Re: net: suspicious RCU usage in nf_hook

[Cong Wang]

On Tue, Jan 31, 2017 at 7:44 AM, Eric Dumazet <eric.dumazet@xxxxxxxxx> wrote:
> On Mon, 2017-01-30 at 22:19 -0800, Cong Wang wrote:
>
>>
>> The context is process context (TX path before hitting qdisc), and
>> BH is not disabled, so in_interrupt() doesn't catch it. Hmm, this
>> makes me thinking maybe we really need to disable BH in this
>> case for nf_hook()? But it is called in RX path too, and BH is
>> already disabled there.
>
> ipt_do_table() and similar netfilter entry points disable BH.
>
> Maybe it is done too late.

I think we need a fix like the following one for minimum impact.

diff --git a/net/core/dev.c b/net/core/dev.c
index 727b6fd..eee7d63 100644
--- a/net/core/dev.c
+++ b/net/core/dev.c
@@ -1720,12 +1720,10 @@ EXPORT_SYMBOL(net_enable_timestamp);
 void net_disable_timestamp(void)
 {
 #ifdef HAVE_JUMP_LABEL
-       if (in_interrupt()) {
-               atomic_inc(&netstamp_needed_deferred);
-               return;
-       }
-#endif
+       atomic_inc(&netstamp_needed_deferred);
+#else
        static_key_slow_dec(&netstamp_needed);
+#endif
 }
 EXPORT_SYMBOL(net_disable_timestamp);