netdev - Re: [Patch net] net: saving irq context for peernet2id()

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Date:   Thu, 20 Oct 2016 15:07:35 -0400
From:   Paul Moore <paul@...l-moore.com>
To:     Cong Wang <xiyou.wangcong@...il.com>
Cc:     Stephen Smalley <sds@...ho.nsa.gov>,
        Linux Kernel Network Developers <netdev@...r.kernel.org>,
        Elad Raz <e@...draz.com>, Richard Guy Briggs <rgb@...hat.com>
Subject: Re: [Patch net] net: saving irq context for peernet2id()

On Thu, Oct 20, 2016 at 2:29 PM, Cong Wang <xiyou.wangcong@...il.com> wrote:
> On Thu, Oct 20, 2016 at 7:58 AM, Stephen Smalley <sds@...ho.nsa.gov> wrote:
>> On 10/20/2016 02:52 AM, Cong Wang wrote:
>>> A kernel warning inside __local_bh_enable_ip() was reported by people
>>> running SELinux, this is caused due to some SELinux functions
>>> (indirectly) call peernet2id() with IRQ disabled in process context,
>>> when we re-enable BH with IRQ disabled kernel complains. Shut up this
>>> warning by saving IRQ context in peernet2id(), BH is still implicitly
>>> disabled.
>>
>> Not sure this suffices; kill_fasync() -> send_sigio() ->
>> send_sigio_to_task() -> sigio_perm() -> security_file_send_sigiotask()
>> -> selinux_file_send_sigiotask() -> ... -> audit_log() -> ... ->
>> peernet2id()
>
> Oh, this is a new one. kill_fasync() is called in IRQ handler, so we actually
> do multicast in IRQ context.... It makes no sense, netlink multicast could
> be very expensive if we have many listeners.

I'm sure there are a few others I don't know about, but I believe the
only commonly used audit multicast listener is systemd.

> I am Cc'ing Richard who added that multicast in audit_log_end(). It seems
> not easy to just move the multicast to a workqueue, since the skb is copied
> from audit_buffer which is freed immediately after that, probably need another
> queue like audit_skb_queue.

This approach would double the queue size which is something I want to
avoid.  I would suggest sticking with a single queue and dealing with
the netlink message link fixup and multicast send in the existing
netlink unicast thread; basically we would just be moving the
multicast code from audit_log_end() into kauditd_thread().  This is
the same approach I mentioned earlier off-list.

However, that isn't something I want to mess with as a regression fix,
mostly because I really want to see this regression gone by -rc2 as it
is making SELinux testing a real pain.  If the patch posted at the top
of this thread isn't a suitable fix, we really should revert the
original patch.

-- 
paul moore
www.paul-moore.com