Re: [PATCH v7 6/8] blktrace: fix debugfs use after free

Bart Van Assche
2020-06-20T17:31:58Z

On 2020-06-19 13:47, Luis Chamberlain wrote:
> This goes tested with:
^^^^
got?

> diff --git a/kernel/trace/blktrace.c b/kernel/trace/blktrace.c
> index 7ff2ea5cd05e..e6e2d25fdbd6 100644
> --- a/kernel/trace/blktrace.c
> +++ b/kernel/trace/blktrace.c
> @@ -524,10 +524,18 @@ static int do_blk_trace_setup(struct request_queue *q, char *name, dev_t dev,
> if (!bt->msg_data)
> goto err;
>
> - ret = -ENOENT;
> -
> - dir = debugfs_lookup(buts->name, blk_debugfs_root);
> - if (!dir)
> +#ifdef CONFIG_BLK_DEBUG_FS
> + /*
> + * When tracing whole make_request drivers (multiqueue) block devices,
> + * reuse the existing debugfs directory created by the block layer on
> + * init. For request-based block devices, all partitions block devices,
^^^^^^^^^^^^^^^^^^^^^
It seems like a word is missing from the comment? Or did you perhaps
want to refer to "all partition block devices"?

> + * and scsi-generic block devices we create a temporary new debugfs
> + * directory that will be removed once the trace ends.
> + */
> + if (queue_is_mq(q) && bdev && bdev == bdev->bd_contains)
> + dir = q->debugfs_dir;
> + else
> +#endif
> bt->dir = dir = debugfs_create_dir(buts->name, blk_debugfs_root);

Can it happen that two different threads each try to set up block
tracing and hence that debugfs_create_dir() fails because a directory
with name buts->name already exists?

> bt->dev = dev;
> @@ -565,8 +573,6 @@ static int do_blk_trace_setup(struct request_queue *q, char *name, dev_t dev,
>
> ret = 0;
> err:
> - if (dir && !bt->dir)
> - dput(dir);
> if (ret)
> blk_trace_free(bt);
> return ret;

Shouldn't bt->dir be removed in this error path for make_request drivers?

Thanks,

Bart.