On Mon, 2016-11-28 at 12:05 -0300, Arnaldo Carvalho de Melo wrote:
> Em Mon, Nov 28, 2016 at 06:47:14AM -0800, Eric Dumazet escreveu:
> > On Mon, 2016-11-28 at 11:40 -0300, Arnaldo Carvalho de Melo wrote:
> > > Em Mon, Nov 28, 2016 at 06:26:49AM -0800, Eric Dumazet escreveu:
> > > > From: Eric Dumazet <edumazet@xxxxxxxxxx>
> > > >
> > > > pskb_may_pull() can reallocate skb->head, we need to reload dh pointer
> > > > in dccp_invalid_packet() or risk use after free.
> > > >
> > > > Bug found by Andrey Konovalov using syzkaller.
> > > >
> > > > Signed-off-by: Eric Dumazet <edumazet@xxxxxxxxxx>
> > > > Reported-by: Andrey Konovalov <andreyknvl@xxxxxxxxxx>
> > >
> > > Acked-by: Arnaldo Carvalho de Melo <acme@xxxxxxxxxx>
> > >
> > > I was about to send exactly this patch, and while looking at it I think
> > > the patch below needs to go in as well, no? To follow the advice of that
> > > Warning line there :-)
> > >
> > > From: Arnaldo Carvalho de Melo <acme@xxxxxxxxxx>
> > >
> > > pskb_may_pull() can reallocate skb->head, so we can't access
> > > iph->frag_off or risk use after free, save it to a variable and us that
> > > later.
> > >
> > > Cc: Andrey Konovalov <andreyknvl@xxxxxxxxxx>
> > > Cc: Eric Dumazet <edumazet@xxxxxxxxxx>
> > > Signed-off-by: Arnaldo Carvalho de Melo <acme@xxxxxxxxxx>
> > >
> > > diff --git a/net/ipv4/af_inet.c b/net/ipv4/af_inet.c
> > > index 5ddf5cda07f4..9462070561a3 100644
> > > --- a/net/ipv4/af_inet.c
> > > +++ b/net/ipv4/af_inet.c
> > > @@ -1198,6 +1198,7 @@ struct sk_buff *inet_gso_segment(struct sk_buff *skb,
> > > struct iphdr *iph;
> > > int proto, tot_len;
> > > int nhoff;
> > > + u16 frag_off;
> > > int ihl;
> > > int id;
> > >
> > > @@ -1213,6 +1214,7 @@ struct sk_buff *inet_gso_segment(struct sk_buff *skb,
> > >
> > > id = ntohs(iph->id);
> > > proto = iph->protocol;
> > > + frag_off = iph->frag_off;
> > >
> > > /* Warning: after this point, iph might be no longer valid */
> > > if (unlikely(!pskb_may_pull(skb, ihl)))
> > > @@ -1233,7 +1235,7 @@ struct sk_buff *inet_gso_segment(struct sk_buff *skb,
> > > fixedid = !!(skb_shinfo(skb)->gso_type & SKB_GSO_TCP_FIXEDID);
> > >
> > > /* fixed ID is invalid if DF bit is not set */
> > > - if (fixedid && !(iph->frag_off & htons(IP_DF)))
> > > + if (fixedid && !(frag_off & htons(IP_DF)))
> > > goto out;
> > > }
> > >
> >
> >
> > I do not see why this patch would be needed ?
>
> Where is iph being reloaded after that pskb_may_pull() and thus at line 1236 we
> could use after free? The warning at line 1217?
>
> 1209 iph = ip_hdr(skb);
> 1210 ihl = iph->ihl * 4;
> 1211 if (ihl < sizeof(*iph))
> 1212 goto out;
> 1213
> 1214 id = ntohs(iph->id);
> 1215 proto = iph->protocol;
> 1216
> 1217 /* Warning: after this point, iph might be no longer valid */
> 1218 if (unlikely(!pskb_may_pull(skb, ihl)))
> 1219 goto out;
> 1220 __skb_pull(skb, ihl);
> 1221
> 1222 encap = SKB_GSO_CB(skb)->encap_level > 0;
> 1223 if (encap)
> 1224 features &= skb->dev->hw_enc_features;
> 1225 SKB_GSO_CB(skb)->encap_level += ihl;
> 1226
> 1227 skb_reset_transport_header(skb);
> 1228
> 1229 segs = ERR_PTR(-EPROTONOSUPPORT);
> 1230
> 1231 if (!skb->encapsulation || encap) {
> 1232 udpfrag = !!(skb_shinfo(skb)->gso_type & SKB_GSO_UDP);
> 1233 fixedid = !!(skb_shinfo(skb)->gso_type & SKB_GSO_TCP_FIXEDID);
> 1234
> 1235 /* fixed ID is invalid if DF bit is not set */
> 1236 if (fixedid && !(iph->frag_off & htons(IP_DF)))
> 1237 goto out;
> 1238 }
>
Arg, I was looking at an old tree.
Please then add
Fixes: cbc53e08a793b ("GSO: Add GSO type for fixed IPv4 ID")
To ease stable backports.
Also, it looks comments are not read, we might kill this one and reload
iph.
( Saving 3 fields is now more expensive than simply reloading iph )
Thanks.
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