Re: [PATCH v2 01/17] psp: add documentation

Jakub Kicinski
2025-06-26T13:49:34Z

On Thu, 26 Jun 2025 07:55:34 -0400 Daniel Zahka wrote:
> >> +after ``psp-versions-ena`` has been disabled. User may also disable
> >> +``psp-versions-ena`` while there are active associations, which will
> >> +break all PSP Rx processing.
> >> +
> >> +Drivers are expected to ensure that device key is usable upon init
> >> +(working keys can be allocated), and that no duplicate keys may be generated
> >> +(reuse of SPI without key rotation). Drivers may achieve this by rotating
> >> +keys twice before registering the PSP device.
> > Since the device returns a { session_key, spi } pair, risk of reuse
> > is purely in firmware.

I don't think this is a requirement put forward in the spec?
Specifically if a device wants to allow partitioning of the SPI
space it may let the host pick the SPI. To me the device allocating
the SPIs seemed more like a convenience thing that a security feature
to prevent reuse.

> > I don't follow the need for the extra double rotation.
>
> Indeed that last sentence is superfluous. Re-initializing a device
> shouldn't leave a device key from a previous initialization, while
> resetting the spi space. If something like that were possible, it should
> probably be obvious to the driver writer to do something like double
> rotate the keys.