Re: [PATCH net v2 2/6] net/sched: act_mpls: pull the inner Ethernet header after POP

[Willem de Bruijn <willemdebruijn.kernel@gmail.com>]

Jamal Hadi Salim wrote:
> The issue:
> act_mpls POP can leave the inner Ethernet header in a page fragment after
> skb_mpls_pop(), either on an L3 device (mac_len == 0) or chained after
> act_vlan POP_ETH.  A following eth_type_trans() can then BUG in
> __skb_pull().
> 
> The Fix:
> After the pop, if the original MAC header is shorter than ETH_HLEN, pull
> the inner Ethernet header into the linear area and drop only when
> pskb_may_pull(skb, ETH_HLEN) fails.  skb_mpls_pop() does nothing when the
> skb was not MPLS, so no separate state is needed.
> 
> Conditions to recreate the bug:
> - egress tc filter on lo: action vlan pop_eth action mpls pop
>   (chained), or on an L3 device where mac_len == 0.
> - AF_PACKET SOCK_RAW with PACKET_VNET_HDR, hdr_len=28, and a non-linear
>   eth / mpls / inner-eth frame.
> - The inner header is in page frags; a following eth_type_trans() can BUG
>   in __skb_pull().
> 
> Fixes: 2a2ea50870ba ("net: sched: add mpls manipulation actions to TC")
> Reported-by: Vega <vega@nebusec.ai>
> Link: https://lore.kernel.org/r/40fe2d7dfce13411bfbe9271c0f7c54fb7e88e3b.1782548651.git.bronzed_45_vested@icloud.com/
> Link: https://lore.kernel.org/netdev/CAL4WiiqSMdYQYeeW0Fk_GK6ekR4xiS4zPrGKrbB9wZknpqf1-w@mail.gmail.com/
> Cc: stable@vger.kernel.org
> Suggested-by: Jakub Kicinski <kuba@kernel.org>
> Suggested-by: Willem de Bruijn <willemdebruijn.kernel@gmail.com>
> Suggested-by: Eric Dumazet <edumazet@kernel.org>
> Reviewed-by: Victor Nogueira <victor@mojatatu.com>
> Signed-off-by: Jamal Hadi Salim <jhs@mojatatu.com>
> ---
>  net/sched/act_mpls.c | 2 ++
>  1 file changed, 2 insertions(+)
> 
> diff --git a/net/sched/act_mpls.c b/net/sched/act_mpls.c
> index 4ea8b2e08c3a..5363bca892fb 100644
> --- a/net/sched/act_mpls.c
> +++ b/net/sched/act_mpls.c
> @@ -79,6 +79,8 @@ TC_INDIRECT_SCOPE int tcf_mpls_act(struct sk_buff *skb,
>  		if (skb_mpls_pop(skb, p->tcfm_proto, mac_len,
>  				 skb->dev && skb->dev->type == ARPHRD_ETHER))
>  			goto drop;
> +		if (mac_len < ETH_HLEN && !pskb_may_pull(skb, ETH_HLEN))
> +			goto drop;

Is this first condition needed?

More importantly, can it hide a bug?

Simply running pskb_may_pull is cheap if no pull is needed.

>  		break;
>  	case TCA_MPLS_ACT_PUSH:
>  		new_lse = tcf_mpls_get_lse(NULL, p, !eth_p_mpls(skb_protocol(skb, true)));
> -- 
> 2.43.0
> 
>