saver: re-fetch screen private after CheckScreenPrivate in CreateSaverWindow

CreateSaverWindow stores pPriv (the ScreenSaverScreenPrivatePtr) in a local
variable via the SetupScreen macro at function entry. When an existing saver
window is being replaced, the function sets pPriv->hasWindow = FALSE and
calls CheckScreenPrivate(). If at this point pPriv->attr is NULL (cleared
by a prior UnsetAttributes call), pPriv->events is NULL, and
pPriv->installedMap is None, then CheckScreenPrivate determines the screen
private is unused, frees it, and sets the screen private pointer to NULL.

The function then continues to dereference the now-freed pPriv on the very
next line (pPriv->attr), resulting in a use-after-free. On glibc 2.34+,
the tcache key at offset 8 within the freed block makes pPriv->attr appear
non-NULL, causing the function to continue operating on garbage data and
eventually crash.

The attack sequence is:
  1. SetAttributes (creates pPriv with pPriv->attr set)
  2. ForceScreenSaver(Active) (creates saver window, pPriv->hasWindow=TRUE)
  3. UnsetAttributes (sets pPriv->attr = NULL)
  4. ForceScreenSaver(Active) (re-enters CreateSaverWindow → UAF)

Fix by re-fetching pPriv from the screen private after CheckScreenPrivate
returns, so the subsequent NULL check correctly detects the freed state.

ScreenSaverFreeAttr has the same pattern, force pPriv to NULL there too
even though it has no real effect.

This vulnerability was discovered by:
Anonymous working with TrendAI Zero Day Initiative

ZDI-CAN-30168

Assisted-by: Claude:claude-opus-4-6
(cherry picked from commit ecc634f1b2)

Part-of: <https://gitlab.freedesktop.org/xorg/xserver/-/merge_requests/2229>

Xext/saver.c

@@ -348,6 +348,9 @@ ScreenSaverFreeAttr(void *value, XID id)
         dixSaveScreens(serverClient, SCREEN_SAVER_FORCER, ScreenSaverActive);
     }
     CheckScreenPrivate(pScreen);
+    /* CheckScreenPrivate may have freed pPriv (same pattern as
+     * CreateSaverWindow fix for ZDI-CAN-30168). */
+    pPriv = NULL;
     return TRUE;
 }
 
@@ -479,6 +482,8 @@ CreateSaverWindow(ScreenPtr pScreen)
             UninstallSaverColormap(pScreen);
             pPriv->hasWindow = FALSE;
             CheckScreenPrivate(pScreen);
+            /* Re-fetch pPriv since CheckScreenPrivate may have freed it */
+            pPriv = GetScreenPrivate(pScreen);
         }
     }