From mboxrd@z Thu Jan  1 00:00:00 1970
From: David Miller <davem@davemloft.net>
Subject: Re: [PATCH v4] af_netlink: ensure that NLMSG_DONE never fails in
 dumps
Date: Sat, 11 Nov 2017 23:21:01 +0900 (KST)
Message-ID: <20171111.232101.352298023245450121.davem@davemloft.net>
References: <20171109040444.1728-1-Jason@zx2c4.com>
        <20171111.230924.412544731735070222.davem@davemloft.net>
        <1510409721.12037.3.camel@sipsolutions.net>
Mime-Version: 1.0
Content-Type: Text/Plain; charset=us-ascii
Content-Transfer-Encoding: 7bit
Cc: Jason@zx2c4.com, netdev@vger.kernel.org,
        linux-kernel@vger.kernel.org
To: johannes@sipsolutions.net
Return-path: <linux-kernel-owner@vger.kernel.org>
In-Reply-To: <1510409721.12037.3.camel@sipsolutions.net>
Sender: linux-kernel-owner@vger.kernel.org
List-Id: netdev.vger.kernel.org

From: Johannes Berg <johannes@sipsolutions.net>
Date: Sat, 11 Nov 2017 15:15:21 +0100

> On Sat, 2017-11-11 at 23:09 +0900, David Miller wrote:
>> From: "Jason A. Donenfeld" <Jason@zx2c4.com>
>> Date: Thu,  9 Nov 2017 13:04:44 +0900
>> 
>> > @@ -2195,13 +2197,15 @@ static int netlink_dump(struct sock *sk)
>> >               return 0;
>> >       }
>> >  
>> > -     nlh = nlmsg_put_answer(skb, cb, NLMSG_DONE, sizeof(len), NLM_F_MULTI);
>> > -     if (!nlh)
>> > +     nlh = nlmsg_put_answer(skb, cb, NLMSG_DONE,
>> > +                            sizeof(nlk->dump_done_errno), NLM_F_MULTI);
>> > +     if (WARN_ON(!nlh))
>> >               goto errout_skb;
>> 
>> If you're handling this by forcing another read() to procude the
>> NLMSG_DONE, then you have no reason to WARN_ON() here.
>> 
>> In fact you are adding a WARN_ON() which is trivially triggerable by
>> any user.
> 
> I added this in my suggestion for how this could work, but I don't
> think you're right, since we previously check if there's enough space.
> The patch is missing the full context, but this is:
 ...
> So unless the nlmsg_total_size() vs. nlmsg_put_answer() suddenly gets a
> different idea of how much space is needed, nlh shouldn't ever be NULL
> once we get here.

Aha, that's what I missed.  Indeed, it cannot happen.

My bad.