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

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Date:   Fri, 19 Jun 2020 15:36:20 +0000
From:   Luis Chamberlain <mcgrof@...nel.org>
To:     Bart Van Assche <bvanassche@....org>
Cc:     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, 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 Fri, Jun 12, 2020 at 07:42:12PM -0700, Bart Van Assche wrote:
> 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()?

I've simplified this and so this is no longer a valid comment.

> >  	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()?

I found a way to not have to do any of this, the fix will be short and
sweet now.

  Luis