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

From: Matthew Wilcox <willy@infradead.org>
To: Salvatore Mesoraca <s.mesoraca16@gmail.com>
Cc: linux-kernel@vger.kernel.org,
	Kernel Hardening <kernel-hardening@lists.openwall.com>,
	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: Wed, 22 Nov 2017 05:22:36 -0800	[thread overview]
Message-ID: <20171122132235.GA30635@bombadil.infradead.org> (raw)
In-Reply-To: <1511337706-8297-3-git-send-email-s.mesoraca16@gmail.com>

On Wed, Nov 22, 2017 at 09:01:46AM +0100, Salvatore Mesoraca wrote:
> +An O_CREAT open missing the O_EXCL flag in a sticky directory is,
> +often, a bug or a synthom of the fact that the program is not
> +using appropriate procedures to access sticky directories.
> +This protection allow to detect and possibly block these unsafe
> +open invocations, even if the files don't exist yet.
> +Though should be noted that, sometimes, it's OK to open a file
> +with O_CREAT and without O_EXCL (e.g. shared lock files based
> +on flock()), for this reason values above 2 should be set
> +with care.
> +
> +When set to "0" the protection is disabled.
> +
> +When set to "1", notify about O_CREAT open missing the O_EXCL flag
> +in world writable sticky directories.
> +
> +When set to "2", notify about O_CREAT open missing the O_EXCL flag
> +in world or group writable sticky directories.
> +
> +When set to "3", block O_CREAT open missing the O_EXCL flag
> +in world writable sticky directories and notify (but don't block)
> +in group writable sticky directories.
> +
> +When set to "4", block O_CREAT open missing the O_EXCL flag
> +in world writable and group writable sticky directories.

This seems insufficiently flexible.  For example, there is no way for me
to specify that I want to block O_CREAT without O_EXCL in world-writable,
but not be notified about O_CREAT without O_EXCL in group-writable.

And maybe I want to be notified that blocking has happened?

Why not make it bits?  So:

0 => notify in world
1 => block in world
2 => notify in group
3 => block in group

So you'd have the following meaningful values:

 0 - permit all (your option 0)
 1 - notify world; permit group (your option 1)
 2 - block world; permit group
 3 - block,notify world; permit group
 4 - permit world; notify group (?)
 5 - notify world; notify group (your option 2)
 6 - block world; notify group (your option 3)
 7 - block,notify world; notify group
 8 - permit world; block group (?)
 9 - notify world; block group (?)
10 - block world; block group (your option 4)
11 - block,notify world; block group
12 - permit world; block, notify group (?)
13 - notify world; block, notify group (?)
14 - block world; block, notify group
15 - block, notify world; block, notify group

Some of these don't make a lot of sense (marked with ?), but I don't see
the harm in permitting a sysadmin to do something that seems nonsensical
to me.