[PATCH v2 1/6] writeback: protect race between bdi release and bdi_debug_stats_show

[Kemeng Shi <shikemeng@huaweicloud.com>]

There is a race between bdi release and bdi_debug_stats_show:
/* get debug info */		/* bdi release */
bdi_debug_stats_show
  bdi = m->private;
  wb = &bdi->wb;
				bdi_unregister
				bdi_put
				  release_bdi
				    kfree(bdi)
  /* use after free */
  spin_lock(&wb->list_lock);

Search bdi on bdi_list under rcu lock in bdi_debug_stats_show to ensure
the bdi is not freed to fix the issue.

Signed-off-by: Kemeng Shi <shikemeng@huaweicloud.com>
---
 mm/backing-dev.c | 33 +++++++++++++++++++++++++++++++--
 1 file changed, 31 insertions(+), 2 deletions(-)

diff --git a/mm/backing-dev.c b/mm/backing-dev.c
index 5f2be8c8df11..70f02959f3bd 100644
--- a/mm/backing-dev.c
+++ b/mm/backing-dev.c
@@ -46,16 +46,44 @@ static void bdi_debug_init(void)
 	bdi_debug_root = debugfs_create_dir("bdi", NULL);
 }
 
-static int bdi_debug_stats_show(struct seq_file *m, void *v)
+static struct backing_dev_info *lookup_bdi(struct seq_file *m)
 {
+	const struct file *file = m->file;
 	struct backing_dev_info *bdi = m->private;
-	struct bdi_writeback *wb = &bdi->wb;
+	struct backing_dev_info *exist;
+
+	list_for_each_entry_rcu(exist, &bdi_list, bdi_list) {
+		if (exist != bdi)
+			continue;
+
+		if (exist->debug_dir == file->f_path.dentry->d_parent)
+			return bdi;
+		else
+			return NULL;
+	}
+
+	return NULL;
+}
+
+
+static int bdi_debug_stats_show(struct seq_file *m, void *v)
+{
+	struct backing_dev_info *bdi;
+	struct bdi_writeback *wb;
 	unsigned long background_thresh;
 	unsigned long dirty_thresh;
 	unsigned long wb_thresh;
 	unsigned long nr_dirty, nr_io, nr_more_io, nr_dirty_time;
 	struct inode *inode;
 
+	rcu_read_lock();
+	bdi = lookup_bdi(m);
+	if (!bdi) {
+		rcu_read_unlock();
+		return -EEXIST;
+	}
+
+	wb = &bdi->wb;
 	nr_dirty = nr_io = nr_more_io = nr_dirty_time = 0;
 	spin_lock(&wb->list_lock);
 	list_for_each_entry(inode, &wb->b_dirty, i_io_list)
@@ -101,6 +129,7 @@ static int bdi_debug_stats_show(struct seq_file *m, void *v)
 		   nr_dirty_time,
 		   !list_empty(&bdi->bdi_list), bdi->wb.state);
 
+	rcu_read_unlock();
 	return 0;
 }
 DEFINE_SHOW_ATTRIBUTE(bdi_debug_stats);
-- 
2.30.0