irmap: Get root mntfd before releasing tasks on predump

We have a use-after-free in predump code:

1st the free_pstree() is called in pre_dump_tasks(), then we
go to irmap_predump_run() which may call the lookup_irmap()
which, in turn, dereferences the root_item to get the root
mount ns fd.

But the problem is bigger than that. After we've released the
tasks (done before freeing pstree on predump) we can no longer
access them by PIDs, so keeping the root-item after irmap
scan is not a fix.

Fix is to get the root fd before releasing the tasks and using
one in irmap scanner.

Caught recently on iterative inotify_irmap test.
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>
Acked-by: Andrew Vagin <avagin@parallels.com>

cr-dump.c

@@ -1693,6 +1693,9 @@ int cr_pre_dump_tasks(pid_t pid)
 		if (pre_dump_one_task(item, &ctls))
 			goto err;
 
+	if (irmap_predump_prep())
+		goto err;
+
 	ret = 0;
 err:
 	pstree_switch_state(root_item,

include/irmap.h

@@ -4,6 +4,7 @@ char *irmap_lookup(unsigned int s_dev, unsigned long i_ino);
 struct _FhEntry;
 int irmap_queue_cache(unsigned int dev, unsigned long ino,
 		struct _FhEntry *fh);
+int irmap_predump_prep(void);
 int irmap_predump_run(void);
 int check_open_handle(unsigned int s_dev, unsigned long i_ino,
 		struct _FhEntry *f_handle);

irmap.c

@@ -217,6 +217,8 @@ invalid:
 	return 1;
 }
 
+static bool doing_predump = false;
+
 char *irmap_lookup(unsigned int s_dev, unsigned long i_ino)
 {
 	struct irmap *c, *h, **p;
@@ -227,7 +229,14 @@ char *irmap_lookup(unsigned int s_dev, unsigned long i_ino)
 
 	pr_debug("Resolving %x:%lx path\n", s_dev, i_ino);
 
-	if (__mntns_get_root_fd(root_item->pid.real) < 0)
+	/*
+	 * If we're in predump, then processes already run
+	 * and the root_item is already freed by that time.
+	 * But the root service fd is already set by the
+	 * irmap_predump_prep, so we just go ahead and scan.
+	 */
+	if (!doing_predump &&
+			__mntns_get_root_fd(root_item->pid.real) < 0)
 		goto out;
 
 	timing_start(TIME_IRMAP_RESOLVE);
@@ -296,6 +305,22 @@ int irmap_queue_cache(unsigned int dev, unsigned long ino,
 	return 0;
 }
 
+int irmap_predump_prep(void)
+{
+	/*
+	 * Tasks are about to get released soon, but
+	 * we'll need to do FS scan for irmaps. In this
+	 * scan we will need to know the root dir tasks
+	 * live in. Need to make sure the respective fd
+	 * (service) is set to that root, so that the
+	 * scan works and doesn't race with the tasks
+	 * dying or changind root.
+	 */
+
+	doing_predump = true;
+	return __mntns_get_root_fd(root_item->pid.real) < 0 ? -1 : 0;
+}
+
 int irmap_predump_run(void)
 {
 	int ret = 0;