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

[Eric Sandeen]

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