linux-kernel - Re: [PATCH] workqueue: Fix UAF report by KASAN in pwq_release

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Date:   Fri, 8 Dec 2023 15:32:04 +0800
From:   Xuewen Yan <xuewen.yan94@...il.com>
To:     Z qiang <qiang.zhang1211@...il.com>
Cc:     Tejun Heo <tj@...nel.org>, jiangshanlai@...il.com,
        linux-kernel@...r.kernel.org, guohua.yan@...soc.com
Subject: Re: [PATCH] workqueue: Fix UAF report by KASAN in pwq_release_workfn()

Hi qiang && Tejun

On Tue, Sep 19, 2023 at 7:30 PM Z qiang <qiang.zhang1211@...il.com> wrote:
>
> >
> > On Thu, Sep 07, 2023 at 10:13:23AM +0800, Z qiang wrote:
> > > >
> > > > On Wed, Sep 06, 2023 at 10:12:34AM +0800, Z qiang wrote:
> > > > > Flush the pwq_release_worker is insufficient, the call_rcu() is
> > > > > invoked to release wq
> > > > > in pwq_release_workfn(), this is also asynchronous.
> > > >
> > > > But rcu_free_pwq() doesn't access wq or anything. The last access is from
> > > > the work function.
> > >
> > > The rcu_free_wq() will access wq->cpu_pwq or unbound_attrs,
> > > but  at this time, the kfree(wq) may have been called in alloc_workqueue().
> >
> > I'm not following. The only way alloc_and_link fails is if
> > apply_wqattrs_prepare() fails and if prepare fails, none of the pwq's are
> > installed and pwq_unbound_release_workfn() won't try to free the wq as the
> > pwq's don't have any reference on it. So, if you flush the pwq release work
> > items, there can be no rcu_free_wq() in flight. Can you please try to see
> > whether the problem is reproducible with flushing?
> >
>
> you are right .  sorry,  I ignore if apply_wqattrs_prepare() fails,
> none of the pwq is installed,
> the install_unbound_pwq() is not invoked. I will resend v2 and test.

I want to ask a question, why delete the v1?
Although add the flush_work, the release work would release the lockdep key,
and at the last of alloc_workqueue, it would also unregister_lockdep_key twice.

And other question, why not free lockdep and just unregister_lockdep?

4136         if (is_last) {
4137                 wq_unregister_lockdep(wq);
4138                 call_rcu(&wq->rcu, rcu_free_wq);
4139         }

I would really appreciate it if you could help explain it.

Thanks!
BR



>
> Thanks
> Zqiang
>
> >
> > Thanks.
> >
> > --
> > tejun