netdev - Re: [PATCH net-next] net: ethtool: fix NULL pointer dereference in stats_prepare

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Message-ID: <20230125004517.74c7ssj47zykciuv@skbuf>
Date:   Wed, 25 Jan 2023 02:45:17 +0200
From:   Vladimir Oltean <vladimir.oltean@....com>
To:     Jakub Kicinski <kuba@...nel.org>
Cc:     netdev@...r.kernel.org, "David S. Miller" <davem@...emloft.net>,
        Eric Dumazet <edumazet@...gle.com>,
        Paolo Abeni <pabeni@...hat.com>,
        Michal Kubecek <mkubecek@...e.cz>
Subject: Re: [PATCH net-next] net: ethtool: fix NULL pointer dereference in
 stats_prepare_data()

On Tue, Jan 24, 2023 at 04:23:47PM -0800, Jakub Kicinski wrote:
> On Tue, 24 Jan 2023 13:08:01 +0200 Vladimir Oltean wrote:
> > In the following call path:
> > 
> > ethnl_default_dumpit
> > -> ethnl_default_dump_one
> >    -> ctx->ops->prepare_data
> >       -> stats_prepare_data  
> > 
> > struct genl_info *info will be passed as NULL, and stats_prepare_data()
> > dereferences it while getting the extended ack pointer.
> > 
> > To avoid that, just set the extack to NULL if "info" is NULL, since the
> > netlink extack handling messages know how to deal with that.
> > 
> > The pattern "info ? info->extack : NULL" is present in quite a few other
> > "prepare_data" implementations, so it's clear that it's a more general
> > problem to be dealt with at a higher level, but the code should have at
> > least adhered to the current conventions to avoid the NULL dereference.
> 
> Choose one:
>  - you disagree with my comment on the report
>  - you don't think that we should mix the immediate fix with the
>    structural change
>  - you agree but "don't have the time" to fix this properly

Yeah, sorry, I shouldn't have left your question unanswered ("should we make
a fake struct genl_info * to pass here?") - but I don't think I'm qualified
enough to have an opinion, either. Whereas the immediate fix is neutral
enough to not be controversial, or so I thought.

The problem is not so much "the time to fix this properly", but rather,
I'm not even sure how to trigger the ethtool dumpit() code...