linux-kernel - Re: [PATCH net-next v2] net: dsa: avoid potential use-after-free error

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Message-ID: <20201120210436.scmic7ygrzviy53o@skbuf>
Date:   Fri, 20 Nov 2020 23:04:36 +0200
From:   Vladimir Oltean <olteanv@...il.com>
To:     Jakub Kicinski <kuba@...nel.org>
Cc:     Christian Eggers <ceggers@...i.de>, Andrew Lunn <andrew@...n.ch>,
        Florian Fainelli <f.fainelli@...il.com>,
        Vivien Didelot <vivien.didelot@...il.com>,
        "David S . Miller" <davem@...emloft.net>, netdev@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH net-next v2] net: dsa: avoid potential use-after-free
 error

On Fri, Nov 20, 2020 at 12:59:21PM -0800, Jakub Kicinski wrote:
> On Fri, 20 Nov 2020 20:01:49 +0200 Vladimir Oltean wrote:
> > On Thu, Nov 19, 2020 at 12:09:06PM +0100, Christian Eggers wrote:
> > > If dsa_switch_ops::port_txtstamp() returns false, clone will be freed
> > > immediately. Shouldn't store a pointer to freed memory.
> > >
> > > Signed-off-by: Christian Eggers <ceggers@...i.de>
> > > Fixes: 146d442c2357 ("net: dsa: Keep a pointer to the skb clone for TX timestamping")
> > > ---
> >
> > IMO this is one of the cases to which the following from
> > Documentation/process/stable-kernel-rules.rst does not apply:
> >
> >  - It must fix a real bug that bothers people (not a, "This could be a
> >    problem..." type thing).
> >
> > Therefore, specifying "net-next" as the target tree here as opposed to
> > "net" is the correct choice.
>
> The commit message doesn't really explain what happens after.
>
> Is the dangling pointer ever accessed?

Nothing happens afterwards. He explained that he accessed it once while
working on his ksz9477 PTP series. There's no code affected by this in
mainline.