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 /* Warning: after this point, iph might be no longer valid */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 }
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