Re: [PATCH] RDMA/uverbs: Consider capability of the process that opens the file

["Serge E. Hallyn" <serge@hallyn.com>]

On Fri, Apr 25, 2025 at 01:54:07PM +0000, Parav Pandit wrote:
> 
> > From: Jason Gunthorpe <jgg@nvidia.com>
> > Sent: Friday, April 25, 2025 7:00 PM
> > 
> > On Fri, Apr 25, 2025 at 01:14:35PM +0000, Parav Pandit wrote:
> > 
> > > 1. In uobject creation syscall, I will add the check current->nsproxy->net-
> > >user_ns capability using ns_capable().
> > > And we don't hold any reference for user ns.
> > 
> > This is the thing that makes my head ache.. Is that really the right way to get
> > the user_ns of current? 
> 
> > Is it possible that current has multiple user_ns's? 
> I don't think so.
> 
> > We
> > are picking nsproxy because ib_dev has a net namespace affiliation?
> > 
> Yes.
> 
> After ruling out file's user ns, I believe there are two user ns.
> 
> 1. current_user_ns() 
> 2. current->nsproxy->net->user_ns.
> 
> In most cases #1 and #2 should be same to my knowledge.
> 
> When/if user wants to do have nested user ns, and don't want to create a new net ns, #2 can be of use.
> For example,
> a. Process1 starts in user_ns_1 which created net_ns_1
> b. rdma device is in net_ns_1
> c. Process1 unshare and moves to user_ns_2.
> d. For some reason user_ns_2 does not have the cap.

(d) is important.  "user_ns_2 does not have the cap" is imprecise.  Process1
after the unshare does have the cap against user_ns_2.  It does not have
it against user_ns_1, and since net_ns_1->user_ns == user_ns_1, that
means it loses privilege over net_ns_1.  Which is what we need.  Because
otherwise, an unprivileged user could simply unshare the user_ns, be root
there, and now tweak networking.

This all stems from the original requirements for user namespaces, which
were (off top of my head)

* unprivileged users must be able to create user namespaces
* root in a user namespace must be privileged over its resources
* root in a user namespace must have no privilege over any other resources
* user namespaces must nest

> By current UTS and other namespace semantics, since rdma device belongs to net ns, net ns's creator user ns to be considered.
> 
> I am unsure if doing #1 breaks any existing model.
> I like to get Eric/Serge's view also, if we should consider #1 or #2.