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SubjectRe: [PATCH v5 5/7] blktrace: fix debugfs use after free
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On 2020-05-26 20:12, Luis Chamberlain wrote:
> + /*
> + * Blktrace needs a debugsfs name even for queues that don't register
> + * a gendisk, so it lazily registers the debugfs directory. But that
> + * can get us into a situation where a SCSI device is found, with no
> + * driver for it (yet). Then blktrace is used on the device, creating
> + * the debugfs directory, and only after that a drivers is loaded. In
^^^^^^^
driver?

> @@ -494,6 +490,38 @@ static int do_blk_trace_setup(struct request_queue *q, char *name, dev_t dev,
> */
> strreplace(buts->name, '/', '_');
>
> + /*
> + * We also have to use a partition directory if a partition is
> + * being worked on, even though the same request_queue is shared.
> + */
> + if (bdev && bdev != bdev->bd_contains)
> + dir = bdev->bd_part->debugfs_dir;

Please balance braces in if-statements as required by the kernel coding style.

> + else {
> + /*
> + * For queues that do not have a gendisk attached to them, the
> + * debugfs directory will not have been created at setup time.
> + * Create it here lazily, it will only be removed when the
> + * queue is torn down.
> + */

Is the above comment perhaps a reference to blk_register_queue()? If so, please
mention the name of that function explicitly.

> + if (!q->debugfs_dir) {
> + q->debugfs_dir =
> + debugfs_create_dir(buts->name,
> + blk_debugfs_root);
> + }
> + dir = q->debugfs_dir;
> + }
> +
> + /*
> + * As blktrace relies on debugfs for its interface the debugfs directory
> + * is required, contrary to the usual mantra of not checking for debugfs
> + * files or directories.
> + */
> + if (IS_ERR_OR_NULL(q->debugfs_dir)) {
> + pr_warn("debugfs_dir not present for %s so skipping\n",
> + buts->name);
> + return -ENOENT;
> + }

How are do_blk_trace_setup() calls serialized against the debugfs directory
creation code in blk_register_queue()? Perhaps via q->blk_trace_mutex? Are
mutex lock and unlock calls for that mutex perhaps missing from
compat_blk_trace_setup()?

How about adding a lockdep_assert_held(&q->blk_trace_mutex) statement in
do_blk_trace_setup()?

Thanks,

Bart.

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