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

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Message-ID: <20161022155752.GD26044@egarver>
Date:   Sat, 22 Oct 2016 11:57:52 -0400
From:   Eric Garver <e@...g.me>
To:     Arnd Bergmann <arnd@...db.de>
Cc:     Jiri Pirko <jiri@...nulli.us>,
        "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@...vckh6395k16k5.yundunddos.com>,
        Amir Vadai <amir@...ai.me>, netdev@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] flow_dissector: avoid uninitialized variable access

On Sat, Oct 22, 2016 at 12:16:29AM +0200, Arnd Bergmann wrote:
> 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.

Only skb_flow_dissect_flow_keys_buf() calls this function with skb ==
NULL. It uses flow_keys_buf_dissector_keys which does not specify
FLOW_DISSECTOR_KEY_VLAN, so the if statement is false.

A similar assumption is made for FLOW_DISSECTOR_KEY_ETH_ADDRS higher up.

> 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.

I see no harm in moving _vlan to the same scope as vlan.