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

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Date:   Tue, 9 Jun 2020 10:32:18 -0700
From:   Christoph Hellwig <hch@...radead.org>
To:     Luis Chamberlain <mcgrof@...nel.org>
Cc:     Christoph Hellwig <hch@...radead.org>, 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 Tue, Jun 09, 2020 at 05:29:22PM +0000, Luis Chamberlain wrote:
> Is scsi-generic is the only unwanted ugly child blktrace has to deal
> with? For some reason I thought drivers/md/md.c was one but it seems
> like it is not. Do we have an easy way to search for these? I think
> this would just affect how we express the comment only.

grep for blk_trace_setup.  For all blk devices that setup comes in
through the block device ioctl path, and that relies on having a
struct block_device and queue.  sg on the other hand calls
blk_trace_setup directly with a NULL bdev argument.

> >  		 */
> > -		dir = q->sg_debugfs_dir;
> > +		dir = debugfs_create_dir(buts->name, blk_debugfs_root);
> > +		bt->dir = dir;
> 
> The other chicken and egg problem to consider at least in the comments
> is that the debugfs directory for these types of devices *have* an
> exposed path, but the data structure is rather opaque to the device and
> even blktrace.  Fortunately given the recent set of changes around the
> q->blk_trace and clarifications around its use we have made it clear now
> that so long as hold the q->blk_trace_mutex *and* check q->blk_trace we
> *should* not race against two separate creations of debugfs directories,
> so I think this is safe, so long as these indpendent drivers don't end
> up re-using the same path for some other things later in the future, and
> since we have control over what goes under debugfsroot block / I think
> we should be good.
> 
> But I think that the concern for race on names may still be worth
> explaining a bit here.

Feel free to add more comments, but please try to keep them short
and crisp.  At the some point long comments really distract from what
is going on.