vm: Allow W|X before kernel startup ends.

The altcodepatch mechanism needs to overwrite parts of the kernel
image. This can't be done by setting it to RW-only and not RWX,
as we are already running within the kernel when this occurs,
and so instruction fetches can and will occur between the points
of +W and -W.

As gKernelStartup is turned off before the scheduler is started,
this is not much of a lifted restriction, as no modules are loaded,
no secondary threads started, etc.

Fixes #14751.

src/system/kernel/vm/vm.cpp

@@ -4914,7 +4914,7 @@ vm_set_area_memory_type(area_id id, phys_addr_t physicalBase, uint32 type)
 
 
 /*!	This function enforces some protection properties:
-	 - kernel areas must be W^X
+	 - kernel areas must be W^X (after kernel startup)
 	 - if B_WRITE_AREA is set, B_KERNEL_WRITE_AREA is set as well
 	 - if only B_READ_AREA has been set, B_KERNEL_READ_AREA is also set
 	 - if no protection is specified, it defaults to B_KERNEL_READ_AREA
@@ -4923,8 +4923,10 @@ vm_set_area_memory_type(area_id id, phys_addr_t physicalBase, uint32 type)
 static void
 fix_protection(uint32* protection)
 {
-	if ((*protection & B_KERNEL_WRITE_AREA) != 0
-		&& (*protection & B_KERNEL_EXECUTE_AREA) != 0)
+	if ((*protection & B_KERNEL_EXECUTE_AREA) != 0
+		&& ((*protection & B_KERNEL_WRITE_AREA) != 0
+			|| (*protection & B_WRITE_AREA) != 0)
+		&& !gKernelStartup)
 		panic("kernel areas cannot be both writable and executable!");
 
 	if ((*protection & B_KERNEL_PROTECTION) == 0) {