Re: [PATCH -next] block: fix uaf for flush rq while iterating tags

[Yu Kuai]

Hi,Ming and Christoph

在 2024/11/05 19:19, Christoph Hellwig 写道:
> On Mon, Nov 04, 2024 at 07:00:05PM +0800, Yu Kuai wrote:
> > From: Yu Kuai <yukuai3@xxxxxxxxxx>
> >
> > blk_mq_clear_flush_rq_mapping() is not called during scsi probe, by
> > checking blk_queue_init_done(). However, QUEUE_FLAG_INIT_DONE is cleared
> > in del_gendisk by commit aec89dc5d421 ("block: keep q_usage_counter in
> > atomic mode after del_gendisk"), hence for disk like scsi, following
> > blk_mq_destroy_queue() will not clear flush rq from tags->rqs[] as well,
> > cause following uaf that is found by our syzkaller for v6.6:
>
> Which means we leave the flush request lingering after del_gendisk,
> which sounds like the real bug.  I suspect we just need to move the
> call to blk_mq_clear_flush_rq_mapping so that it is called from
> del_gendisk and doesn't leave the flush tag lingering around.

This remind me that del_gendisk is still too late to do that. Noted that
flush_rq can acquire different tags, so if the multiple flush_rq is done
and those tags are not reused, the flush_rq can exist in multiple
entries in tags->rqs[]. The consequence I can think of is that iterating
tags can found the same flush_rq multiple times, and the flush_rq can be
inflight.

Therefor, I think the best solution is to clear the flush_rq mapping
when flush request is done.

Thanks,
Kuai
> .