Re: net: suspicious RCU usage in nf_hook

[Cong Wang <xiyou.wangcong@gmail.com>]

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On Wed, Feb 1, 2017 at 1:16 PM, Eric Dumazet <eric.dumazet@gmail.com> wrote:
> On Wed, 2017-02-01 at 12:51 -0800, Cong Wang wrote:
>> On Tue, Jan 31, 2017 at 7:44 AM, Eric Dumazet <eric.dumazet@gmail.com> 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);
>
> This would permanently leave the kernel in the netstamp_needed state.
>
> I would prefer the patch using a process context to perform the
> cleanup ? Note there is a race window, but probably not a big deal.

Not sure if it is better. The difference is caught up in net_enable_timestamp(),
which is called setsockopt() path and sk_clone() path, so we could be
in netstamp_needed state for a long time too until user-space exercises
these paths.

I am feeling we probably need to get rid of netstamp_needed_deferred,
and simply defer the whole static_key_slow_dec(), like the attached patch
(compile only).

What do you think?

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diff --git a/net/core/dev.c b/net/core/dev.c
index 727b6fd..0ef1734 100644
--- a/net/core/dev.c
+++ b/net/core/dev.c
@@ -1694,38 +1694,28 @@ EXPORT_SYMBOL_GPL(net_dec_egress_queue);
 #endif
 
 static struct static_key netstamp_needed __read_mostly;
-#ifdef HAVE_JUMP_LABEL
-/* We are not allowed to call static_key_slow_dec() from irq context
- * If net_disable_timestamp() is called from irq context, defer the
- * static_key_slow_dec() calls.
- */
-static atomic_t netstamp_needed_deferred;
-#endif
 
-void net_enable_timestamp(void)
+static void netstamp_clear(struct work_struct *work)
 {
-#ifdef HAVE_JUMP_LABEL
-	int deferred = atomic_xchg(&netstamp_needed_deferred, 0);
+	static_key_slow_dec(&netstamp_needed);
+}
 
-	if (deferred) {
-		while (--deferred)
-			static_key_slow_dec(&netstamp_needed);
-		return;
-	}
-#endif
+static DECLARE_WORK(netstamp_work, netstamp_clear);
+
+void net_enable_timestamp(void)
+{
+	flush_work(&netstamp_work);
 	static_key_slow_inc(&netstamp_needed);
 }
 EXPORT_SYMBOL(net_enable_timestamp);
 
 void net_disable_timestamp(void)
 {
-#ifdef HAVE_JUMP_LABEL
-	if (in_interrupt()) {
-		atomic_inc(&netstamp_needed_deferred);
-		return;
-	}
-#endif
-	static_key_slow_dec(&netstamp_needed);
+	/* We are not allowed to call static_key_slow_dec() from irq context
+	 * If net_disable_timestamp() is called from irq context, defer the
+	 * static_key_slow_dec() calls.
+	 */
+	schedule_work(&netstamp_work);
 }
 EXPORT_SYMBOL(net_disable_timestamp);