linux-kernel - RE: [PATCH v2] unix: properly account for FDs passed over unix sockets

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Date:	Wed, 3 Feb 2016 12:19:11 +0000
From:	David Laight <David.Laight@...LAB.COM>
To:	'Linus Torvalds' <torvalds@...ux-foundation.org>,
	Hannes Frederic Sowa <hannes@...essinduktion.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>,
	???? ????????? <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

From: Linus Torvalds
> Sent: 02 February 2016 20:45
> 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?
> 
> So I don't think you can later look up the cred based on the pid.
> 
> 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"?

Is that going to work if the sending process exits before the message is read?
You need a reference count against the structure than contains the count.
I think this is 'struct user' not 'struct cred'

	David