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

Waiman Long
2022-11-28T15:54:41Z

On 11/28/22 10:42, Jens Axboe wrote:
> On 11/28/22 8:38?AM, Waiman Long wrote:
>> 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.
> But what prevents it from going away here since you're not under RCU
> lock for the tryget? Doesn't help that the freeing side is done in an
> RCU safe manner, if the ref attempt is not.

You are right. blkcg_destroy_blkgs() shouldn't be called with all the
blkcg references gone. Will work on a revised patch.

Cheers,
Longman