linux-kernel - Re: [PATCH] crypto: ccp: introduce SEV_GET

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Date:   Fri, 22 Feb 2019 12:37:07 +0800
From:   Herbert Xu <herbert@...dor.apana.org.au>
To:     "Singh, Brijesh" <brijesh.singh@....com>
Cc:     Nathaniel McCallum <npmccallum@...hat.com>,
        "linux-crypto@...r.kernel.org" <linux-crypto@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "Natarajan, Janakarajan" <Janakarajan.Natarajan@....com>,
        "Hook, Gary" <Gary.Hook@....com>,
        "Lendacky, Thomas" <Thomas.Lendacky@....com>
Subject: Re: [PATCH] crypto: ccp: introduce SEV_GET_ID2 command

On Thu, Feb 14, 2019 at 07:33:29PM +0000, Singh, Brijesh wrote:
> 
> 
> On 2/14/19 10:57 AM, Nathaniel McCallum wrote:
> > I'm a little concerned that this immediately disables SEV_GET_ID.
> > IMHO, we should continue to support both for a period of time. One
> > justification for immediate disablement would be that if keeping it
> > around is likely to enabled incorrect or insecure userspace behavior
> > with a firmware change. 
> 
> 
> There are not many programs using the GET_ID today, my preference
> is to force userspace running on a kernel which supports the GET_ID2
> to use GET_ID2 and not fallback to GET_ID.
> 
> The current GET_ID is *broken*.
> 
> Here is one case I am trying to navigate:
> - AMD releases a new CPU
> - The kernel used in your distro does not support this CPU yet.
>    You updated the kernel to get the CPU support.
> - The GET_ID on this CPU returned a 10 bytes (instead of 64)
> - You gave the 64-bytes of data to AMD to get the certificate.
>    AMD server rejects the request because ID given to it does not
>    exist in its database.
> 
> If we drop the support for GET_ID in kernel, then GET_ID will fail and
> user will required to take action.

Sorry, but we can't drop a kernel API just to force userspace
to upgrade to a new one.

So I agree with Nathaniel that we should keep compatibility until
such a time when user-space is no longer using the old API.

You can use other mechanisms to encourage user-space to switch
over to the new API, e.g., a once-only warning if the old API
is used.

Cheers,
-- 
Email: Herbert Xu <herbert@...dor.apana.org.au>
Home Page: http://gondor.apana.org.au/~herbert/
PGP Key: http://gondor.apana.org.au/~herbert/pubkey.txt