Messages in this thread | | | Date | Mon, 18 Dec 2000 20:14:02 +0100 | From | Harald Welte <> | Subject | ip_defrag / ip_conntrack issues (was Re: [PATCH] Fix netfilter locking) |
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On Mon, Dec 18, 2000 at 10:11:14AM -0800, David S. Miller wrote: > From: Rusty Russell <rusty@linuxcare.com.au> > Date: Mon, 18 Dec 2000 14:15:52 +1100 > > Alexey is right, locking is screwed (explains some reports of > occasional failure during rmmod). > > Patch applied, thank you. > > I have a patch which compiles for non-linear skb mods to netfilter > (NAT uses linear packets still, but connection tracking and packet > filtering only linearize minimal requirements). Waiting for > DaveM's solution to ip_ct_gather_frags()... > > I feel it's best to just skb_clone() the skb arg to ip_defrag > and this will close the whole thing, I think.
no. The clone()d skb will still have a skb->dev pointing to NULL.
The problem occurs only for locally-generated outgoing packets, which need to be fragmented:
- ip_build_xmit() discoveres it has to fragment - ip_build_xmit_slow() generates fragments and calls - NF_HOOK() for NF_IP_LOCAL_OUT - ip_conntrack_local() is called, which in turn calls - ip_conntrack_in(), which calls - ip_ct_gather_frags(), which calls - ip_defrag(), which calls
[now we have two possible oops - caues]
a ip_find(), which calls a ip_frag_create(), which initialises a timer with the function a ip_expire(), which dereferences qp->iif
b ip_frag_queue(), which dereferences it in qp->iif= skb->dev->ifindex
as andi kleen pointed out:
> > Also is it sure that the backtrace involves ip_rcv ? A more likely > > guess is that it happens during the IP_LOCAL_OUT hook, when skb->dev > > isn't set yet, but conntrack already has to already reassemble fragments.
> Actually, I do not understand how current code could even have worked > in the past. Once the SKB is passed to ip_defrag, it is nobody's > buisness to reference that SKB anymore. This ip_defrag call is (from
mmh... we really don't do this. We use the return value of ip_defrag(), which is what ip_frag_reasm() returns (== the new datagram consisting out of all its fragments).
> Alexey, what have I missed? I don't like the ip_fragment.c proposed > fix for this reason, what netfilter is doing with ip_defrag here looks > just wrong.
Well, my conclusion is:
- the defragmentation code in the ipv4 stack assumes that skb->dev points to a valid device. It does this primaryly to send icmp reassembly errors if a fragment reassembly timeout occurs
- netfilter wants to use the ip_defrag for defragmentation, not only for incoming packets from the network at NF_IP_PRE_ROUTING, but also for locally-generated fragmented packets (at NF_IP_LOCAL_OUT). Unfortunately we don't have a valid skb->dev at this point.
I don't think that there is any skb_clone()ing needed, nor this would solve the problem. The solution is
a) netfilter conntrack (more exactly: ip_ct_gather_frags) sets skb->dev to something valid (skb->dst->dev). Dirty hack.
b) make ip_defrag(), ... aware of the case where skb->dev == NULL. Sounds like a good idea, since it is only one if(skb->dev) clause.
c) netfilter stops using ip_defrag() for this case. Bad idea, it had to reinvent the wheel :(
> David S. Miller > davem@redhat.com
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