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

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Message-ID: <20210315115058.GA4296@linux.home>
Date:   Mon, 15 Mar 2021 12:50:58 +0100
From:   Guillaume Nault <gnault@...hat.com>
To:     lyl2019@...l.ustc.edu.cn
Cc:     paulus@...ba.org, davem@...emloft.net, linux-ppp@...r.kernel.org,
        netdev@...r.kernel.org, linux-kernel@...r.kernel.org,
        Tom Parkin <tparkin@...alix.com>
Subject: Re: [BUG] net/ppp: A use after free in ppp_unregister_channe

On Thu, Mar 11, 2021 at 08:34:44PM +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.

No. It's freed only if if the refcount drops to 0. And the caller of
ppp_unregister_channel() must hold its own refcount. So
ppp_unbridge_channels() is not going to drop the last refcount in this
case.

> I checked the commit history and found that this problem is introduced from
> 4cf476ced45d7 ("ppp: add PPPIOCBRIDGECHAN and PPPIOCUNBRIDGECHAN ioctls").

Next time, please also Cc. the author of the patch.

> I have no idea about how to generate a suitable patch, sorry.

There's no patch to send as far as I can see.