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

[Eric Sandeen]

On 4/4/20 10:39 PM, Bart Van Assche wrote:
> 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?

Honestly I think the whole "misusing blktrace" narrative is not relevant
here; the kernel has to deal with whatever ioctls it receives, right.

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?

>> 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?

Seems to be:

"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."

?

-Eric