[Devel] Re: [RFC][PATCH] ns: Syscalls for better namespace sharing control.

[Serge E. Hallyn]

Quoting Eric W. Biederman (ebiederm at xmission.com):
> 
> I have take an snapshot of my development tree and placed it at.
> 
> 
> git://git.kernel.org/pub/scm/linux/people/ebiederm/linux-2.6.33-nsfd-v5.git
> 
> 
> >> I am going to explore a bit more.  Given that nsfd is using the same
> >> permission checks as a proc file, I think I can just make it a proc
> >> file.  Something like "/proc/<pid>/ns/net".  With a little luck that
> >> won't suck too badly.
> >>   
> > Ah ! yes. Good idea.
> 
> It is a hair more code to use proc files but nothing worth counting.
> 
> Probably the biggest thing I am aware of right now in my development
> tree is in getting uids to pass properly between unix domain sockets
> I would up writing this cred_to_ucred function.
> 
> Serge can you take a look and check my logic, and do you have
> any idea of where we should place something like pid_vnr but
> for the uid namespace?

Well my first thought was user_namespace, but I'm thinking kernel/cred.c is
the best place for it.

> void cred_to_ucred(struct pid *pid, const struct cred *cred,
> 		   struct ucred *ucred)
> {
> 	ucred->pid = pid_vnr(pid);
> 	ucred->uid = ucred->gid = -1;
> 	if (cred) {
> 		struct user_namespace *cred_ns = cred->user->user_ns;
> 		struct user_namespace *current_ns = current_user_ns();
> 		struct user_namespace *tmp;
> 
> 		if (likely(cred_ns == current_ns)) {
> 			ucred->uid = cred->euid;
> 			ucred->gid = cred->egid;
> 		} else {
> 			/* Is cred in a child user namespace */
> 			tmp = cred_ns;
> 			do {
> 				tmp = tmp->creator->user_ns;
> 				if (tmp == current_ns) {

	Hmm, I think you want to catch one level up - so the creator itself
	is in current_user_ns, so

	do {
		if (tmp->creator->user_ns == current_ns) {
			ucred->uid = tmp->creator->uid;
			ucred->gid = tmp->creator_gid;
			return;
		}
		tmp = tmp->creator->user_ns;
	} while (tmp != &init_user_ns);

> 					ucred->uid = tmp->creator->uid;
> 					ucred->gid = overflowgid;

			should we start recording a user_ns->creator_gid
			instead?

> 					return;
> 				}
> 			} while (tmp != &init_user_ns);
> 
> 			/* Is cred the creator of my user namespace,
> 			 * or the creator of one of it's parents?
> 			 */
> 			for( tmp = current_ns; tmp != &init_user_ns;
> 			     tmp = tmp->creator->user_ns) {
> 				if (cred->user == tmp->creator) {
> 					ucred->uid = 0;
> 					ucred->gid = 0;
> 					return;
> 				}
> 			}

That looks right.

> 			/* No user namespace relationship so no mapping */
> 			ucred->uid = overflowuid;
> 			ucred->gid = overflowgid;
> 		}
> 	}
> }
> 
> Eric
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