On Monday 31 December 2007 4:46:09 pm James Morris wrote:
> On Mon, 31 Dec 2007, Paul Moore wrote:
> > I'm pretty certain this is an uninitialized value problem now and not a
> > use-after-free issue. The invalid/garbage ->iif value seems to only
> > happen on packets that are generated locally and sent back into the stack
> > for local consumption, e.g. loopback. These local packets also need to
> > have been cloned at some point, either on the output or input path.
>
> I think we need to find out exactly what's happening, first.
The more I've looked at the code this afternoon, I'm certain this is the case.
I've also been running a patched kernel (using option #2 from below) and all
of the skbs coming up the stack have valid ->iif values. Granted, I haven't
examined the code from the avahi daemon or the tcl test cases and traced the
entire code path through the kernel but I _am_ certain that at some point in
that code path the packet is cloned and due to a problem in skb_clone()
the ->iif field is not copied correctly causing the problems we have all
seen.
How much smoke needs to be coming from the gun? :)
> > The problem appears to be a skb_clone() function which does not clear the
> > skb structure properly and fails to copy the ->iif value from the
> > original skb to the cloned skb. From what I can tell, there are two
> > possible solutions to this problem:
> >
> > 1. Clear all of the cloned skb fields in skb_clone() via memset()
>
> Sounds like it's not going to fly for performance reasons in any case.
That was my gut feeling. I was also a little unsure where exactly the correct
placement should be for the memset() call.
> > 2. Copy the ->iif field in __copy_skb_header()
>
> Seems valid.
Okay, I'll stick with this approach. I'll post a patch backed against
net-2.6.25 tomorrow as an RFC to see if anyone on netdev has any strong
feelings. If no one complains, I'll add it to the lblnet git tree.
--
paul moore
linux security @ hp
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