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

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Message-ID: <CANn89iKTN0NgEcUAhBf19siC2FJ9hGpQppHf4wKmH7HgAtkn9g@mail.gmail.com>
Date: Thu, 26 Sep 2024 11:19:44 +0200
From: Eric Dumazet <edumazet@...gle.com>
To: Willem de Bruijn <willemdebruijn.kernel@...il.com>
Cc: "David S . Miller" <davem@...emloft.net>, Jakub Kicinski <kuba@...nel.org>, 
	Paolo Abeni <pabeni@...hat.com>, 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
Subject: Re: [PATCH net 2/2] net: add more sanity checks to qdisc_pkt_len_init()

On Thu, Sep 26, 2024 at 11:17 AM Willem de Bruijn
<willemdebruijn.kernel@...il.com> wrote:
>
> 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.
>

It is unclear if we can change a field in skb here, I hope that in the
future we can make virtio_net_hdr_to_skb() safer.

Role of qdisc_pkt_len_init() is to set a private skb->cb[] field for
qdisc layer.

Thanks !