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

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Message-ID: <20200610233116.GI13911@42.do-not-panic.com>
Date:   Wed, 10 Jun 2020 23:31:16 +0000
From:   Luis Chamberlain <mcgrof@...nel.org>
To:     Christoph Hellwig <hch@...radead.org>
Cc:     Jan Kara <jack@...e.cz>, axboe@...nel.dk, viro@...iv.linux.org.uk,
        bvanassche@....org, gregkh@...uxfoundation.org,
        rostedt@...dmis.org, mingo@...hat.com, ming.lei@...hat.com,
        nstange@...e.de, akpm@...ux-foundation.org, mhocko@...e.com,
        yukuai3@...wei.com, martin.petersen@...cle.com, jejb@...ux.ibm.com,
        linux-block@...r.kernel.org, linux-fsdevel@...r.kernel.org,
        linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        Omar Sandoval <osandov@...com>,
        Hannes Reinecke <hare@...e.com>,
        Michal Hocko <mhocko@...nel.org>,
        syzbot+603294af2d01acfdd6da@...kaller.appspotmail.com
Subject: Re: [PATCH v6 6/6] blktrace: fix debugfs use after free

On Wed, Jun 10, 2020 at 09:52:13PM +0000, Luis Chamberlain wrote:
> 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))

And this should be instead:

if (queue_is_mq(q) && bdev && bdev == bdev->bd_contains)

> +		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;