User: Password:
|
|
Subscribe / Log in / New account

Re: [PATCH v2011.1] fs: symlink restrictions on sticky directories

From:  Kees Cook <keescook-AT-chromium.org>
To:  Linus Torvalds <torvalds-AT-linux-foundation.org>
Subject:  Re: [PATCH v2011.1] fs: symlink restrictions on sticky directories
Date:  Wed, 7 Dec 2011 10:54:55 -0800
Message-ID:  <CAGXu5j+F0Zr=xORUxXfqDsivAfv4WH872pnUERC8zPE=GET64Q@mail.gmail.com>
Cc:  Andrew Morton <akpm-AT-linux-foundation.org>, Al Viro <viro-AT-zeniv.linux.org.uk>, linux-kernel-AT-vger.kernel.org, linux-fsdevel-AT-vger.kernel.org, linux-doc-AT-vger.kernel.org, Randy Dunlap <rdunlap-AT-xenotime.net>, Rik van Riel <riel-AT-redhat.com>, Federica Teodori <federica.teodori-AT-googlemail.com>, Lucian Adrian Grijincu <lucian.grijincu-AT-gmail.com>, Ingo Molnar <mingo-AT-elte.hu>, Peter Zijlstra <a.p.zijlstra-AT-chello.nl>, Eric Paris <eparis-AT-redhat.com>, Dan Rosenberg <drosenberg-AT-vsecurity.com>, kernel-hardening-AT-lists.openwall.com
Archive-link:  Article

On Wed, Dec 7, 2011 at 10:41 AM, Linus Torvalds
<torvalds@linux-foundation.org> wrote:
> On Tue, Dec 6, 2011 at 3:58 PM, Kees Cook <keescook@chromium.org> wrote:
>> A long-standing class of security issues is the symlink-based
>> time-of-check-time-of-use race, most commonly seen in world-writable
>> directories like /tmp.
>
> Ugh. I really dislike the implementation.
>
> Wouldn't it be much nicer to instead actually use the symlink
> protection fields, and make the rules be:
>
>  - creating a symlink as root does the traditional "lrwxrwxrwx" thing
>  - creating a symlink in a directory you own similarly does "lrwxrwxrwx"
>  - creating a symlink anywhere else (which implies either sticky or
> world-writable directory) does "lrwx------", so only you can use it.
>
> That seems to be much nicer semantics, and makes the protection
> *visible* instead of some kind of hacky run-time random behavior
> depending on some invisible config option that people aren't even
> aware of.
>
> Of course, it needs to handle renames too (and maybe lchown?), and it
> would still need to be an option you enable explicitly, but it seems
> much more polite to make things like this something you can actually
> see.
>
> No, I have not thought this through deeply, but I really dislike your
> kind of "change random semantics in ways that are very hidden and
> subtle". The symlink protection approach seems to be much less hidden
> and subtle, and should result in largely the same result.
>
> Notably, you can install a system without it on, and turn it on even
> after install - who cares if there are *long-term* symlinks with
> lrwxrwxrwx and that thus allow all access, it's the racily-created
> ones we need to worry about, so it should actually be perfectly ok to
> enable the symlink creation protection dynamically. In fact, it could
> be a inheritable per-*process* (or per-container) flag rather than a
> global flag that says how symlink creation acts.
>
> I dunno.

This seems like a much deeper and wider change. This would expand it
to non-sticky, non-world-writable directories as well, meaning
symlinks on fileservers couldn't be managed by non-directory owners
any more, etc. I think there would be a lot of negative fall-out from
that.

Also it wouldn't track the state of a potentially dangerous
environment. Creation isn't the problem; existing is the problem. As
an attacker, I could: "mkdir /tmp/foo; ln -s /etc/shadow
/tmp/foo/evil-symlink; mv /tmp/foo/evil-symlink /tmp" and suddenly the
lrwxrwxrwx would follow the mv. To fix this we'd have to change the
permissions across a move, or other weird hidden behaviors.

Given that the patch is tweaking what "sticky directory" means, I
don't think it's impolite. Only attackers have ever depended on this
behavior, so let's just eliminate it.

-Kees

-- 
Kees Cook
ChromeOS Security
--
To unsubscribe from this list: send the line "unsubscribe linux-fsdevel" in
the body of a message to majordomo@vger.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html



(Log in to post comments)


Copyright © 2011, Eklektix, Inc.
Comments and public postings are copyrighted by their creators.
Linux is a registered trademark of Linus Torvalds