x86, kmmio/mmiotrace: Fix double free of kmmio_fault_pages

After every iounmap mmiotrace has to free kmmio_fault_pages, but
it can't do it directly, so it defers freeing by RCU.

It usually works, but when mmiotraced code calls ioremap-iounmap
multiple times without sleeping between (so RCU won't kick in
and start freeing) it can be given the same virtual address, so
at every iounmap mmiotrace will schedule the same pages for
release. Obviously it will explode on second free.

Fix it by marking kmmio_fault_pages which are scheduled for
release and not adding them second time.

Signed-off-by: Marcin Slusarz <[e-mail]>
Tested-by: Marcin Kocielnicki <[e-mail]>
Tested-by: Shinpei KATO <[e-mail]>
Acked-by: Pekka Paalanen <[e-mail]>
Cc: Stuart Bennett <[e-mail]>
Cc: Marcin Kocielnicki <[e-mail]>
Cc: [e-mail]
Cc: <[e-mail]>
LKML-Reference: <[e-mail]>
Signed-off-by: Ingo Molnar <[e-mail]>

arch/x86/mm/kmmio.c

···
 	 * Protected by kmmio_lock, when linked into kmmio_page_table.
 	 */
 	int count;
+
+	bool scheduled_for_release;
 };
 
 struct kmmio_delayed_release {
···
 	BUG_ON(f->count < 0);
 	if (!f->count) {
 		disarm_kmmio_fault_page(f);
-		f->release_next = *release_list;
-		*release_list = f;
+		if (!f->scheduled_for_release) {
+			f->release_next = *release_list;
+			*release_list = f;
+			f->scheduled_for_release = true;
+		}
 	}
 }
 
···
 			prevp = &f->release_next;
 		} else {
 			*prevp = f->release_next;
+			f->release_next = NULL;
+			f->scheduled_for_release = false;
 		}
-		f = f->release_next;
+		f = *prevp;
 	}
 	spin_unlock_irqrestore(&kmmio_lock, flags);
 
···
 	list_del_rcu(&p->list);
 	kmmio_count--;
 	spin_unlock_irqrestore(&kmmio_lock, flags);
+
+	if (!release_list)
+		return;
 
 	drelease = kmalloc(sizeof(*drelease), GFP_ATOMIC);
 	if (!drelease) {

arch/x86/mm/testmmiotrace.c

···
 	iounmap(p);
 }
 
+/*
+ * Tests how mmiotrace behaves in face of multiple ioremap / iounmaps in
+ * a short time. We had a bug in deferred freeing procedure which tried
+ * to free this region multiple times (ioremap can reuse the same address
+ * for many mappings).
+ */
+static void do_test_bulk_ioremapping(void)
+{
+	void __iomem *p;
+	int i;
+
+	for (i = 0; i < 10; ++i) {
+		p = ioremap_nocache(mmio_address, PAGE_SIZE);
+		if (p)
+			iounmap(p);
+	}
+
+	/* Force freeing. If it will crash we will know why. */
+	synchronize_rcu();
+}
+
 static int __init init(void)
 {
 	unsigned long size = (read_far) ? (8 << 20) : (16 << 10);
···
 		   "and writing 16 kB of rubbish in there.\n",
 		   size >> 10, mmio_address);
 	do_test(size);
+	do_test_bulk_ioremapping();
 	pr_info("All done.\n");
 	return 0;
 }