Re: [PATCH 0/7] Initial support for user namespace owned mounts

Andy Lutomirski
2015-07-16T22:28:14Z

On Thu, Jul 16, 2015 at 2:42 PM, Casey Schaufler <casey@schaufler-ca.com> wrote:
> You want to provide a mechanism whereby an unprivileged user (Seth)
> can mount a filesystem for his own use. You want full filesystem
> semantics, but you're willing to accept restrictions on certain
> filesystem features to avoid opening security holes. You are not
> willing to accept restrictions that make the filesystem unusable,
> such as making it read-only.
>
> I am going to present a suggestion. Feel free to correct my
> assumptions and my reasoning. For simplicity let's use loop-back
> mounting of a filesystem contained in a file as an example. The
> principles should apply to newly created memory based filesystems
> or disk partitions "owned" by Seth.
>
> Seth wants to mount a file (~seth/myfs) which contains an ext4
> filesystem. There is already a filesystem object, with security
> attributes, that the system knows how to deal with. If Seth mounts
> this as a filesystem he, and potentially other people, will be
> able to access the content of this object without accessing the
> object itself.
>
> seth$ mount --justforme -t ext4 ~seth/myfs /tmp/seth
> seth$ chmod 777 /tmp/seth
> seth$ ls -la /tmp/seth
> drwxrwxrwx. 3 seth seth 260 Jul 16 12:59 .
> drwxrwxrwxt 18 root root 4069 Jul 16 11:13 ..
> seth$
>
> Everything's fine at this point. Wilma is also using the system,
> being the sort who likes to hide things in out of the way places
>
> wilma$ cp ~/scandals /tmp/seth
> wilma$ chmod 600 /tmp/seth/scandals

This is already impossible as described. Seth can only mount the
filesystem in a private mount namespace inside a user namespace that
he created. Wilma can't see it unless Seth passes an fd to Wilma and
Wilma accepts and uses it.

>
> puts her list of scandals on the unsuspecting filesystem, and changes
> the mode to ensure that no one can find out what went on after the
> office party.
>
> Seth unmounts /tmp/seth. He looks in ~seth/myfs, finds out what really
> happened at the office party, and the story goes from there.
>
> Wilma did everything correctly according to the system security policy,
> but the system security policy did not protect her as advertised. The
> system was tricked into behaving as if it was in control of the content
> of the filesystem when in fact it was not.

I would argue that, if Wilma writes to some place described by an fd
and doesn't verify where she's writing to, then she has no expectation
of privacy. After all, she could just *tell* Seth directly whatever
she wants (assuming she can communicate with Seth in the first place).

>
> One way to fix this problem is for unprivileged mounts to recognize the
> attributes of the object mounted and to propagate those attributes to all
> the objects they present. All files on /tmp/seth would be owned by seth
> and protected by the mode bits, ACL and LSM requirements of ~/seth/myfs.

This is impossible to enforce, because Seth could use FUSE instead of ext4.

> opening a file on /tmp/seth would require the same permissions as opening
> the file containing the mounted filesystem. These attributes would have to
> be immutable, or at least demonstrably more restrictive (chmod might be
> allowed in some cases, but chown would never be) when changed. I don't see
> how a user other than seth could create a new file, as you'd either have
> a magical change in ownership or a false sense of security.

This would be a very harsh restriction. Seth might legitimately want
to give a user access to a file on backing store he owns without
giving that user access to the backing store. Root on a normal system
does that all the time.

> If you can mount a filesystem such that the labels are ignored you
> are effectively specifying that the Smack label on the files be
> determined by the defaulting rules. With CAP_MAC_ADMIN that's fine.
> Without it, it's not.

Can you explain what the threat model is here? I don't see what it is
that you're trying to prevent.

>> Your point is taken about my less-than-expert opinion about the other
>> security modules. We should at minimum get acks from the maintainers of
>> those modules that unprivileged mounts will not compromise MAC.
>
> I am the Smack maintainer. Unprivileged mounts as you have
> described them compromise MAC. They compromise DAC, too.
>

How do they compromise DAC?

--Andy