Re: [PATCH-block v2] bdi, blk-cgroup: Fix potential UAF of blkcg

[Waiman Long <longman@redhat.com>]

On 11/30/22 10:16, Michal Koutný wrote:
> On Tue, Nov 29, 2022 at 03:34:00PM -0500, Waiman Long <longman@redhat.com> wrote:
>> The reproducing system can no longer produce a warning with this patch.
>> All the runnable block/0* tests including block/027 were run successfully
>> without failure.
> Thanks for the test!
>
>> @@ -1088,7 +1088,15 @@ static void blkcg_destroy_blkgs(struct blkcg *blkcg)
>>   
>>   	might_sleep();
>>   
>> -	css_get(&blkcg->css);
>> +	/*
>> +	 * blkcg_destroy_blkgs() shouldn't be called with all the blkcg
>> +	 * references gone and rcu_read_lock not held.
>> +	 */
>> +	if (!css_tryget(&blkcg->css)) {
>> +		WARN_ON_ONCE(!rcu_read_lock_held());
>> +		return;
>> +	}
> As I followed the previous discussion, the principle is that obtaining a
> reference or being inside an RCU read section is sufficient.
>
> Consequently, I'd expect the two situations handled equally but here the
> no-ref but RCU bails out. (Which is OK because blkg_list must be empty?)
>
> However, the might_sleep() in (non-sleepable) RCU reader section combo
> makes me wary anyway (not with the early return but tools would likely
> complain).
>
> All in all, can't the contract of blkcg_destroy_blkgs() declare that
> a caller must pass blkcg with a valid reference? (The body of
> blkcg_destroy_blkgs then wouldn't need to get neither put the inner
> reference).

You are right. I should have pushed the might_sleep down(). Will post a 
new version to fix that.

Thanks,
Longman