linux-kernel - Re: [PATCH 2/5] blktrace: fix debugfs use after free

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Message-ID: <20200416062054.GL11244@42.do-not-panic.com>
Date:   Thu, 16 Apr 2020 06:20:54 +0000
From:   Luis Chamberlain <mcgrof@...nel.org>
To:     Ming Lei <ming.lei@...hat.com>
Cc:     axboe@...nel.dk, viro@...iv.linux.org.uk, bvanassche@....org,
        gregkh@...uxfoundation.org, rostedt@...dmis.org, mingo@...hat.com,
        jack@...e.cz, 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 2/5] blktrace: fix debugfs use after free

On Thu, Apr 16, 2020 at 01:47:50PM +0800, Ming Lei wrote:
> On Thu, Apr 16, 2020 at 05:25:24AM +0000, Luis Chamberlain wrote:
> > On Thu, Apr 16, 2020 at 10:10:36AM +0800, Ming Lei wrote:
> > > In theory, multiple partitions can be traced concurrently, but looks
> > > it never works, so it won't cause trouble for multiple partition trace.
> > > 
> > > One userspace visible change is that blktrace debugfs dir name is switched 
> > > to disk name from partition name in case of partition trace, will it
> > > break some utilities?
> > 
> > How is this possible, its not clear to me, we go from:
> > 
> > -	q->debugfs_dir = debugfs_create_dir(kobject_name(q->kobj.parent),
> > -					    blk_debugfs_root);
> > 
> > To this:
> > 
> > +	q->debugfs_dir = debugfs_create_dir(kobject_name(q->kobj.parent),
> > +					    blk_debugfs_root);
> > 
> > 
> > Maybe I am overlooking something.
> 
> Your patch removes the blktrace debugfs dir:
> 
> do_blk_trace_setup()
> 
> -       dir = debugfs_lookup(buts->name, blk_debugfs_root);
> -       if (!dir)
> -               bt->dir = dir = debugfs_create_dir(buts->name, blk_debugfs_root);
> -
> 
> Then create blktrace attributes under the dir of q->debugfs_dir.
> 
> However, buts->name could be one partition device name, but

I can see how buts->name is set to bdevname() which expands to
disk_name(bdev->bd_disk, bdev->bd_part->partno, buf).

> q->debugfs_dir has to be disk name.

I can't see this, can you point me to where it is clear the
request_queue kobject's parent is sure to be the disk name?

If it is different, the issue I don't think should be debugfs, but
the bigger issue would be that blktrace on two different partitions
would clash.

Also, the *old* lookup intent on partitions always would fail on mq
and we'd end up creating a directory.

I think we'd need to create a directory per partition, even when we
don't use blktrace. That makes this more complex than I'd hope for.

  Luis