Can User Level Thread read/write Kernel thread? As far as I read in a OS text book (Operating Systems by Silberschatz) Kernel mode is for privileged task, so it it true to claim that &quot;User Level Thread can read/write Kernel threads&quot; ?
Generally speaking, Is there any kind of protection between user and kernel level threads?
No.  In most operating systems, kernel memory is protected using virtual memory mechanisms, so a user process cannot read or write to kernel memory.
A user thread is part of a user process, and has the same permissions as the process.  So, a user thread will not be able to write to kernel memory.