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

[Yu Kuai]

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@xxxxxxxxxx>
>
> 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