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

["Eric W. Biederman" <ebiederm@xmission.com>]

Parav Pandit <parav@nvidia.com> writes:

>> From: Serge E. Hallyn <serge@hallyn.com>
>> Sent: Friday, April 25, 2025 8:37 PM
>> 
>> On Fri, Apr 25, 2025 at 11:24:29AM -0300, Jason Gunthorpe wrote:
>> > On Fri, Apr 25, 2025 at 09:01:44AM -0500, Serge E. Hallyn wrote:
>> > > On Fri, Apr 25, 2025 at 10:29:30AM -0300, Jason Gunthorpe wrote:
>> > > > 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? We are picking nsproxy because
>> > > > ib_dev has a net namespace affiliation?
>> > >
>> > > It's not that "current has multiple user_ns's", it's that the
>> > > various resources, including other namespaces, which current has or
>> > > belongs to have associated namespaces.
>> >
>> > That seems like splitting nits. Can I do current->XXX->user_ns and get
>> > different answers? Sounds like yes?
>> 
>> I don't think it's splitting nits.  current->nsproxy->net_ns->user_ns is not
>> current's user namespace.
>> 
>> > > current_user_ns() is the user namespace to which current belongs.
>> > > But if you want to check if it can have privilege over a resource,
>> > > you have to check whether current has ns_capable(resource->userns,
>> CAP_X).
>> >
>> > So what is the resource here?
>> 
>> That's what I've been trying to get answered :)
>>
> A. When a raw socket is created to send a packet vis netdev
> (resource), the cap is checked against the process in [1].

No.

> [1] https://elixir.bootlin.com/linux/v6.14.3/source/net/ipv4/af_inet.c#L314
> Not against the netdev's dev_net().

There isn't a netdevide to check against.

The resource is the network namespace aka net.

The permission check is not solely against the processes credentials
in current->cred.

> B. Netdev's dev_net() is crossed checked to see if current process can access netdev ifindex or not in send() call.
>
> So resource was netdev, but literally the check is against the process
> cap.

Not at all.    The process (user_ns,cap) pair in current->cred is checked
against the user_ns that owns the network namespace aka net->user_ns.

That current is used to find the network namespace is irrelevant for
the permission check.

> And considering above is right,
>
> I try to draw the parallels between the two types of network devices,
>
> the check for rdma raw QP (equivalent of raw socket), 
> Should check similarly against the process's net->user_ns during QP creation time.
> This will match #A.
>
> And process to rdma access check should be a separate check #B.

The way you described it sounds wrong, but your conclusion of
what needs to be checked seems correct.

Eric