netdev - Re: [PATCH net 2/2] net: add more sanity checks to qdisc_pkt_len

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Message-ID: <66f525aab17bb_8456129490@willemb.c.googlers.com.notmuch>
Date: Thu, 26 Sep 2024 05:13:14 -0400
From: Willem de Bruijn <willemdebruijn.kernel@...il.com>
To: Eric Dumazet <edumazet@...gle.com>, 
 "David S . Miller" <davem@...emloft.net>, 
 Jakub Kicinski <kuba@...nel.org>, 
 Paolo Abeni <pabeni@...hat.com>
Cc: David Ahern <dsahern@...nel.org>, 
 netdev@...r.kernel.org, 
 Willem de Bruijn <willemb@...gle.com>, 
 Jonathan Davies <jonathan.davies@...anix.com>, 
 eric.dumazet@...il.com, 
 Eric Dumazet <edumazet@...gle.com>
Subject: Re: [PATCH net 2/2] net: add more sanity checks to
 qdisc_pkt_len_init()

Eric Dumazet wrote:
> One path takes care of SKB_GSO_DODGY, assuming
> skb->len is bigger than hdr_len.
> 
> virtio_net_hdr_to_skb() does not fully dissect TCP headers,
> it only make sure it is at least 20 bytes.
> 
> It is possible for an user to provide a malicious 'GSO' packet,
> total length of 80 bytes.
> 
> - 20 bytes of IPv4 header
> - 60 bytes TCP header
> - a small gso_size like 8
> 
> virtio_net_hdr_to_skb() would declare this packet as a normal
> GSO packet, because it would see 40 bytes of payload,
> bigger than gso_size.
> 
> We need to make detect this case to not underflow
> qdisc_skb_cb(skb)->pkt_len.
> 
> Fixes: 1def9238d4aa ("net_sched: more precise pkt_len computation")
> Signed-off-by: Eric Dumazet <edumazet@...gle.com>
> ---
>  net/core/dev.c | 10 +++++++---
>  1 file changed, 7 insertions(+), 3 deletions(-)
> 
> diff --git a/net/core/dev.c b/net/core/dev.c
> index f2c47da79f17d5ebe6b334b63d66c84c84c519fc..35b8bcfb209bd274c81380eaf6e445641306b018 100644
> --- a/net/core/dev.c
> +++ b/net/core/dev.c
> @@ -3766,10 +3766,14 @@ static void qdisc_pkt_len_init(struct sk_buff *skb)
>  				hdr_len += sizeof(struct udphdr);
>  		}
>  
> -		if (shinfo->gso_type & SKB_GSO_DODGY)
> -			gso_segs = DIV_ROUND_UP(skb->len - hdr_len,
> -						shinfo->gso_size);
> +		if (unlikely(shinfo->gso_type & SKB_GSO_DODGY)) {
> +			int payload = skb->len - hdr_len;
>  
> +			/* Malicious packet. */
> +			if (payload <= 0)
> +				return;
> +			gso_segs = DIV_ROUND_UP(payload, shinfo->gso_size);
> +		}

Especially for a malicious packet, should gso_segs be reinitialized to
a sane value? As sane as feasible when other fields cannot be fully
trusted..

>  		qdisc_skb_cb(skb)->pkt_len += (gso_segs - 1) * hdr_len;
>  	}
>  }
> -- 
> 2.46.0.792.g87dc391469-goog
>