x86/domctl: two functional fixes to XEN_DOMCTL_[gs]etvcpuextstate

Interacting with the vcpu itself should be protected by vcpu_pause().
Buggy/naive toolstacks might encounter adverse interaction with a vcpu context
switch, or increase of xcr0_accum.  There are no much problems with current
in-tree code.

Explicitly permit a NULL guest handle as being a request for size.  It is the
prevailing Xen style, and without it, valgrind's ioctl handler is unable to
determine whether evc->buffer actually got written to.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>

xen/arch/x86/domctl.c

@@ -1173,45 +1173,48 @@ long arch_do_domctl(
              ((v = d->vcpu[evc->vcpu]) == NULL) )
             goto vcpuextstate_out;
 
+        ret = -EINVAL;
+        if ( v == current ) /* no vcpu_pause() */
+            goto vcpuextstate_out;
+
         if ( domctl->cmd == XEN_DOMCTL_getvcpuextstate )
         {
-            unsigned int size = PV_XSAVE_SIZE(v->arch.xcr0_accum);
+            unsigned int size;
 
-            if ( !evc->size && !evc->xfeature_mask )
+            ret = 0;
+            vcpu_pause(v);
+
+            size = PV_XSAVE_SIZE(v->arch.xcr0_accum);
+            if ( (!evc->size && !evc->xfeature_mask) ||
+                 guest_handle_is_null(evc->buffer) )
             {
                 evc->xfeature_mask = xfeature_mask;
                 evc->size = size;
-                ret = 0;
+                vcpu_unpause(v);
                 goto vcpuextstate_out;
             }
+
             if ( evc->size != size || evc->xfeature_mask != xfeature_mask )
-            {
                 ret = -EINVAL;
-                goto vcpuextstate_out;
-            }
-            if ( copy_to_guest_offset(domctl->u.vcpuextstate.buffer,
-                                      offset, (void *)&v->arch.xcr0,
-                                      sizeof(v->arch.xcr0)) )
-            {
+
+            if ( !ret && copy_to_guest_offset(evc->buffer, offset,
+                                              (void *)&v->arch.xcr0,
+                                              sizeof(v->arch.xcr0)) )
                 ret = -EFAULT;
-                goto vcpuextstate_out;
-            }
+
             offset += sizeof(v->arch.xcr0);
-            if ( copy_to_guest_offset(domctl->u.vcpuextstate.buffer,
-                                      offset, (void *)&v->arch.xcr0_accum,
-                                      sizeof(v->arch.xcr0_accum)) )
-            {
+            if ( !ret && copy_to_guest_offset(evc->buffer, offset,
+                                              (void *)&v->arch.xcr0_accum,
+                                              sizeof(v->arch.xcr0_accum)) )
                 ret = -EFAULT;
-                goto vcpuextstate_out;
-            }
+
             offset += sizeof(v->arch.xcr0_accum);
-            if ( copy_to_guest_offset(domctl->u.vcpuextstate.buffer,
-                                      offset, (void *)v->arch.xsave_area,
-                                      size - 2 * sizeof(uint64_t)) )
-            {
+            if ( !ret && copy_to_guest_offset(evc->buffer, offset,
+                                              (void *)v->arch.xsave_area,
+                                              size - 2 * sizeof(uint64_t)) )
                 ret = -EFAULT;
-                goto vcpuextstate_out;
-            }
+
+            vcpu_unpause(v);
         }
         else
         {
@@ -1259,12 +1262,14 @@ long arch_do_domctl(
 
             if ( evc->size <= PV_XSAVE_SIZE(_xcr0_accum) )
             {
+                vcpu_pause(v);
                 v->arch.xcr0 = _xcr0;
                 v->arch.xcr0_accum = _xcr0_accum;
                 if ( _xcr0_accum & XSTATE_NONLAZY )
                     v->arch.nonlazy_xstate_used = 1;
                 memcpy(v->arch.xsave_area, _xsave_area,
                        evc->size - 2 * sizeof(uint64_t));
+                vcpu_unpause(v);
             }
             else
                 ret = -EINVAL;