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

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Message-ID: <66f526c06b0fa_851bd294af@willemb.c.googlers.com.notmuch>
Date: Thu, 26 Sep 2024 05:17:52 -0400
From: Willem de Bruijn <willemdebruijn.kernel@...il.com>
To: Willem de Bruijn <willemdebruijn.kernel@...il.com>, 
 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()

Willem de Bruijn wrote:
> 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..

Never mind. I guess the best thing we can do is to leave pkt_len as
skb->len, indeed.

Reviewed-by: Willem de Bruijn <willemb@...gle.com>

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