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

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Date:   Fri, 12 Jun 2020 19:42:12 -0700
From:   Bart Van Assche <bvanassche@....org>
To:     Luis Chamberlain <mcgrof@...nel.org>, axboe@...nel.dk,
        viro@...iv.linux.org.uk, gregkh@...uxfoundation.org,
        rostedt@...dmis.org, mingo@...hat.com, jack@...e.cz,
        ming.lei@...hat.com, nstange@...e.de, akpm@...ux-foundation.org
Cc:     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 2020-06-08 10:01, Luis Chamberlain wrote:
> +	/*
> +	 * Blktrace needs a debugfs name even for queues that don't register
> +	 * a gendisk, so it lazily registers the debugfs directory.  But that
> +	 * can get us into a situation where a SCSI device is found, with no
> +	 * driver for it (yet).  Then blktrace is used on the device, creating
> +	 * the debugfs directory, and only after that a driver is loaded. In
> +	 * that case we might already have a debugfs directory registered here.
> +	 * Even worse we could be racing with blktrace to register it.
> +	 */

There are LLD and ULD drivers in the SCSI subsystem. Please mention the
driver type explicitly. I assume that you are referring to SCSI ULDs
since only SCSI ULD drivers call device_add_disk()?

Could the above comment be made shorter by only mentioning that blktrace
may have been set up before or concurrently with device_add_disk() and
that device_add_disk() calls blk_register_queue()?

>  	case BLKTRACESETUP:
> +		if (!sdp->device->request_queue->sg_debugfs_dir)
> +			blk_sg_debugfs_init(sdp->device->request_queue,
> +					    sdp->disk->disk_name);

How about moving the sg_debugfs_dir check into blk_sg_debugfs_init()?

Thanks,

Bart.