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From: Bart Van Assche <bvanassche@acm.org>
To: 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: Sat, 4 Apr 2020 20:39:47 -0700	[thread overview]
Message-ID: <3640b16b-abda-5160-301a-6a0ee67365b4@acm.org> (raw)
In-Reply-To: <20200402000002.7442-3-mcgrof@kernel.org>

On 2020-04-01 17:00, Luis Chamberlain wrote:
> korg#205713 then was used to create CVE-2019-19770 and claims that
> the bug is in a use-after-free in the debugfs core code. The
> implications of this being a generic UAF on debugfs would be
> much more severe, as it would imply parent dentries can sometimes
> not be possitive, which is something claim is not possible.
         ^^^^^^^^^  ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
         positive?  is there perhaps a word missing here?

> It turns out that the issue actually is a mis-use of debugfs for
> the multiqueue case, and the fragile nature of how we free the
> directory used to keep track of blktrace debugfs files. Omar's
> commit assumed the parent directory would be kept with
> debugfs_lookup() but this is not the case, only the dentry is
> kept around. We also special-case a solution for multiqueue
> given that for multiqueue code we always instantiate the debugfs
> directory for the request queue. We were leaving it only to chance,
> if someone happens to use blktrace, on single queue block devices
> for the respective debugfs directory be created.

Since the legacy block layer is gone, the above explanation may have to
be rephrased.

> We can fix the UAF by simply using a debugfs directory which is
> always created for singlequeue and multiqueue block devices. This
> simplifies the code considerably, with the only penalty now being
> that we're always creating the request queue directory debugfs
> directory for the block device on singlequeue block devices.

Same comment here - the legacy block layer is gone. I think that today
all block drivers are either request-based and multiqueue or so-called
make_request drivers. See also the output of git grep -nHw
blk_alloc_queue for examples of the latter category.

> This patch then also contends the severity of CVE-2019-19770 as
> this issue is only possible using root to shoot yourself in the
> foot by also misuing blktrace.
               ^^^^^^^
               misusing?

> diff --git a/block/blk-mq-debugfs.c b/block/blk-mq-debugfs.c
> index b3f2ba483992..bda9378eab90 100644
> --- a/block/blk-mq-debugfs.c
> +++ b/block/blk-mq-debugfs.c
> @@ -823,9 +823,6 @@ void blk_mq_debugfs_register(struct request_queue *q)
>  	struct blk_mq_hw_ctx *hctx;
>  	int i;
>  
> -	q->debugfs_dir = debugfs_create_dir(kobject_name(q->kobj.parent),
> -					    blk_debugfs_root);
> -
>  	debugfs_create_files(q->debugfs_dir, q, blk_mq_debugfs_queue_attrs);
>  
>  	/*

[ ... ]

>  static void blk_mq_debugfs_register_ctx(struct blk_mq_hw_ctx *hctx,
> diff --git a/block/blk-sysfs.c b/block/blk-sysfs.c
> index fca9b158f4a0..20f20b0fa0b9 100644
> --- a/block/blk-sysfs.c
> +++ b/block/blk-sysfs.c
> @@ -895,6 +895,7 @@ static void __blk_release_queue(struct work_struct *work)
>  
>  	blk_trace_shutdown(q);
>  
> +	blk_q_debugfs_unregister(q);
>  	if (queue_is_mq(q))
>  		blk_mq_debugfs_unregister(q);

Does this patch change the behavior of the block layer from only
registering a debugfs directory for request-based block devices to
registering a debugfs directory for request-based and make_request based
block devices? Is that behavior change an intended behavior change?

Thanks,

Bart.

  parent reply	other threads:[~2020-04-05  3:39 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 [this message]
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
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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