UML specifies that its signals will be delivered on a dedicated signal stack, which in kernel terms is the process' kernel stack. This can not be write-protected since the host would just kill the UML thread with a segfault if it tried to deliver a signal on a write-protected stack.
This stack is also used for system calls, and in this case, it is initialized by the tracing thread.
However, any modifications to these pages by the process would just be lost because UML initializes them completely before using them and its behavior doesn't depend on their previous contents.