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

[Jens Axboe <axboe@kernel.dk>]

On 11/30/22 8:16?AM, Michal Koutn? 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).

Totally agree, the proposed patch feels more like a hacky workaround
rather than a true solution. Either the contract should be that it's
ALWAYS entered with RCU lock held and hence the tryget is fine, OR that
a reference always is held when entered.

I'm going to revert the offending patch for now, and then we can queue
up a proper patch when that exists.

-- 
Jens Axboe