAGS_AC));
+        switch ( v->arch.smap_check_policy )
+        {
+        case SMAP_CHECK_HONOR_CPL_AC:
+            hvm_get_segment_register(v, x86_seg_ss, &seg);
+
+            /*
+             * SMAP: kernel-mode data accesses from user-mode mappings
+             * should fault.
+             * A fault is considered as a SMAP violation if the following
+             * conditions come true:
+             *   - X86_CR4_SMAP is set in CR4
+             *   - A user page is accessed
+             *   - CPL = 3 or X86_EFLAGS_AC is clear
+             *   - Page fault in kernel mode
+             */
+            smap = hvm_smap_enabled(v) &&
+                   ((seg.attr.fields.dpl == 3) ||
+                    !(regs->eflags & X86_EFLAGS_AC));
+            break;
+        case SMAP_CHECK_ENABLED:
+            smap = hvm_smap_enabled(v);
+            break;
+        default:
+            ASSERT(v->arch.smap_check_policy == SMAP_CHECK_DISABLED);
+            break;
+        }
     }
 
     if ( smep || smap )

xen/include/asm-x86/domain.h

@@ -382,6 +382,12 @@ struct pv_vcpu
     struct vcpu_time_info pending_system_time;
 };
 
+typedef enum __packed {
+    SMAP_CHECK_HONOR_CPL_AC,    /* honor the guest's CPL and AC */
+    SMAP_CHECK_ENABLED,         /* enable the check */
+    SMAP_CHECK_DISABLED,        /* disable the check */
+} smap_check_policy_t;
+
 struct arch_vcpu
 {
     /*
@@ -438,6 +444,12 @@ struct arch_vcpu
      * and thus should be saved/restored. */
     bool_t nonlazy_xstate_used;
 
+    /*
+     * The SMAP check policy when updating runstate_guest(v) and the
+     * secondary system time.
+     */
+    smap_check_policy_t smap_check_policy;
+
     struct vmce vmce;
 
     struct paging_vcpu paging;
@@ -448,11 +460,14 @@ struct arch_vcpu
     XEN_GUEST_HANDLE(vcpu_time_info_t) time_info_guest;
 } __cacheline_aligned;
 
+smap_check_policy_t smap_policy_change(struct vcpu *v,
+                                       smap_check_policy_t new_policy);
+
 /* Shorthands to improve code legibility. */
 #define hvm_vmx         hvm_vcpu.u.vmx
 #define hvm_svm         hvm_vcpu.u.svm
 
-bool_t update_runstate_area(const struct vcpu *);
+bool_t update_runstate_area(struct vcpu *);
 bool_t update_secondary_system_time(const struct vcpu *,
                                     struct vcpu_time_info *);