Subject: Re: [PATCH V4 0/5] mlx5 ConnectX control misc driver

From: Jason Gunthorpe <[email-masked]>
Date: Wed, 14 Feb 2024 14:37:55 -0400

On Wed, Feb 14, 2024 at 10:11:26AM -0800, Jakub Kicinski wrote:
> On Wed, 14 Feb 2024 13:57:35 -0400 Jason Gunthorpe wrote:
> > There is a clear split in my mind between:
> >  - inspection debugging
> >  - invasive mutating debugging
> >  - configuration
> 
> Yes there's a clear split, and how are you going to enforce it on 
> an opaque interface? Put an "evil" bit in the common header?

The interface is opaque through a subsystem, it doesn't mean it is
completely opaque to every driver layer in the kernel. There is still a
HW specific kernel driver that delivers the FW command to the actual
HW.

In the mlx5 model the kernel driver stamps the command with "uid"
which is effectively a security scope label. This cannot be avoided by
userspace and is fundamental to why mlx5ctl is secure in a lockdown
kernel.

For example mlx5's FW interface has the concept of security scopes. We
have several defined today:
 - Kernel
 - Userspace rdma
 - Userspace rdma with CAP_NET_RAW
 - Userspace rdma with CAP_SYS_RAWIO

So we trivally add three more for the scopes I listed above. The
mlx5ctl driver as posted already introduced a new scope, for example.

Userspace will ask the kernel for an appropriate security scope after
opening the char-device. If userspace asks for invasive then you get a
taint. Issuing an invasive command without a kernel applied invasive
security label will be rejected by the FW.

We trust the kernel to apply the security label for the origin of the
command. We trust the the device FW to implement security scopes,
because these are RDMA devices and all of RDMA and all of SRIOV
virtualization are totally broken if the device FW cannot be trusted
to maintain security separation between scopes.

Jason