Re: [PATCH v10 7/9] platform: cznic: turris-omnia-mcu: Add support for digital message signing via debugfs

[Greg Kroah-Hartman <gregkh@linuxfoundation.org>]

On Fri, May 10, 2024 at 01:31:58PM +0200, Marek Behún wrote:
> On Fri, 10 May 2024 11:52:56 +0100
> Greg Kroah-Hartman <gregkh@linuxfoundation.org> wrote:
> 
> > On Fri, May 10, 2024 at 12:18:17PM +0200, Marek Behún wrote:
> > > Add support for digital message signing with private key stored in the
> > > MCU. Boards with MKL MCUs have a NIST256p ECDSA private key created
> > > when manufactured. The private key is not readable from the MCU, but
> > > MCU allows for signing messages with it and retrieving the public key.
> > > 
> > > As described in a similar commit 50524d787de3 ("firmware:
> > > turris-mox-rwtm: support ECDSA signatures via debugfs"):
> > >   The optimal solution would be to register an akcipher provider via
> > >   kernel's crypto API, but crypto API does not yet support accessing
> > >   akcipher API from userspace (and probably won't for some time, see
> > >   https://www.spinics.net/lists/linux-crypto/msg38388.html).
> > > 
> > > Therefore we add support for accessing this signature generation
> > > mechanism via debugfs for now, so that userspace can access it.  
> > 
> > Having a "real" user/kernel api in debugfs feels wrong here, why would
> > you not do this properly?  On most, if not all, systems, debugfs is
> > locked down so you do not have access to it, as it is only there for
> > debugging.  So how is a user supposed to use this feature if they can't
> > get access to it?
> > 
> > And debugfs files can be changed at any time, so how can you ensure that
> > your new api will always be there?
> > 
> > In other words, please solve this properly, do not just add a hack into
> > debugfs that no one can use as that is not a good idea.
> 
> Hi Greg,
> 
> this is the same thing we discussed 5 years ago, I wanted to implement
> it via crypto's akcipher, but was refused due to
>   https://www.spinics.net/lists/linux-crypto/msg38388.html
> 
> I've then exposed this via debugfs in the turris-mox-rwtm driver 4
> years ago, and we have supported this in our utility scripts, with the
> plan that to reimplement it in the kernel via the correct ABI once
> akcipher (or other ABI) is available to userspace, but AFAIK after 5
> years this is still not the case :-(
> 
> If not debugfs and not akcipher, another option is to expose this via
> sysfs, but that also doesn't seem right, and if I recall correctly you
> also disapproved of this 5 years ago.

Yeah, sysfs is not ok for this either.

> The last option would be to create another device, something like
> /dev/turris-crypto for this. I wanted to avoid that and wait for
> akcipher to be exposed do crypto since another /dev device must be
> supported forever, while debugfs implementation can be removed once
> this is supported via standardized ABI.
> 
> Do you have any suggestions?

Not really, I can't see the link above (no internet connection right
now) but this should just be fixed properly at the crypto subsystem
instead of these horrible debugfs hacks.

thanks,

greg k-h