Re: [RFC v2 01/26] mm/x86: Introduce kernel address space isolation

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On 7/11/19 11:33 PM, Thomas Gleixner wrote:
On Thu, 11 Jul 2019, Alexandre Chartre wrote:
+/*
+ * When isolation is active, the address space doesn't necessarily map
+ * the percpu offset value (this_cpu_off) which is used to get pointers
+ * to percpu variables. So functions which can be invoked while isolation
+ * is active shouldn't be getting pointers to percpu variables (i.e. with
+ * get_cpu_var() or this_cpu_ptr()). Instead percpu variable should be
+ * directly read or written to (i.e. with this_cpu_read() or
+ * this_cpu_write()).
+ */
+
+int asi_enter(struct asi *asi)
+{
+	enum asi_session_state state;
+	struct asi *current_asi;
+	struct asi_session *asi_session;
+
+	state = this_cpu_read(cpu_asi_session.state);
+	/*
+	 * We can re-enter isolation, but only with the same ASI (we don't
+	 * support nesting isolation). Also, if isolation is still active,
+	 * then we should be re-entering with the same task.
+	 */
+	if (state == ASI_SESSION_STATE_ACTIVE) {
+		current_asi = this_cpu_read(cpu_asi_session.asi);
+		if (current_asi != asi) {
+			WARN_ON(1);
+			return -EBUSY;
+		}
+		WARN_ON(this_cpu_read(cpu_asi_session.task) != current);
+		return 0;
+	}
+
+	/* isolation is not active so we can safely access the percpu pointer */
+	asi_session = &get_cpu_var(cpu_asi_session);

get_cpu_var()?? Where is the matching put_cpu_var() ? get_cpu_var()
contains a preempt_disable ...

What's wrong with a simple this_cpu_ptr() here?


Oups, my mistake, I should be using this_cpu_ptr(). I will replace all get_cpu_var()
with this_cpu_ptr().


+void asi_exit(struct asi *asi)
+{
+	struct asi_session *asi_session;
+	enum asi_session_state asi_state;
+	unsigned long original_cr3;
+
+	asi_state = this_cpu_read(cpu_asi_session.state);
+	if (asi_state == ASI_SESSION_STATE_INACTIVE)
+		return;
+
+	/* TODO: Kick sibling hyperthread before switching to kernel cr3 */
+	original_cr3 = this_cpu_read(cpu_asi_session.original_cr3);
+	if (original_cr3)

Why would this be 0 if the session is active?


Correct, original_cr3 won't be 0. I think this is a remain from a previous version
where original_cr3 was handled differently.


+		write_cr3(original_cr3);
+
+	/* page-table was switched, we can now access the percpu pointer */
+	asi_session = &get_cpu_var(cpu_asi_session);

See above.


Will fix that.


Thanks,

alex.

+	WARN_ON(asi_session->task != current);
+	asi_session->state = ASI_SESSION_STATE_INACTIVE;
+	asi_session->asi = NULL;
+	asi_session->task = NULL;
+	asi_session->original_cr3 = 0;
+}

Thanks,

	tglx




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