netdev - Re: [PATCH 4.9.y] net: sched: prevent UAF on tc_ctl_tfilter when temporarily dropping rtnl

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Date:   Tue, 3 May 2022 11:24:01 -0300
From:   Thadeu Lima de Souza Cascardo <cascardo@...onical.com>
To:     Greg KH <gregkh@...uxfoundation.org>
Cc:     stable@...r.kernel.org, netdev@...r.kernel.org, kuba@...nel.org,
        davem@...emloft.net, jiri@...nulli.us, xiyou.wangcong@...il.com,
        jhs@...atatu.com, vladbu@...lanox.com
Subject: Re: [PATCH 4.9.y] net: sched: prevent UAF on tc_ctl_tfilter when
 temporarily dropping rtnl_lock

On Tue, May 03, 2022 at 03:49:15PM +0200, Greg KH wrote:
> On Mon, May 02, 2022 at 05:49:24PM -0300, Thadeu Lima de Souza Cascardo wrote:
> > When dropping the rtnl_lock for looking up for a module, the device may be
> > removed, releasing the qdisc and class memory. Right after trying to load
> > the module, cl_ops->put is called, leading to a potential use-after-free.
> > 
> > Though commit e368fdb61d8e ("net: sched: use Qdisc rcu API instead of
> > relying on rtnl lock") fixes this, it involves a lot of refactoring of the
> > net/sched/ code, complicating its backport.
> 
> What about 4.14.y?  We can not take a commit for 4.9.y with it also
> being broken in 4.14.y, and yet fixed in 4.19.y, right?  Anyone who
> updates from 4.9 to 4.14 will have a regression.
> 
> thanks,
> 
> greg k-h

4.14.y does not call cl_ops->put (the get/put and class refcount has been done
with on 4.14.y). However, on the error path after the lock has been dropped,
tcf_chain_put is called. But it does not touch the qdisc, but only the chain
and block objects, which cannot be released on a race condition, as far as I
was able to investigate.

Cascardo.