Re: [PATCH -next v2 7/9] blk-iocost: fix UAF in ioc_pd_free

Yu Kuai
2022-12-06T07:53:40Z

Hi, Tejun!

在 2022/12/01 4:42, Tejun Heo 写道:
> On Wed, Nov 30, 2022 at 09:21:54PM +0800, Li Nan wrote:
>> T1 T2 T3
>> //delete device
>> del_gendisk
>> bdi_unregister
>> bdi_remove_from_list
>> synchronize_rcu_expedited
>>
>> //rmdir cgroup
>> blkcg_destroy_blkgs
>> blkg_destroy
>> percpu_ref_kill
>> blkg_release
>> call_rcu
>> rq_qos_exit
>> ioc_rqos_exit
>> kfree(ioc)
>> __blkg_release
>> blkg_free
>> blkg_free_workfn
>> pd_free_fn
>> ioc_pd_free
>> spin_lock_irqsave
>> ->ioc is freed
>>
>> Fix the problem by moving the operation on ioc in ioc_pd_free() to
>> ioc_pd_offline(), and just free resource in ioc_pd_free() like iolatency
>> and throttle.
>>
>> Signed-off-by: Li Nan <linan122@huawei.com>
>
> I wonder what we really wanna do is pinning ioc while blkgs are still around
> but I think this should work too.
>

I just found that this is not enough, other problems still exist:

t1:
bio_init
bio_associate_blkg
//get blkg->refcnt
......
submit_bio
blk_throtl_bio
// bio is throttlled, user thread can exit
t2:
// blkcg online_pin is zero
blkcg_destroy_blkgs
blkg_destroy
ioc_pd_offline
list_del_init(&iocg->active_list)
t3:
ioc_rqos_throttle
blkg_to_iocg
// got the iocg that is offlined
iocg_activate
// acitvate the iocg again

For consequence, kernel can crash due to access unexpected
address. Fortunately, bfq already handle similar problem by checking
blkg->online in bfq_bio_bfqg(), this problem can be fixed by checking
it in iocg_activate().

BTW, I'm still working on checking if other policies have the same
problem.

Thanks,
Kuai