selinux: make default_noexec read-only after init

SELinux checks whether VM_EXEC is set in the VM_DATA_DEFAULT_FLAGS
during initialization and saves the result in default_noexec for use
in its mmap and mprotect hook function implementations to decide
whether to apply EXECMEM, EXECHEAP, EXECSTACK, and EXECMOD checks.
Mark default_noexec as ro_after_init to prevent later clearing it
and thereby disabling these checks.  It is only set legitimately from
init code.

Signed-off-by: Stephen Smalley <[email protected]>
Signed-off-by: Paul Moore <[email protected]>

security/selinux/hooks.c

@@ -3647,7 +3647,7 @@ static int selinux_file_ioctl(struct file *file, unsigned int cmd,
 	return error;
 }
 
-static int default_noexec;
+static int default_noexec __ro_after_init;
 
 static int file_map_prot_check(struct file *file, unsigned long prot, int shared)
 {