netdev - Re: Re: [BUG] net/ppp: A use after free in ppp_unregister

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Date:   Mon, 15 Mar 2021 16:58:07 +0000
From:   Tom Parkin <tparkin@...alix.com>
To:     Guillaume Nault <gnault@...hat.com>
Cc:     lyl2019@...l.ustc.edu.cn, paulus@...ba.org, davem@...emloft.net,
        linux-ppp@...r.kernel.org, netdev@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: Re: [BUG] net/ppp: A use after free in ppp_unregister_channe

On  Mon, Mar 15, 2021 at 13:18:24 +0100, Guillaume Nault wrote:
> On Fri, Mar 12, 2021 at 10:47:53PM +0800, lyl2019@...l.ustc.edu.cn wrote:
> > 
> > 
> > 
> > > -----原始邮件-----
> > > 发件人: "Tom Parkin" <tparkin@...alix.com>
> > > 发送时间: 2021-03-12 18:12:58 (星期五)
> > > 收件人: lyl2019@...l.ustc.edu.cn
> > > 抄送: paulus@...ba.org, davem@...emloft.net, linux-ppp@...r.kernel.org, netdev@...r.kernel.org, linux-kernel@...r.kernel.org
> > > 主题: Re: [BUG] net/ppp: A use after free in ppp_unregister_channe
> > > 
> > > Thanks for the report!
> > > 
> > > On  Thu, Mar 11, 2021 at 20:34:44 +0800, lyl2019@...l.ustc.edu.cn wrote:
> > > > File: drivers/net/ppp/ppp_generic.c
> > > > 
> > > > In ppp_unregister_channel, pch could be freed in ppp_unbridge_channels()
> > > > but after that pch is still in use. Inside the function ppp_unbridge_channels,
> > > > if "pchbb == pch" is true and then pch will be freed.
> > > 
> > > Do you have a way to reproduce a use-after-free scenario?
> > > 
> > > From static analysis I'm not sure how pch would be freed in
> > > ppp_unbridge_channels when called via. ppp_unregister_channel.
> > > 
> > > In theory (at least!) the caller of ppp_register_net_channel holds 
> > > a reference on struct channel which ppp_unregister_channel drops.
> > > 
> > > Each channel in a bridged pair holds a reference on the other.
> > > 
> > > Hence on return from ppp_unbridge_channels, the channel should not have
> > > been freed (in this code path) because the ppp_register_net_channel
> > > reference has not yet been dropped.
> > > 
> > > Maybe there is an issue with the reference counting or a race of some
> > > sort?
> > > 
> > > > I checked the commit history and found that this problem is introduced from
> > > > 4cf476ced45d7 ("ppp: add PPPIOCBRIDGECHAN and PPPIOCUNBRIDGECHAN ioctls").
> > > > 
> > > > I have no idea about how to generate a suitable patch, sorry.
> > 
> > This issue was reported by a path-sensitive static analyzer developed by our Lab,
> > thus i have not a crash or bug log.
> > 
> > As the return type of ppp_unbridge_channels() is a int, can we return a value to
> > inform caller that the channel is freed?
> 
> I don't think this is going to improve anything, as
> ppp_unregister_channel() couldn't take any corrective action anyway.

I agree with you to be honest.

I think the best ppp_unregister_channel could to is to not access pch
again.

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