Re: [RFC 2/3] blktrace: fix debugfs use after free

[Bart Van Assche <bvanassche@acm.org>]

On 2020-04-05 18:27, Eric Sandeen wrote:
> The thing I can't figure out from reading the change log is
> 
> 1) what the root cause of the problem is, and
> 2) how this patch fixes it?

I think that the root cause is that do_blk_trace_setup() uses
debugfs_lookup() and that debugfs_lookup() may return a pointer
associated with a previous incarnation of the block device.
Additionally, I think the following changes fix that problem by using
q->debugfs_dir in the blktrace code instead of debugfs_lookup():

[ ... ]
--- a/kernel/trace/blktrace.c
+++ b/kernel/trace/blktrace.c
@@ -311,7 +311,6 @@ static void blk_trace_free(struct blk_trace *bt)
 	debugfs_remove(bt->msg_file);
 	debugfs_remove(bt->dropped_file);
 	relay_close(bt->rchan);
-	debugfs_remove(bt->dir);
 	free_percpu(bt->sequence);
 	free_percpu(bt->msg_data);
 	kfree(bt);
[ ... ]
@@ -509,21 +510,19 @@ static int do_blk_trace_setup(struct request_queue
*q, char *name, dev_t dev,

 	ret = -ENOENT;

-	dir = debugfs_lookup(buts->name, blk_debugfs_root);
-	if (!dir)
-		bt->dir = dir = debugfs_create_dir(buts->name, blk_debugfs_root);
-
 	bt->dev = dev;
 	atomic_set(&bt->dropped, 0);
 	INIT_LIST_HEAD(&bt->running_list);

 	ret = -EIO;
-	bt->dropped_file = debugfs_create_file("dropped", 0444, dir, bt,
+	bt->dropped_file = debugfs_create_file("dropped", 0444,
+					       q->debugfs_dir, bt,
 					       &blk_dropped_fops);
[ ... ]

Bart.