Re: net/ipv6: use-after-free in __call_rcu/in6_dev_finish_destroy_rcu

[Andrey Konovalov]

On Tue, May 2, 2017 at 4:44 AM, David Ahern <dsa@xxxxxxxxxxxxxxxxxxx> wrote:
> On 4/26/17 9:15 AM, Andrey Konovalov wrote:
>> +David
>>
>> I've enabled CONFIG_DEBUG_OBJECTS_RCU_HEAD and this is what I get.
>>
>> Apparently the rcu warning is related to the fib6_del_route bug I've
>> been trying to reproduce:
>> https://groups.google.com/forum/#!msg/syzkaller/3SS80JbVPKA/2tfIAcW7DwAJ
>>
>> Adding David, who provided the fix:
>> https://patchwork.ozlabs.org/patch/754913/
>>
>> I've managed to extract a reproducer, attached together with the
>> .config that I used.
>>
>> On commit 5a7ad1146caa895ad718a534399e38bd2ba721b7 (4.11-rc8) with
>> David's patch applied.
>>
>> ------------[ cut here ]------------
>> WARNING: CPU: 1 PID: 5911 at lib/debugobjects.c:289
>> debug_print_object+0x175/0x210
>> ODEBUG: activate active (active state 1) object type: rcu_head hint:
>>         (null)
>> Modules linked in:
>> CPU: 1 PID: 5911 Comm: a.out Not tainted 4.11.0-rc8+ #271
>> Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS Bochs 01/01/2011
>> Call Trace:
>>  __dump_stack lib/dump_stack.c:16
>>  dump_stack+0x192/0x22d lib/dump_stack.c:52
>>  __warn+0x19f/0x1e0 kernel/panic.c:549
>>  warn_slowpath_fmt+0xe0/0x120 kernel/panic.c:564
>>  debug_print_object+0x175/0x210 lib/debugobjects.c:286
>>  debug_object_activate+0x574/0x7e0 lib/debugobjects.c:442
>>  debug_rcu_head_queue kernel/rcu/rcu.h:75
>>  __call_rcu.constprop.76+0xff/0x9c0 kernel/rcu/tree.c:3229
>>  call_rcu_sched+0x12/0x20 kernel/rcu/tree.c:3288
>>  rt6_rcu_free net/ipv6/ip6_fib.c:158
>>  rt6_release+0x1ea/0x290 net/ipv6/ip6_fib.c:188
>>  fib6_del_route net/ipv6/ip6_fib.c:1461
>
> I think I got to the bottom of this one.
>
> With your config, ip6_tunnel is compiled in.
>
> The program runs in a very tight loop, calling 'unshare -n' and then
> spawns 2 sets of 14 threads running random ioctl calls. The networking
> sequence:
>
> 1. New network namespace created via unshare -n
> - ip6tnl0 device is created in down state
>
> 2. address added to ip6tnl0 (equivalent to ip -6 addr add dev ip6tnl0
> fd00::bb/1)
> - the host route is created and inserted into FIB
>
> 3. ip6tnl0 is brought up - starts DAD on the address
>
> 4. exit namespace
> - teardown / cleanup sequence starts
> - lo teardown appears to happen BEFORE teardown of ip6tunl0
>   + removes host route from FIB
>   + host route added to rcu callback list: call_rcu(&rt->dst.rcu_head,
> dst_rcu_free);
>   + rcu callback has not run yet, so rt is NOT on the gc list so it has
>     NOT been marked obsolete
>
> 5. worker_thread runs addrconf_dad_completed
> - calls ipv6_ifa_notify which inserts the host route
>
> All of that happens very quickly. The result is that a route that has
> been deleted and added to the RCU list is re-inserted into the FIB. What
> happens next depends on order -- in this case the exit namespace
> eventually gets to cleaning up ip6tnl0 which removes the host route from
> the FIB, calls the rcu function for cleanup -- and triggers the double
> rcu trace.
>
> I have a hack that flags this sequence and prevents the re-insertion
> following DAD. That allows the command to run until it consumes all 2G
> of memory the VM has -- about 600+ iterations without triggering any
> stack traces.

Hi David,

Thanks for looking into this!

Do you have a patch that I could test?

I also reported another issue recently, that might also be related to this one:
https://groups.google.com/forum/#!topic/syzkaller/Rt0pgY4wfiw

Thanks!