linux-kernel - Re: [PATCH v3 2/2] Protected O_CREAT open in sticky directories

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Date:   Thu, 30 Nov 2017 15:39:48 +0100
From:   Salvatore Mesoraca <s.mesoraca16@...il.com>
To:     Solar Designer <solar@...nwall.com>
Cc:     David Laight <David.Laight@...lab.com>,
        Alan Cox <gnomes@...rguk.ukuu.org.uk>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Kernel Hardening <kernel-hardening@...ts.openwall.com>,
        "linux-fsdevel@...r.kernel.org" <linux-fsdevel@...r.kernel.org>,
        Alexander Viro <viro@...iv.linux.org.uk>,
        Jann Horn <jannh@...gle.com>,
        Kees Cook <keescook@...omium.org>,
        "Eric W. Biederman" <ebiederm@...ssion.com>
Subject: Re: [PATCH v3 2/2] Protected O_CREAT open in sticky directories

2017-11-27 1:26 GMT+01:00 Solar Designer <solar@...nwall.com>:
> On Fri, Nov 24, 2017 at 12:43:47PM +0100, Salvatore Mesoraca wrote:
> > 2017-11-24 11:53 GMT+01:00 David Laight <David.Laight@...lab.com>:
> > > From: Alan Cox
> > >> Sent: 22 November 2017 16:52
> > >>
> > >> On Wed, 22 Nov 2017 09:01:46 +0100 Salvatore Mesoraca <s.mesoraca16@...il.com> wrote:
> > >>
> > >> > Disallows O_CREAT open missing the O_EXCL flag, in world or
> > >> > group writable directories, even if the file doesn't exist yet.
> > >> > With few exceptions (e.g. shared lock files based on flock())
>
> Why would "shared lock files based on flock()" need O_CREAT without
> O_EXCL?  Where specifically are they currently used that way?

I don't think that they *need* to act like this, but this is how
util-linux's flock(1) currently works.
And it doesn't seem an unreasonable behavior from their perspective,
so maybe other programs do that too.
I was citing that case just to make it clear that, if someone gets
a warning because of flock(1), they shouldn't be worried about it.
That behavior can be certainly avoided, but of course it isn't a
security problem per se.

> If a program does
> that, we could want to find out and revise it (if O_CREAT|O_EXCL fails,
> retry without these to open the existing file, then flock() either way).

Yes, this would probably be the best thing to do, good idea.
Thanks again for your time,

Salvatore