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From: Eric Sandeen <sandeen@sandeen.net>
To: Bart Van Assche <bvanassche@acm.org>,
	Luis Chamberlain <mcgrof@kernel.org>,
	axboe@kernel.dk, viro@zeniv.linux.org.uk,
	gregkh@linuxfoundation.org, rostedt@goodmis.org,
	mingo@redhat.com, jack@suse.cz, ming.lei@redhat.com,
	nstange@suse.de
Cc: mhocko@suse.com, linux-block@vger.kernel.org,
	linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org,
	Omar Sandoval <osandov@fb.com>, Hannes Reinecke <hare@suse.com>,
	Michal Hocko <mhocko@kernel.org>,
	syzbot+603294af2d01acfdd6da@syzkaller.appspotmail.com
Subject: Re: [RFC 2/3] blktrace: fix debugfs use after free
Date: Mon, 6 Apr 2020 09:29:40 -0500	[thread overview]
Message-ID: <4a84844d-ef87-c609-9963-4dea17bc506c@sandeen.net> (raw)
In-Reply-To: <75aa4cff-1b90-ebd4-17a4-c1cb6d390b30@acm.org>

On 4/5/20 11:25 PM, Bart Van Assche wrote:
> On 2020-04-05 18:27, Eric Sandeen wrote:
>> The thing I can't figure out from reading the change log is
>>
>> 1) what the root cause of the problem is, and
>> 2) how this patch fixes it?
> 
> I think that the root cause is that do_blk_trace_setup() uses
> debugfs_lookup() and that debugfs_lookup() may return a pointer
> associated with a previous incarnation of the block device.
> Additionally, I think the following changes fix that problem by using
> q->debugfs_dir in the blktrace code instead of debugfs_lookup():

Yep, I gathered that from reading the patch, was just hoping for a commit log
that makes it clear.

> [ ... ]
> --- a/kernel/trace/blktrace.c
> +++ b/kernel/trace/blktrace.c
> @@ -311,7 +311,6 @@ static void blk_trace_free(struct blk_trace *bt)
>  	debugfs_remove(bt->msg_file);
>  	debugfs_remove(bt->dropped_file);
>  	relay_close(bt->rchan);
> -	debugfs_remove(bt->dir);
>  	free_percpu(bt->sequence);
>  	free_percpu(bt->msg_data);
>  	kfree(bt);
> [ ... ]
> @@ -509,21 +510,19 @@ static int do_blk_trace_setup(struct request_queue
> *q, char *name, dev_t dev,
> 
>  	ret = -ENOENT;
> 
> -	dir = debugfs_lookup(buts->name, blk_debugfs_root);
> -	if (!dir)
> -		bt->dir = dir = debugfs_create_dir(buts->name, blk_debugfs_root);
> -
>  	bt->dev = dev;
>  	atomic_set(&bt->dropped, 0);
>  	INIT_LIST_HEAD(&bt->running_list);
> 
>  	ret = -EIO;
> -	bt->dropped_file = debugfs_create_file("dropped", 0444, dir, bt,
> +	bt->dropped_file = debugfs_create_file("dropped", 0444,
> +					       q->debugfs_dir, bt,
>  					       &blk_dropped_fops);

One thing I'm not sure about, the block_trace *bt still points to a dentry that
could get torn down via debugfs_remove_recursive when the queue is released, right,
but could later be sent to blk_trace_free again?  And yet this does seem to fix the
use after free in my testing, so I must be missing something.

Thanks,
-Eric

  parent reply	other threads:[~2020-04-06 14:29 UTC|newest]

Thread overview: 29+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-04-01 23:59 [RFC 0/3] block: address blktrace use-after-free Luis Chamberlain
2020-04-02  0:00 ` [RFC 1/3] block: move main block debugfs initialization to its own file Luis Chamberlain
2020-04-05  3:12   ` Bart Van Assche
2020-04-06 14:23     ` Luis Chamberlain
2020-04-02  0:00 ` [RFC 2/3] blktrace: fix debugfs use after free Luis Chamberlain
2020-04-02  1:57   ` Eric Sandeen
2020-04-02 16:14     ` Luis Chamberlain
2020-04-05  3:39   ` Bart Van Assche
2020-04-06  1:27     ` Eric Sandeen
2020-04-06  4:25       ` Bart Van Assche
2020-04-06  9:18         ` Nicolai Stange
2020-04-06 15:19           ` Luis Chamberlain
2020-04-07  8:15             ` Luis Chamberlain
2020-04-06 14:29         ` Eric Sandeen [this message]
2020-04-07  8:09           ` Luis Chamberlain
2020-04-06 15:14     ` Luis Chamberlain
2020-04-02  0:00 ` [RFC 3/3] block: avoid deferral of blk_release_queue() work Luis Chamberlain
2020-04-02  3:39   ` Bart Van Assche
2020-04-02 14:49     ` Nicolai Stange
2020-04-06  9:11       ` Nicolai Stange
2020-04-09 18:11       ` Luis Chamberlain
2020-04-02  7:44 ` [RFC 0/3] block: address blktrace use-after-free Greg KH
2020-04-03  8:19 ` Ming Lei
2020-04-03 14:06   ` Luis Chamberlain
2020-04-03 14:13   ` Bart Van Assche
2020-04-03 19:49     ` Luis Chamberlain
2020-04-07  2:47   ` yukuai (C)
2020-04-07 19:00     ` Luis Chamberlain
2020-04-09 20:59       ` Luis Chamberlain

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