xen/arm: Boot modules should always be scrubbed if bootscrub={on, idle}

The function to initialize the pages (see init_heap_pages()) will request
scrub when the admin request idle bootscrub (default) and state ==
SYS_STATE_active. When bootscrub=on, Xen will scrub any free pages in
heap_init_late().

Currently, the boot modules (e.g. kernels, initramfs) will be discarded/
freed after heap_init_late() is called and system_state switched to
SYS_STATE_active. This means the pages associated with the boot modules
will not get scrubbed before getting re-purposed.

If the memory is assigned to an untrusted domU, it may be able to
retrieve secrets from the modules.

This is part of XSA-372 / CVE-2021-28693.

Fixes: 1774e9b1df ("xen/arm: introduce create_domUs")
Signed-off-by: Julien Grall <jgrall@amazon.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Stefano Stabellini <sstabellini@kernel.org>
Tested-by: Stefano Stabellini <sstabellini@kernel.org>
(cherry picked from commit fd5dc41cea)

xen/arch/arm/setup.c

@@ -75,7 +75,6 @@ static __used void init_done(void)
     /* Must be done past setting system_state. */
     unregister_init_virtual_region();
 
-    discard_initial_modules();
     free_init_memory();
     startup_cpu_idle_loop();
 }
@@ -964,6 +963,12 @@ void __init start_xen(unsigned long boot_phys_offset,
 
     create_domUs();
 
+    /*
+     * This needs to be called **before** heap_init_late() so modules
+     * will be scrubbed (unless suppressed).
+     */
+    discard_initial_modules();
+
     heap_init_late();
 
     init_trace_bufs();