Re: [PATCH net v2] ipv6/addrconf: fix a null-ptr-deref bug for ip6_ptr

[Eric Dumazet]

On Tue, Jul 26, 2022 at 1:50 PM Ziyang Xuan
<william.xuanziyang@xxxxxxxxxx> wrote:
>
> Change net device's MTU to smaller than IPV6_MIN_MTU or unregister
> device while matching route. That may trigger null-ptr-deref bug
> for ip6_ptr probability as following.
>
> =========================================================
> BUG: KASAN: null-ptr-deref in find_match.part.0+0x70/0x134
> Read of size 4 at addr 0000000000000308 by task ping6/263
>
> CPU: 2 PID: 263 Comm: ping6 Not tainted 5.19.0-rc7+ #14
> Call trace:
>  dump_backtrace+0x1a8/0x230
>  show_stack+0x20/0x70
>  dump_stack_lvl+0x68/0x84
>  print_report+0xc4/0x120
>  kasan_report+0x84/0x120
>  __asan_load4+0x94/0xd0
>  find_match.part.0+0x70/0x134
>  __find_rr_leaf+0x408/0x470
>  fib6_table_lookup+0x264/0x540
>  ip6_pol_route+0xf4/0x260
>  ip6_pol_route_output+0x58/0x70
>  fib6_rule_lookup+0x1a8/0x330
>  ip6_route_output_flags_noref+0xd8/0x1a0
>  ip6_route_output_flags+0x58/0x160
>  ip6_dst_lookup_tail+0x5b4/0x85c
>  ip6_dst_lookup_flow+0x98/0x120
>  rawv6_sendmsg+0x49c/0xc70
>  inet_sendmsg+0x68/0x94
>
> Reproducer as following:
> Firstly, prepare conditions:
> $ip netns add ns1
> $ip netns add ns2
> $ip link add veth1 type veth peer name veth2
> $ip link set veth1 netns ns1
> $ip link set veth2 netns ns2
> $ip netns exec ns1 ip -6 addr add 2001:0db8:0:f101::1/64 dev veth1
> $ip netns exec ns2 ip -6 addr add 2001:0db8:0:f101::2/64 dev veth2
> $ip netns exec ns1 ifconfig veth1 up
> $ip netns exec ns2 ifconfig veth2 up
> $ip netns exec ns1 ip -6 route add 2000::/64 dev veth1 metric 1
> $ip netns exec ns2 ip -6 route add 2001::/64 dev veth2 metric 1
>
> Secondly, execute the following two commands in two ssh windows
> respectively:
> $ip netns exec ns1 sh
> $while true; do ip -6 addr add 2001:0db8:0:f101::1/64 dev veth1; ip -6 route add 2000::/64 dev veth1 metric 1; ping6 2000::2; done
>
> $ip netns exec ns1 sh
> $while true; do ip link set veth1 mtu 1000; ip link set veth1 mtu 1500; sleep 5; done
>
> It is because ip6_ptr has been assigned to NULL in addrconf_ifdown() firstly,
> then ip6_ignore_linkdown() accesses ip6_ptr directly without NULL check.
>
>         cpu0                    cpu1
> fib6_table_lookup
> __find_rr_leaf
>                         addrconf_notify [ NETDEV_CHANGEMTU ]
>                         addrconf_ifdown
>                         RCU_INIT_POINTER(dev->ip6_ptr, NULL)
> find_match
> ip6_ignore_linkdown
>
> So we can add NULL check for ip6_ptr before using in ip6_ignore_linkdown() to
> fix the null-ptr-deref bug.
>
> Fixes: 6d3d07b45c86 ("ipv6: Refactor fib6_ignore_linkdown")

If we need to backport, I guess dcd1f572954f ("net/ipv6: Remove fib6_idev")
already had the bug.

> Signed-off-by: Ziyang Xuan <william.xuanziyang@xxxxxxxxxx>
>
> ---
> v2:
>   - Use NULL check in ip6_ignore_linkdown() but synchronize_net() in
>     addrconf_ifdown()
>   - Add timing analysis of the problem
>
> ---
>  include/net/addrconf.h | 3 +++
>  1 file changed, 3 insertions(+)
>
> diff --git a/include/net/addrconf.h b/include/net/addrconf.h
> index f7506f08e505..c04f359655b8 100644
> --- a/include/net/addrconf.h
> +++ b/include/net/addrconf.h
> @@ -405,6 +405,9 @@ static inline bool ip6_ignore_linkdown(const struct net_device *dev)
>  {
>         const struct inet6_dev *idev = __in6_dev_get(dev);
>
> +       if (unlikely(!idev))
> +               return true;
> +

Note that we might read a non NULL pointer here, but read it again
later in rt6_score_route(),
since another thread could switch the pointer under us ?

>         return !!idev->cnf.ignore_routes_with_linkdown;
>  }
>
> --
> 2.25.1
>