netdev - Re: [PATCH] net: skbuff: Fix length validation in skb_vlan

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Message-ID: <CAOrHB_Dy3osOea19e+iTxHFpCgJOYe1fUJJnaN-PB-hOq0-rmA@mail.gmail.com>
Date:   Mon, 19 Sep 2016 13:46:10 -0700
From:   pravin shelar <pshelar@....org>
To:     Shmulik Ladkani <shmulik.ladkani@...il.com>
Cc:     Jiri Pirko <jiri@...lanox.com>,
        "David S . Miller" <davem@...emloft.net>,
        Linux Kernel Network Developers <netdev@...r.kernel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Jamal Hadi Salim <jhs@...atatu.com>
Subject: Re: [PATCH] net: skbuff: Fix length validation in skb_vlan_pop()

On Mon, Sep 19, 2016 at 1:04 PM, Shmulik Ladkani
<shmulik.ladkani@...il.com> wrote:
> Hi Pravin,
>
> On Sun, 18 Sep 2016 13:26:30 -0700 pravin shelar <pshelar@....org> wrote:
>> > +++ b/net/core/skbuff.c
>> > @@ -4537,7 +4537,7 @@ int skb_vlan_pop(struct sk_buff *skb)
>> >         } else {
>> >                 if (unlikely((skb->protocol != htons(ETH_P_8021Q) &&
>> >                               skb->protocol != htons(ETH_P_8021AD)) ||
>> > -                            skb->len < VLAN_ETH_HLEN))
>> > +                            skb->mac_len < VLAN_ETH_HLEN))
>>
>> There is already check in __skb_vlan_pop() to validate skb for a vlan
>> header. So it is safe to drop this check entirely.
>
> Yep, I submitted a v2 with your suggestion, however I withdrew it, as
> there is a slight behavior difference noticable by 'skb_vlan_pop' callers.
>
> Suppose the rare case where skb->len is too small.
>
> pre:
>   skb_vlan_pop returns 0 (at least for the correct tx path).
>   Meaning, callers do not see it as a failure.
> post:
>   skb_ensure_writable fails (!pskb_may_pull), therefore -ENOMEM returned
>   to the callers of 'skb_vlan_pop'.
>
> For ovs, it means do_execute_actions's loop is terminated, no further
> actions are executed, and skb gets freed.
>
> For tc act vlan, it means skb gets dropped.
>
> This actually makes sense, but do we want to present this change?
>
I think this is correct behavior over existing code. And under memory
pressure chances of packet drop are higher even without the change
anyways.