linux-kernel - Re: [PATCH] flow_dissector: avoid uninitialized variable access

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Message-ID: <3516805.IZ705sLgKU@wuerfel>
Date:   Sat, 22 Oct 2016 00:16:29 +0200
From:   Arnd Bergmann <arnd@...db.de>
To:     Jiri Pirko <jiri@...nulli.us>
Cc:     "David S. Miller" <davem@...emloft.net>,
        Alexander Duyck <aduyck@...antis.com>,
        Tom Herbert <tom@...bertland.com>,
        Jiri Pirko <jiri@...lanox.com>,
        Hadar Hen Zion <hadarh@...lanox.com>,
        Gao Feng <fgao@...ai8.com>, Eric Garver <e@...g.me>,
        Amir Vadai <amir@...ai.me>, netdev@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] flow_dissector: avoid uninitialized variable access

On Friday, October 21, 2016 11:05:45 PM CEST Arnd Bergmann wrote:
> 
> Can you explain why "dissector_uses_key(flow_dissector,
> FLOW_DISSECTOR_KEY_VLAN) && skb_vlan_tag_present(skb)" implies
> "eth_type_vlan(proto))"?
> 
> If I add uninitialized_var() here, I would at least put that in
> a comment here.

Found it now myself: if skb_vlan_tag_present(skb), then we don't
access 'vlan', otherwise we know it is initialized because
eth_type_vlan(proto) has to be true.
 
> On a related note, I also don't see how
> "dissector_uses_key(flow_dissector, FLOW_DISSECTOR_KEY_VLAN)"
> implies that skb is non-NULL. I guess this is related to the
> first one.

I'm still unsure about this one.

I also found something else that is suspicious: 'vlan' points
to the local _vlan variable, but that has gone out of scope
by the time we access the pointer, which doesn't seem safe.

	Arnd