Re: [PATCH 0/3] Allow access to confidential computing secret area in SEV guests

From: James Bottomley <jejb@linux.ibm.com>
To: Greg KH <gregkh@linuxfoundation.org>
Cc: Dov Murik <dovmurik@linux.ibm.com>,
	linux-efi@vger.kernel.org, Borislav Petkov <bp@suse.de>,
	Ashish Kalra <ashish.kalra@amd.com>,
	Brijesh Singh <brijesh.singh@amd.com>,
	Tom Lendacky <thomas.lendacky@amd.com>,
	Ard Biesheuvel <ardb@kernel.org>,
	James Morris <jmorris@namei.org>,
	"Serge E. Hallyn" <serge@hallyn.com>,
	Andi Kleen <ak@linux.intel.com>,
	"Dr. David Alan Gilbert" <dgilbert@redhat.com>,
	Tobin Feldman-Fitzthum <tobin@linux.ibm.com>,
	Jim Cadden <jcadden@ibm.com>,
	linux-coco@lists.linux.dev,
	linux-security-module@vger.kernel.org,
	linux-kernel@vger.kernel.org
Subject: Re: [PATCH 0/3] Allow access to confidential computing secret area in SEV guests
Date: Thu, 02 Sep 2021 08:19:51 -0700	[thread overview]
Message-ID: <6cb65cb3bd69ae69bde044f809525e478bdb8512.camel@linux.ibm.com> (raw)
In-Reply-To: <YTDoS5XycY3gO4MM@kroah.com>

On Thu, 2021-09-02 at 17:05 +0200, Greg KH wrote:
> On Thu, Sep 02, 2021 at 07:35:10AM -0700, James Bottomley wrote:
> > On Thu, 2021-09-02 at 14:57 +0200, Greg KH wrote:
> > [...]
> > > Wait, why are you using securityfs for this?
> > > 
> > > securityfs is for LSMs to use. 
> > 
> > No it isn't ... at least not exclusively; we use it for non LSM
> > security purposes as well, like for the TPM BIOS log and for
> > IMA.  What makes you think we should start restricting securityfs
> > to LSMs only?  That's not been the policy up to now.
> 
> Well that was the original intent of the filesystem when it was
> created, but I guess it's really up to the LSM maintainers now what
> they want it for.
> 
> > >  If you want your own filesystem to play around with stuff like
> > > this, great, write your own, it's only 200 lines or less these
> > > days.  We used to do it all the time until people realized they
> > > should just use sysfs for driver stuff.
> > 
> > This is a security purpose (injected key retrieval), so securityfs
> > seems to be the best choice.  It's certainly possible to create a
> > new filesystem, but I really think things with a security purpose
> > should use securityfs so people know where to look for them.
> 
> knowing where to look should not be an issue, as that should be
> documented in Documentation/ABI/ anyway, right?
> 
> It's just the overlap / overreach of using an existing filesystem for
> things that don't seem to be LSM-related that feels odd to me.
> 
> Why not just make a cocofs if those people want a filesystem
> interface?
> It's 200 lines or so these days, if not less, and that way you only
> mount what you actually need for the system.

Secrets transfer is actually broader than confidential computing,
although confidential computing is a first proposed use, so I think
cocofs would be too narrow.

> Why force this into securityfs if it doesn't have to be?

It's not being forced.  Secrets transfer is a security function in the
same way the bios log is.

James