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Subject: Re: [PATCH v7 6/8] blktrace: fix debugfs use after free
To:     Luis Chamberlain <mcgrof@kernel.org>, axboe@kernel.dk,
        viro@zeniv.linux.org.uk, gregkh@linuxfoundation.org,
        rostedt@goodmis.org, mingo@redhat.com, jack@suse.cz,
        ming.lei@redhat.com, nstange@suse.de, akpm@linux-foundation.org
Cc:     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
References: <20200619204730.26124-1-mcgrof@kernel.org>
 <20200619204730.26124-7-mcgrof@kernel.org>
From:   Bart Van Assche <bvanassche@acm.org>
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Message-ID: <75c3a94d-dcd1-05e4-47c6-db65f074136a@acm.org>
Date:   Sat, 20 Jun 2020 10:31:51 -0700
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On 2020-06-19 13:47, Luis Chamberlain wrote:
> This goes tested with:
       ^^^^
       got?

> diff --git a/kernel/trace/blktrace.c b/kernel/trace/blktrace.c
> index 7ff2ea5cd05e..e6e2d25fdbd6 100644
> --- a/kernel/trace/blktrace.c
> +++ b/kernel/trace/blktrace.c
> @@ -524,10 +524,18 @@ 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)
> +#ifdef CONFIG_BLK_DEBUG_FS
> +	/*
> +	 * 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,
                                                  ^^^^^^^^^^^^^^^^^^^^^
It seems like a word is missing from the comment? Or did you perhaps
want to refer to "all partition 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) && bdev && bdev == bdev->bd_contains)
> +		dir = q->debugfs_dir;
> +	else
> +#endif
>  		bt->dir = dir = debugfs_create_dir(buts->name, blk_debugfs_root);

Can it happen that two different threads each try to set up block
tracing and hence that debugfs_create_dir() fails because a directory
with name buts->name already exists?

>  	bt->dev = dev;
> @@ -565,8 +573,6 @@ 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;

Shouldn't bt->dir be removed in this error path for make_request drivers?

Thanks,

Bart.