RE: [PATCH v2] net: ipv6: fix skb_over_panic in __ip6_append_data

[David Laight <David.Laight@ACULAB.COM>]

From: Willem de Bruijn
> Sent: 10 March 2022 22:43
> 
> On Thu, Mar 10, 2022 at 5:30 PM Jakub Kicinski <kuba@kernel.org> wrote:
> >
> > On Thu, 10 Mar 2022 14:13:28 -0800 Tadeusz Struk wrote:
> > > diff --git a/net/ipv6/ip6_output.c b/net/ipv6/ip6_output.c
> > > index 4788f6b37053..6d45112322a0 100644
> > > --- a/net/ipv6/ip6_output.c
> > > +++ b/net/ipv6/ip6_output.c
> > > @@ -1649,6 +1649,16 @@ static int __ip6_append_data(struct sock *sk,
> > >                       skb->protocol = htons(ETH_P_IPV6);
> > >                       skb->ip_summed = csummode;
> > >                       skb->csum = 0;
> > > +
> > > +                     /*
> > > +                      *      Check if there is still room for payload
> > > +                      */
> >
> > TBH I think the check is self-explanatory. Not worth a banner comment,
> > for sure.
> >
> > > +                     if (fragheaderlen >= mtu) {
> > > +                             err = -EMSGSIZE;
> > > +                             kfree_skb(skb);
> > > +                             goto error;
> > > +                     }
> >
> > Not sure if Willem prefers this placement, but seems like we can lift
> > this check out of the loop, as soon as fragheaderlen and mtu are known.
> >
> > >                       /* reserve for fragmentation and ipsec header */
> > >                       skb_reserve(skb, hh_len + sizeof(struct frag_hdr) +
> > >                                   dst_exthdrlen);
> 
> Just updating this boundary check will do?
> 
>         if (mtu < fragheaderlen ||
>             ((mtu - fragheaderlen) & ~7) + fragheaderlen <
> sizeof(struct frag_hdr))
>                 goto emsgsize;

Both those < should be <=

But I think I'd check:
	if (fragheaderlen >= 1280 - sizeof (struct frag_hdr))
		goto emsgsize;
first (or only!)

	David

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