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

[Waiman Long]

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@xxxxxxxxxx>
> > Signed-off-by: Waiman Long <longman@xxxxxxxxxx>
> > ---
> >   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