Xi: allocate enough XkbActions for our buttons

button->xkb_acts is supposed to be an array sufficiently large for all
our buttons, not just a single XkbActions struct. Allocating
insufficient memory here means when we memcpy() later in
XkbSetDeviceInfo we write into memory that wasn't ours to begin with,
leading to the usual security ooopsiedaisies.

CVE-2023-6377, ZDI-CAN-22412, ZDI-CAN-22413

This vulnerability was discovered by:
Jan-Niklas Sohn working with Trend Micro Zero Day Initiative

(cherry picked from commit 0c1a93d319558fe3ab2d94f51d174b4f93810afd)

2 files changed, 16 insertions, 6 deletions

diff --git a/Xi/exevents.c b/Xi/exevents.c
index dcd4efb3b..54ea11a93 100644
--- a/Xi/exevents.c
+++ b/Xi/exevents.c
@@ -611,13 +611,13 @@ DeepCopyPointerClasses(DeviceIntPtr from, DeviceIntPtr to)
         }
 
         if (from->button->xkb_acts) {
-            if (!to->button->xkb_acts) {
-                to->button->xkb_acts = calloc(1, sizeof(XkbAction));
-                if (!to->button->xkb_acts)
-                    FatalError("[Xi] not enough memory for xkb_acts.\n");
-            }
+            size_t maxbuttons = max(to->button->numButtons, from->button->numButtons);
+            to->button->xkb_acts = xnfreallocarray(to->button->xkb_acts,
+                                                   maxbuttons,
+                                                   sizeof(XkbAction));
+            memset(to->button->xkb_acts, 0, maxbuttons * sizeof(XkbAction));
             memcpy(to->button->xkb_acts, from->button->xkb_acts,
-                   sizeof(XkbAction));
+                   from->button->numButtons * sizeof(XkbAction));
         }
         else {
             free(to->button->xkb_acts);
diff --git a/dix/devices.c b/dix/devices.c
index 5bf956ead..15e46a9a5 100644
--- a/dix/devices.c
+++ b/dix/devices.c
@@ -2525,6 +2525,8 @@ RecalculateMasterButtons(DeviceIntPtr slave)
 
     if (master->button && master->button->numButtons != maxbuttons) {
         int i;
+        int last_num_buttons = master->button->numButtons;
+
         DeviceChangedEvent event = {
             .header = ET_Internal,
             .type = ET_DeviceChanged,
@@ -2535,6 +2537,14 @@ RecalculateMasterButtons(DeviceIntPtr slave)
         };
 
         master->button->numButtons = maxbuttons;
+        if (last_num_buttons < maxbuttons) {
+            master->button->xkb_acts = xnfreallocarray(master->button->xkb_acts,
+                                                       maxbuttons,
+                                                       sizeof(XkbAction));
+            memset(&master->button->xkb_acts[last_num_buttons],
+                   0,
+                   (maxbuttons - last_num_buttons) * sizeof(XkbAction));
+        }
 
         memcpy(&event.buttons.names, master->button->labels, maxbuttons *
                sizeof(Atom));