i965: Pad buffer objects by 2kB in robust contexts to avoid OOB access.

This is an annoyingly big hammer, but it seems less mean than disabling
UBO pushing, and I'm not sure what else to do.

Reviewed-by: Matt Turner <mattst88@gmail.com>

src/mesa/drivers/dri/i965/intel_buffer_objects.c

@@ -74,8 +74,26 @@ static void
 alloc_buffer_object(struct brw_context *brw,
                     struct intel_buffer_object *intel_obj)
 {
-   intel_obj->buffer = brw_bo_alloc(brw->bufmgr, "bufferobj",
-					  intel_obj->Base.Size, 64);
+   const struct gl_context *ctx = &brw->ctx;
+
+   uint64_t size = intel_obj->Base.Size;
+   if (ctx->Const.RobustAccess) {
+      /* Pad out buffer objects with an extra 2kB (half a page).
+       *
+       * When pushing UBOs, we need to safeguard against 3DSTATE_CONSTANT_*
+       * reading out of bounds memory.  The application might bind a UBO that's
+       * smaller than what the program expects.  Ideally, we'd bind an extra
+       * push buffer containing zeros, but we have a limited number of those,
+       * so it's not always viable.  Our only safe option is to pad all buffer
+       * objects by the maximum push data length, so that it will never read
+       * past the end of a BO.
+       *
+       * This is unfortunate, but it should result in at most 1 extra page,
+       * which probably isn't too terrible.
+       */
+      size += 64 * 32; /* max read length of 64 256-bit units */
+   }
+   intel_obj->buffer = brw_bo_alloc(brw->bufmgr, "bufferobj", size, 64);
 
    /* the buffer might be bound as a uniform buffer, need to update it
     */