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

[Greg KH <gregkh@linuxfoundation.org>]

On Tue, May 03, 2022 at 11:24:01AM -0300, Thadeu Lima de Souza Cascardo wrote:
> 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.

So what changed between 4.9 and 4.14 that requires this out-of-tree
change to 4.9 for the issue?  Shouldn't we backport that change instead
of this custom one?

thanks,

greg k-h