Re: [RFC 2/3] blktrace: fix debugfs use after free

[Bart Van Assche <bvanassche@acm.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.