netdev - Re: [PATCH v2] unix: properly account for FDs passed over unix sockets

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Date:	Tue, 2 Feb 2016 21:56:36 +0100
From:	Hannes Frederic Sowa <hannes@...essinduktion.org>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	David Herrmann <dh.herrmann@...il.com>, Willy Tarreau <w@....eu>,
	"David S. Miller" <davem@...emloft.net>,
	netdev <netdev@...r.kernel.org>,
	linux-kernel <linux-kernel@...r.kernel.org>,
	Eric Dumazet <edumazet@...gle.com>,
	Марк Коренберг 
	<socketpair@...il.com>,
	Tetsuo Handa <penguin-kernel@...ove.sakura.ne.jp>,
	Simon McVittie <simon.mcvittie@...labora.co.uk>
Subject: Re: [PATCH v2] unix: properly account for FDs passed over unix
 sockets

On 02.02.2016 21:44, Linus Torvalds wrote:
> On Tue, Feb 2, 2016 at 12:32 PM, Hannes Frederic Sowa
> <hannes@...essinduktion.org> wrote:
>>
>> Unfortunately we never transfer a scm_cookie via the skbs but merely use it
>> to initialize unix_skb_parms structure in skb->cb and destroy it afterwards.
>
> Ok, I obviously didn't check very closely.
>
>> But "struct pid *" in unix_skb_parms should be enough to get us to
>> corresponding "struct cred *" so we can decrement the correct counter during
>> skb destruction.
>
> Umm. I think the "struct cred" may change in between, can't it?

While reviewing the task_struct->cred/real_cred assignments, I noticed 
that, too. I already went the same way and added a "struct cred *" to 
unix_skb_parms.

> So I don't think you can later look up the cred based on the pid.

Yep, it also looked to dangerous to me.

> Could we add the cred pointer (or just the user pointer) to the unix_skb_parms?
>
> Or maybe just add it to the "struct scm_fp_list"?

scm_fp_list seems to be an even better place. I have a look, thanks!

Hannes