From: Bandan Das <bandan.das@stratus.com>
Subject: Re: [PATCH net-next-2.6] net/ipv4: push IP options to CB in
 ip_fragment
Date: Mon, 30 Aug 2010 19:21:47 -0400
Message-ID: <20100830232147.GB10754@stratus.com>
References: <20100830200917.GA10754@stratus.com>
 <1283204118.2405.32.camel@edumazet-laptop>
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Cc: David Miller <davem@davemloft.net>,
	NetDev <netdev@vger.kernel.org>,
	LKML <linux-kernel@vger.kernel.org>,
	Patrick McHardy <kaber@trash.net>
To: Eric Dumazet <eric.dumazet@gmail.com>
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On  0, Eric Dumazet <eric.dumazet@gmail.com> wrote:
> Le lundi 30 ao=C3=BBt 2010 =C3=A0 16:09 -0400, Bandan Das a =C3=A9cri=
t :
> > The bridge layer can overwrite the IPCB using the
> > BR_INPUT_SKB_CB macro. In br_nf_dev_queue_xmit,=20
> > if we recieve a packet greater in size than the bridge=20
> > device MTU, we call ip_fragment which in turn will lead to=20
> > icmp_send calling ip_options_echo if the DF flag is set.=20
> > ip_options_echo will then incorrectly try to parse the IPCB as=20
> > IP options resulting in a buffer overflow.=20
> > This change refills the CB area back with IP=20
> > options before ip_fragment calls icmp_send. If we fail parsing,=20
> > we zero out the IPCB area to guarantee that the stack does=20
> > not get corrupted.
> >=20
> > Test setup that can exhibit this behavior:
> > <Remote Machine>--<Host Machine>--<Bridge>--<Tun/Tap>--<KVM Guest>
> >   1500 mtu          576 mtu        576 mtu   576 mtu    1500 mtu
> >=20
> > Example of crash :
> > Kernel panic - not syncing: stack-protector: Kernel stack is corrup=
ted in: ffffffff8146dff3
> > Pid: 3173, comm: qemu-kvm Not tainted 2.6.32-63.el6.x86_64 #1
> > Call Trace:
> > Message from <IRQ>  syslogd@kvm at  [<ffffffff814c8285>] panic+0x78=
/0x137
> > Aug 30 14:09:50  [<ffffffff8146dff3>] ? icmp_send+0x743/0x780
> >=20
> > Signed-off-by: Bandan Das <bandan.das@stratus.com>
> > ---
> >  net/ipv4/ip_output.c |   11 +++++++++++
> >  1 files changed, 11 insertions(+), 0 deletions(-)
> >=20
> > diff --git a/net/ipv4/ip_output.c b/net/ipv4/ip_output.c
> > index e427620..5fef4a9 100644
> > --- a/net/ipv4/ip_output.c
> > +++ b/net/ipv4/ip_output.c
> > @@ -445,6 +445,7 @@ int ip_fragment(struct sk_buff *skb, int (*outp=
ut)(struct sk_buff *))
> >  	struct iphdr *iph;
> >  	int ptr;
> >  	struct net_device *dev;
> > +	struct ip_options *opt;
> >  	struct sk_buff *skb2;
> >  	unsigned int mtu, hlen, left, len, ll_rs;
> >  	int offset;
> > @@ -462,6 +463,16 @@ int ip_fragment(struct sk_buff *skb, int (*out=
put)(struct sk_buff *))
> > =20
> >  	if (unlikely((iph->frag_off & htons(IP_DF)) && !skb->local_df)) {
> >  		IP_INC_STATS(dev_net(dev), IPSTATS_MIB_FRAGFAILS);
> > +
> > +		/* Refill CB with IP options */
> > +		opt =3D &(IPCB(skb)->opt);
> > +		opt->optlen =3D iph->ihl*4 - sizeof(struct iphdr);
> > +		if (ip_options_compile(dev_net(dev), opt, skb)) {
> > +			IP_INC_STATS(dev_net(dev), IPSTATS_MIB_INHDRERRORS);
> > +			/* If we fail, zero out IPCB and continue */
> > +			memset(IPCB(skb), 0, sizeof(struct inet_skb_parm));
> > +		}
> > +
> >  		icmp_send(skb, ICMP_DEST_UNREACH, ICMP_FRAG_NEEDED,
> >  			  htonl(ip_skb_dst_mtu(skb)));
> >  		kfree_skb(skb);
>=20
>=20
> I wonder if we want this.
>=20
> Maybe setting skb->local_df =3D 1 in bridge should be enough ?
>=20
>=20
Thanks Eric for looking at this. Indeed, setting local_df to 1 seems to=
 be enough! I will
respin and post a different patch.

--=20
Bandan Das