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

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Date:   Thu, 30 Nov 2017 16:53:06 +0000
From:   David Laight <David.Laight@...LAB.COM>
To:     'Salvatore Mesoraca' <s.mesoraca16@...il.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
CC:     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>,
        Solar Designer <solar@...nwall.com>,
        "Eric W. Biederman" <ebiederm@...ssion.com>
Subject: RE: [PATCH v3 2/2] Protected O_CREAT open in sticky directories

From: Salvatore Mesoraca
> Sent: 22 November 2017 08:02
> 
> 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())
> if a program tries to open a file, in a sticky directory,
> with the O_CREAT flag and without the O_EXCL, it probably has a bug.
> This feature allows to detect and potentially block programs that
> act this way, it can be used to find vulnerabilities (like those
> prevented by patch #1) and to do policy enforcement.

(Going back to the original post)

I presume the 'vulnerabilities' are related to symlinks being created
just before the open?

Trouble is this change breaks a lot of general use of /tmp.
I always assumed that code that cared would use O_EXCL and
everything else wasn't worth subverting.

I found code in vi (and elsewhere) that subverted these checks
by opening with O_WRONLY if stat() showed the file existed and
O_CREAT|O_EXCL if it didn't.

I'm pretty sure that traditionally a lot of these opens were done
with O_CREAT|O_TRUNC.
Implementing that as unlink() followed by a create would stop
'random' (ok all) symlinks being followed.

Overall I'm pretty sure this change will break things badly somewhere.

	David