Re: [PATCH-block] blk-cgroup: Use css_tryget() in blkcg_destroy_blkgs()

[Waiman Long <longman@redhat.com>]

On 11/28/22 09:14, Jens Axboe wrote:
> On 11/27/22 8:30 PM, Waiman Long wrote:
>> Commit 951d1e94801f ("blk-cgroup: Flush stats at blkgs destruction
>> path") incorrectly assumes that css_get() will always succeed. That may
>> not be true if there is no blkg associated with the blkcg. If css_get()
>> fails, the subsequent css_put() call may lead to data corruption as
>> was illustrated in a test system that it crashed on bootup when that
>> commit was included. Also blkcg may be freed at any time leading to
>> use-after-free. Fix it by using css_tryget() instead and bail out if
>> the tryget fails.
>>
>> Fixes: 951d1e94801f ("blk-cgroup: Flush stats at blkgs destruction path")
>> Reported-by: Yi Zhang <yi.zhang@redhat.com>
>> Signed-off-by: Waiman Long <longman@redhat.com>
>> ---
>>   block/blk-cgroup.c | 7 ++++++-
>>   1 file changed, 6 insertions(+), 1 deletion(-)
>>
>> diff --git a/block/blk-cgroup.c b/block/blk-cgroup.c
>> index 57941d2a8ba3..74fefc8cbcdf 100644
>> --- a/block/blk-cgroup.c
>> +++ b/block/blk-cgroup.c
>> @@ -1088,7 +1088,12 @@ static void blkcg_destroy_blkgs(struct blkcg *blkcg)
>>   
>>   	might_sleep();
>>   
>> -	css_get(&blkcg->css);
>> +	/*
>> +	 * If css_tryget() fails, there is no blkg to destroy.
>> +	 */
>> +	if (!css_tryget(&blkcg->css))
>> +		return;
>> +
>>   	spin_lock_irq(&blkcg->lock);
>>   	while (!hlist_empty(&blkcg->blkg_list)) {
>>   		struct blkcg_gq *blkg = hlist_entry(blkcg->blkg_list.first,
> This doesn't seem safe to me, but maybe I'm missing something. A tryget
> operation can be fine if we're under RCU lock and the entity is freed
> appropriately, but what makes it safe here? Could blkcg already be gone
> at this point?

The actual freeing of the blkcg structure is under RCU protection. So 
the structure won't be freed immediately even if css_tryget() fails. I 
suspect what Michal found may be the root cause of this problem. If so, 
this is an existing bug which gets exposed by my patch.

Cheers,
Longman