Re: net: heap out-of-bounds in fib6_clean_node/rt6_fill_node/fib6_age/fib6_prune_clone

[Eric Dumazet]

On Sat, 2017-03-04 at 19:57 +0100, Dmitry Vyukov wrote:
> On Fri, Mar 3, 2017 at 8:12 PM, David Ahern <dsa@xxxxxxxxxxxxxxxxxxx> wrote:
> > On 3/3/17 6:39 AM, Dmitry Vyukov wrote:
> >> I am getting heap out-of-bounds reports in
> >> fib6_clean_node/rt6_fill_node/fib6_age/fib6_prune_clone while running
> >> syzkaller fuzzer on 86292b33d4b79ee03e2f43ea0381ef85f077c760. They all
> >> follow the same pattern: an object of size 216 is allocated from
> >> ip_dst_cache slab, and then accessed at offset 272/276 withing
> >> fib6_walk. Looks like type confusion. Unfortunately this is not
> >> reproducible.
> >
> > I'll take a look this weekend or Monday at the latest.
> 
> 
> I've got some additional useful info on this. I think this is
> use-after-free rather than out-of-bounds. I've collected stack where
> the route was disposed with call_rcu, see the last "Disposed" stack.
> The crash happens when cmpxchg in rt_cache_route replaces an existing
> route. And that route seems to have some existing pointers to it
> (rt->dst.rt6_next) which fib6_walk uses to get to it after its
> deletion.

rt_cache_route() deals with IPv4 routes.

We somehow mix IPv4 and IPv6 dsts in IPv6 tree.

We need to add type safety at IPV6 route insertions to catch the
offender.