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

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Date:   Sun, 18 Sep 2016 13:26:30 -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>
Subject: Re: [PATCH] net: skbuff: Fix length validation in skb_vlan_pop()

On Sun, Sep 18, 2016 at 3:09 AM, Shmulik Ladkani
<shmulik.ladkani@...il.com> wrote:
> In 93515d53b1
>   "net: move vlan pop/push functions into common code"
> skb_vlan_pop was moved from its private location in openvswitch to
> skbuff common code.
>
> In case !vlan_tx_tag_present, the original 'pop_vlan()' assured
> that skb->len is sufficient for the existence of a vlan_ethhdr
> (if skb->len < VLAN_ETH_HLEN then pop was a no-op).
>
> This validation was moved as is into the new common 'skb_vlan_pop'.
>
> Alas, in its original location (openvswitch), there's a guarantee that
> 'data' points to the mac_header, therefore the 'skb->len < VLAN_ETH_HLEN'
> condition made sense.
> However there's no such guarantee in the generic 'skb_vlan_pop'.
>
> For short packets received in rx path going through 'skb_vlan_pop',
> this causes 'skb_vlan_pop' to fail pop-ing a valid vlan hdr (in case tag
> is in payload), or to fail moving next tag into hw-accel tag.
>
> Instead, verify that 'skb->mac_len' is sufficient.
>
> Signed-off-by: Shmulik Ladkani <shmulik.ladkani@...il.com>
> ---
>  Spotted by code review while doing work augmenting tc act vlan.
>
>  net/core/skbuff.c | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
>
> diff --git a/net/core/skbuff.c b/net/core/skbuff.c
> index 1e329d4112..cc2c004838 100644
> --- a/net/core/skbuff.c
> +++ 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.