Re: [PATCH v2 1/2] netfilter: ip_tables: guard ipt_unregister_table_pre_exit against NULL ops

[Tristan Madani <tristmd@gmail.com>]

Florian Westphal <fw@strlen.de> wrote:
> Is there a reproducer for this bug?

Syzkaller hit it under failslab. The race is between the lazy
init path in ipt_register_table() and cleanup_net(). The table
becomes visible via xt_register_table() before ops is assigned,
so pre_exit can find it with NULL ops.

Cleaned crash log:

  Oops: general protection fault, probably for non-canonical address 0xdffffc0000000003: 0000 [#1] PREEMPT SMP KASAN NOPTI
  KASAN: null-ptr-deref in range [0x0000000000000018-0x000000000000001f]
  CPU: 1 UID: 0 PID: 604 Comm: kworker/u8:19 Tainted: G            E      6.14.11 #1
  Workqueue: netns cleanup_net
  RIP: 0010:nf_unregister_net_hook net/netfilter/core.c:531 [inline]
  RIP: 0010:nf_unregister_net_hooks+0xbc/0x150 net/netfilter/core.c:613
  Call Trace:
   <TASK>
   ipt_unregister_table_pre_exit+0x8a/0xc0 net/ipv4/netfilter/ip_tables.c:1814
   iptable_mangle_net_pre_exit+0x21/0x30 net/ipv4/netfilter/iptable_mangle.c:99
   ops_pre_exit_list net/core/net_namespace.c:162 [inline]
   cleanup_net+0x4b9/0xbe0 net/core/net_namespace.c:632
   process_one_work+0x98f/0x1750 kernel/workqueue.c:3238
   worker_thread+0x679/0xf50 kernel/workqueue.c:3402
   kthread+0x3f0/0x7e0 kernel/kthread.c:464
   ret_from_fork+0x60/0x90 arch/x86/kernel/process.c:153
   </TASK>

> I'm working on a new unreg scheme to avoid rmmod racing with
> concurrent calls into iptables set/getsockopts.

That sounds like a different issue (rmmod vs sockopt). This one
is init vs cleanup_net -- the NULL ops window exists regardless
of the unreg scheme. V2 is a minimal guard for that.

Thanks,
Tristan