Re: [PATCH-block v3 1/2] bdi, blk-cgroup: Fix potential UAF of blkcg

[Waiman Long <longman@redhat.com>]

On 12/13/22 14:29, Tejun Heo wrote:
> On Tue, Dec 13, 2022 at 01:44:45PM -0500, Waiman Long wrote:
>> Commit 59b57717fff8 ("blkcg: delay blkg destruction until after
>> writeback has finished") delayed call to blkcg_destroy_blkgs() to
>> cgwb_release_workfn(). However, it is done after a css_put() of blkcg
>> which may be the final put that causes the blkcg to be freed as RCU
>> read lock isn't held.
>>
>> Another place where blkcg_destroy_blkgs() can be called indirectly via
>> blkcg_unpin_online() is from the offline_css() function called from
>> css_killed_work_fn(). Over there, the potentially final css_put() call
>> is issued after offline_css().
>>
>> By adding a css_tryget() into blkcg_destroy_blkgs() and warning its
>> failure, the following stack trace was produced in a test system on
>> bootup.
> This doesn't agree with the code anymore. Otherwise
>
> Acked-by: Tejun Heo <tj@kernel.org>

Sorry, I overlooked the commit log in my update. I will update it if I 
need another version, or Jens can make the following edit:

css_tryget() -> percpu_ref_is_zero().

Thanks,
Longman