linux-kernel - Re: [PATCH v3 4/6] blktrace: fix debugfs use after free

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Date:   Fri, 1 May 2020 15:24:45 +0000
From:   Luis Chamberlain <mcgrof@...nel.org>
To:     Greg KH <gregkh@...uxfoundation.org>
Cc:     Christoph Hellwig <hch@...radead.org>, axboe@...nel.dk,
        viro@...iv.linux.org.uk, bvanassche@....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, 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 v3 4/6] blktrace: fix debugfs use after free

On Wed, Apr 29, 2020 at 02:57:26PM +0200, Greg KH wrote:
> On Wed, Apr 29, 2020 at 12:21:52PM +0000, Luis Chamberlain wrote:
> > On Wed, Apr 29, 2020 at 05:04:06AM -0700, Christoph Hellwig wrote:
> > > On Wed, Apr 29, 2020 at 12:02:30PM +0000, Luis Chamberlain wrote:
> > > > > Err, that function is static and has two callers.
> > > > 
> > > > Yes but that is to make it easier to look for who is creating the
> > > > debugfs_dir for either the request_queue or partition. I'll export
> > > > blk_debugfs_root and we'll open code all this.
> > > 
> > > No, please not.  exported variables are usually a bad idea.  Just
> > > skip the somewhat pointless trivial static function.
> > 
> > Alrighty. It has me thinking we might want to only export those symbols
> > to a specific namespace. Thoughts, preferences?
> > 
> > BLOCK_GENHD_PRIVATE ?
> 
> That's a nice add-on issue after this is fixed.  As Christoph and I
> pointed out, you have _less_ code in the file if you remove the static
> wrapper function.  Do that now and then worry about symbol namespaces
> please.

So it turns out that in the old implementation, it was implicit that the
request_queue directory was shared with the scsi drive. So, the
q->debugfs_dir *will* be set, and as we have it here', we'd silently be
overwriting the old q->debugfs_dir, as the queue is the same. To keep
things working as it used to, with both, we just need to use a symlink
here. With the old way, we'd *always* create the sg directory and re-use
that, however since we can only have one blktrace per request_queue, it
still had the same restriction, this was just implicit. Using a symlink
will make this much more obvious and upkeep the old functionality. We'll
need to only export one symbol. I'll roll this in.

  Luis