Re: [PATCH v2] fs: block cross-uid sticky symlinks

[Kees Cook <kees.cook@canonical.com>]

Hi Eric,

On Sun, May 30, 2010 at 08:50:53PM -0700, Eric W. Biederman wrote:
> The name of the sysctl is horrible it is a double negative, which
> makes thinking about it hard.

Hmm, I see your point, the "safe" value is "weak: not".  I was trying to
be descriptive without needing a "true" value, but I guess that's silly;
we already have things like randomize_va_space set to "2" by default, etc.
What would you suggest instead?  "protected_sticky_symlinks" (and reverse
the default and test logic)?

> Why not simply put each user in a different mount namespace and have separate
> /tmp directories per user?  That works today, with no kernel changes.

The key here is "no kernel changes" -- trying to isolate every user
and service from each other using different mount namespaces will not
work quickly in current distributions.  Even doing bind-mount tricks
to keep /tmp away from different users is overkill, especially when
you have situations like "screen" using a common /tmp directory tree
(in the setuid version), etc.  Things (correctly) expect to share /tmp
in some cases.  However, this one kernel change will allow everything
to continue without userspace overhead and without breaking anything
terribly.  Using containers will probably be the future, but I want to
solve this in the general case today.

> Placing this in cap_inode_follow_link is horrible naming.  There is nothing
> capabilities about this.  Either this needs to go into one or several
> of the security modules or this needs to go into the core vfs.

My thinking was that most of the LSMs call down to commoncaps first,
so it's a single place to put this.  When I was looking at this code
originally, I thought that if it doesn't go in security_inode_follow_link,
then a new function would be added to the VFS and both callers
of security_inode_follow_link would need to call it just before
security_inode_follow_link.  It seemed like putting it in there reduced
duplication of logic.

However, on closer examination, it seems that this code could live in
__do_follow_link instead.

fs/namei.c:
...
                error = security_inode_follow_link(path.dentry, &nd);
                if (error)
                        goto exit_dput;
                error = __do_follow_link(&path, &nd, &cookie);
...
        err = security_inode_follow_link(path->dentry, nd);
        if (err)
                goto loop;
        current->link_count++;
        current->total_link_count++;
        nd->depth++;
        err = __do_follow_link(path, nd, &cookie);
...

What would you suggest for the best approach here?

> I can't argue with taking action to close the too frequently security
> issues in /tmp, but this changes appears to be unnecessary, difficult
> to maintain, and difficult to understand.

Well, we disagree about "unnecessary".  :)  Finding an easy to maintain
solution is my goal here, and if it's difficult to understand, then I need
to fix that too.  What could use better clarification?

Thanks!

-Kees

-- 
Kees Cook
Ubuntu Security Team