RE: [PATCH v3 2/2] Protected O_CREAT open in sticky directories

From: David Laight <David.Laight@ACULAB.COM>
To: "'Salvatore Mesoraca'" <s.mesoraca16@gmail.com>
Cc: Alan Cox <gnomes@lxorguk.ukuu.org.uk>,
	"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
	Kernel Hardening <kernel-hardening@lists.openwall.com>,
	"linux-fsdevel@vger.kernel.org" <linux-fsdevel@vger.kernel.org>,
	Alexander Viro <viro@zeniv.linux.org.uk>,
	Jann Horn <jannh@google.com>, Kees Cook <keescook@chromium.org>,
	"Solar Designer" <solar@openwall.com>,
	"Eric W. Biederman" <ebiederm@xmission.com>
Subject: RE: [PATCH v3 2/2] Protected O_CREAT open in sticky directories
Date: Fri, 24 Nov 2017 11:53:01 +0000	[thread overview]
Message-ID: <d1ffc1cd48b545ebb81a7f682fde9f1d@AcuMS.aculab.com> (raw)
In-Reply-To: <CAJHCu1K-xZXaXh38qYaNzJBr+USVxvsaqOReZjtnchxVOyYnww@mail.gmail.com>

From: Salvatore Mesoraca [mailto:s.mesoraca16@gmail.com]
> Sent: 24 November 2017 11:44
> 
> 2017-11-24 11:53 GMT+01:00 David Laight <David.Laight@aculab.com>:
> > From: Alan Cox
> >> Sent: 22 November 2017 16:52
> >>
> >> On Wed, 22 Nov 2017 09:01:46 +0100
> >> Salvatore Mesoraca <s.mesoraca16@gmail.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())
> >>
> >> Enough exceptions to make it a bad idea.
> >>
> >> Firstly if you care this much *stop* having shared writable directories.
> >> We have namespaces, you don't need them. You can give every user their
> >> own /tmp etc.
> >
> > Looks like a very bad idea to me as well.
> >
> > Doesn't this stop all shell redirects into a shared /tmp ?
> > I'm pretty sure most programs use O_CREAT | O_TRUNC for output
> > files - they'll all stop working.
> 
> If some program does such a thing, that's a potential vulnerability.
> With "protected_hardlinks" you are, in most cases, safe.
> But, still, that program has a bug and having this feature enabled will
> help you notice it soon.
> For that matter, I'm using this patch on my system and I don't have any
> program behaving like this.

Hmmm.... a quick strace shows cp and vi doing stat("/tmp/foo") and then
open(O_WRONLY|O_TRUNC) if it exists and O_CREATE|O_EXCL if it doesn't.
I can't help feeling that is just hiding a race.

	David