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

[David Howells]

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.

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?'

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.

		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.

		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.

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

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?

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


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.

David
     
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