[Devel] Re: [PATCH 1/9] Add a user_namespace as creator/owner of uts_namespace

[Eric W. Biederman]

David Howells <dhowells at redhat.com> writes:

> Serge E. Hallyn <serge at hallyn.com> wrote:
>
>>  struct uts_namespace {
>>  	struct kref kref;
>>  	struct new_utsname name;
>> +	struct user_namespace *user_ns;
>>  };
>
> If a uts_namespace belongs to a user_namespace, should CLONE_NEWUSER then
> imply CLONE_NEWUTS?
>
> Or is uts_namespace::user_ns more an implication that the set of users in
> user_namespace are the only ones authorised to alter the uts data.

The later.

> I presume that the uts_namespace of a process must be owned by one of the
> user_namespaces in the alternating inheritance chain of namespaces and their
> creators leading from current_user_ns() to init_user_ns.
>
> With that in mind, looking at patch 3:
>
> -	if (!capable(CAP_SYS_ADMIN))
> +	if (!ns_capable(current->nsproxy->uts_ns->user_ns, CAP_SYS_ADMIN))
>
> what is it you're actually asking?  I presume it's 'does this user have
> CAP_SYS_ADMIN capability over objects belonging to the uts_namespace's
> user_namespace?'

Yes.

> So, to look at the important bit of patch 2:
>
> -int cap_capable(struct task_struct *tsk, const struct cred *cred, int cap,
> -		int audit)
> +int cap_capable(struct task_struct *tsk, const struct cred *cred,
> +		struct user_namespace *targ_ns, int cap, int audit)
>  {
> -	return cap_raised(cred->cap_effective, cap) ? 0 : -EPERM;
> +	for (;;) {
> +		/* The creator of the user namespace has all caps. */
> +		if (targ_ns != &init_user_ns && targ_ns->creator == cred->user)
> +			return 0;
> +
> +		/* Do we have the necessary capabilities? */
> +		if (targ_ns == cred->user->user_ns)
> +			return cap_raised(cred->cap_effective, cap) ? 0 : -EPERM;
> +
> +		/* Have we tried all of the parent namespaces? */
> +		if (targ_ns == &init_user_ns)
> +			return -EPERM;
> +
> +		/* If you have the capability in a parent user ns you have it
> +		 * in the over all children user namespaces as well, so see
> +		 * if this process has the capability in the parent user
> +		 * namespace.
> +		 */
> +		targ_ns = targ_ns->creator->user_ns;
> +	}
> +
> +	/* We never get here */
> +	return -EPERM;
>  }
>
> On entry, as we're called from ns_capable(), cred->user is the user that the
> current process is running as, and, as such, may be in a separate namespace
> from uts_namespace - which may itself be in a separate namespace from
> init_user_ns.
>
> So, assume for the sake of argument that there are three user_namespaces along
> the chain from the calling process to the root, and that the uts_namespace
> belongs to the middle one.

So we have the nested stack of:
user_ns3->creator->user_ns == user_ns2
user_ns2->creator->user_ns == &init_user_ns
uts_ns2->user_ns == user_ns2

>
> 		if (targ_ns != &init_user_ns && targ_ns->creator == cred->user)
> 			return 0;
>
> Can never match because targ_ns->creator cannot be cred->user; even if the
> uts_namespace belongs to our namespace, given that the creator lies outside
> our namespace.

Initially we come in with targ_ns == user_ns2 and cred->user->user_ns in
one of (user_ns3, user_ns2, or &init_user_ns).

targ_ns takes on values user_ns2 and &init_user_ns.

So when targ_ns becomes &init_user_ns.  If the user in question is
uts_ns2->user_ns->creator.  This check will indeed match.

> 		if (targ_ns == cred->user->user_ns)
> 			return cap_raised(cred->cap_effective, cap) ? 0 : -EPERM;
>
> Can only match if we are in the target user_namespace (ie. the one to which
> uts_namespace belongs), whether or not we have CAP_SYS_ADMIN.

As before targ_ns takes on values of user_ns2 and &init_user_ns.

Which means this check will match if we have CAP_SYS_ADMIN in
&init_user_ns or in user_ns2.

> Which means that unless the uts_namespace belongs to our user_namespace, we
> cannot change it.  Is that correct?

No.  If you are root in a parent namespace you can also change it.

> So ns_capable() restricts you to only doing interesting things to objects that
> belong to a user_namespace if they are in your own user_namespace.  Is that
> correct?

No.  Root outside your user namespace is also allowed to do interesting
things.

> If that is so, is the loop required for ns_capable()?

Yes.


> Looking further at patch 2:
>
> 	#define nsown_capable(cap) (ns_capable(current_user_ns(), (cap)))
>
> Given what I've said above, I presume the loop isn't necessary here either.
>
>
> I think you're using ns_capable() in two different ways:
>
>  (1) You're using it to see if a process has power over its descendents in a
>      user_namespace that can be traced back to a clone() that it did with
>      CLONE_NEWUSER.
>
>      For example, automatically granting a process permission to kill
>      descendents in a namespace it created.
>
>  (2) You're using it to see if a process can access objects that might be
>      outside its own user_namespace.
>
>      For example, setting the hostname.
>
> Is it worth giving two different interfaces to make this clearer (even if they
> actually do the same thing)?
>
>
> Sorry if this seems rambly, but I'm trying to get my head round your
> code.

I am all for making that loop a little clearer, because it is something
you have to pause and think about to understand but so far I don't think
the loop is wrong, and it is simple.

Eric
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