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

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Date:   Thu, 11 Mar 2021 20:34:44 +0800 (GMT+08:00)
From:   lyl2019@...l.ustc.edu.cn
To:     paulus@...ba.org, davem@...emloft.net
Cc:     linux-ppp@...r.kernel.org, netdev@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: [BUG] net/ppp: A use after free in ppp_unregister_channe

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.

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.