Re: [PATCH v6 6/6] blktrace: fix debugfs use after free

From: Luis Chamberlain <mcgrof@kernel.org>
To: Christoph Hellwig <hch@infradead.org>
Cc: Jan Kara <jack@suse.cz>,
	axboe@kernel.dk, viro@zeniv.linux.org.uk, bvanassche@acm.org,
	gregkh@linuxfoundation.org, rostedt@goodmis.org,
	mingo@redhat.com, ming.lei@redhat.com, nstange@suse.de,
	akpm@linux-foundation.org, mhocko@suse.com, yukuai3@huawei.com,
	martin.petersen@oracle.com, jejb@linux.ibm.com,
	linux-block@vger.kernel.org, linux-fsdevel@vger.kernel.org,
	linux-mm@kvack.org, linux-kernel@vger.kernel.org,
	Omar Sandoval <osandov@fb.com>, Hannes Reinecke <hare@suse.com>,
	Michal Hocko <mhocko@kernel.org>,
	syzbot+603294af2d01acfdd6da@syzkaller.appspotmail.com
Subject: Re: [PATCH v6 6/6] blktrace: fix debugfs use after free
Date: Wed, 10 Jun 2020 21:52:13 +0000	[thread overview]
Message-ID: <20200610215213.GH13911@42.do-not-panic.com> (raw)
In-Reply-To: <20200610210917.GH11244@42.do-not-panic.com>

So, upon updating the commit log, and moving the empty directory check
into another patch, I realized we might be able to simplify this now even
further still. Patch below. The key of the issue was that the use after free
happens when a recursive removal happens, and then later a specific
dentry removal happens. This happened for make_request block drivers
when using the whole disk, but since we *don't* have any other users of
the directory for the others cases, this in theory shuld not happen for
them either.

I'll try to shoot some bullets at this.

diff --git a/kernel/trace/blktrace.c b/kernel/trace/blktrace.c
index 7ff2ea5cd05e..5cea04c05e09 100644
--- a/kernel/trace/blktrace.c
+++ b/kernel/trace/blktrace.c
@@ -524,10 +524,16 @@ static int do_blk_trace_setup(struct request_queue *q, char *name, dev_t dev,
 	if (!bt->msg_data)
 		goto err;
 
-	ret = -ENOENT;
-
-	dir = debugfs_lookup(buts->name, blk_debugfs_root);
-	if (!dir)
+	/*
+	 * When tracing whole make_request drivers (multiqueue) block devices,
+	 * reuse the existing debugfs directory created by the block layer on
+	 * init. For request-based block devices, all partitions block devices,
+	 * and scsi-generic block devices we create a temporary new debugfs
+	 * directory that will be removed once the trace ends.
+	 */
+	if (queue_is_mq(q))
+		dir = q->debugfs_dir;
+	else
 		bt->dir = dir = debugfs_create_dir(buts->name, blk_debugfs_root);
 
 	bt->dev = dev;
@@ -565,8 +571,6 @@ static int do_blk_trace_setup(struct request_queue *q, char *name, dev_t dev,
 
 	ret = 0;
 err:
-	if (dir && !bt->dir)
-		dput(dir);
 	if (ret)
 		blk_trace_free(bt);
 	return ret;
