linux-kernel - Re: [EXT] Re: [RFC PATCH HBK: 0/8] HW BOUND KEY as TRUSTED KEY

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Message-ID: <843e1f1cbed67ce558e20d1e56a82dfe27732028.camel@pengutronix.de>
Date:   Wed, 07 Sep 2022 10:10:34 +0200
From:   Jan Lübbe <jlu@...gutronix.de>
To:     Pankaj Gupta <pankaj.gupta@....com>,
        Jarkko Sakkinen <jarkko@...nel.org>
Cc:     "a.fatoum@...gutronix.de" <a.fatoum@...gutronix.de>,
        "Jason@...c4.com" <Jason@...c4.com>,
        "jejb@...ux.ibm.com" <jejb@...ux.ibm.com>,
        "zohar@...ux.ibm.com" <zohar@...ux.ibm.com>,
        "dhowells@...hat.com" <dhowells@...hat.com>,
        "sumit.garg@...aro.org" <sumit.garg@...aro.org>,
        "david@...ma-star.at" <david@...ma-star.at>,
        "michael@...le.cc" <michael@...le.cc>,
        "john.ernberg@...ia.se" <john.ernberg@...ia.se>,
        "jmorris@...ei.org" <jmorris@...ei.org>,
        "serge@...lyn.com" <serge@...lyn.com>,
        "herbert@...dor.apana.org.au" <herbert@...dor.apana.org.au>,
        "davem@...emloft.net" <davem@...emloft.net>,
        "j.luebbe@...gutronix.de" <j.luebbe@...gutronix.de>,
        "ebiggers@...nel.org" <ebiggers@...nel.org>,
        "richard@....at" <richard@....at>,
        "keyrings@...r.kernel.org" <keyrings@...r.kernel.org>,
        "linux-crypto@...r.kernel.org" <linux-crypto@...r.kernel.org>,
        "linux-integrity@...r.kernel.org" <linux-integrity@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-security-module@...r.kernel.org" 
        <linux-security-module@...r.kernel.org>,
        Sahil Malhotra <sahil.malhotra@....com>,
        Kshitiz Varshney <kshitiz.varshney@....com>,
        Horia Geanta <horia.geanta@....com>,
        Varun Sethi <V.Sethi@....com>
Subject: Re: [EXT] Re: [RFC PATCH HBK: 0/8] HW BOUND KEY as TRUSTED KEY

On Wed, 2022-09-07 at 07:22 +0000, Pankaj Gupta wrote:
> Even if somehow the key is retrieved from the keyring, the retrieved key 
> would be an encrypted key.
> This encrypted key can only be decrypted by Hardware, which generated it.
> 
> Hence, the retrieved key is unusable outside of the hardware.

NXP's CAAM unit (i.e. on i.MX6) supports several modes of sealed/encrypted keys.
The (un)sealing process uses a key that is normally derived from a per-device
key in eFUSES. One aspect of these modes is whether the plaintext key material
is accessible to the kernel or not.

Ahmad's patch set added support for a mode where the CAAM is used to seal
plaintext known to the kernel to a "blob" (in CAAM terminology) on export to
userspace and the reverse on import. This mode allows the kernel to use the
plaintext for dm-crypt, to encrypt other keyrings and similar.

The CAAM has another sealing mode, where it will not allow writing of the
plaintext key to memory. Instead, it is kept in one of the CAAM-internal key
registers. There, it can be used for cryptographic operations (i.e. AES). This
way, the plaintext key is protected even from the kernel. The kernel could keep
a copy of in sealed form, so it can reload the CAAM's key register when needed.

Pankaj, is that the mode you intend to support with this series?

Could you describe the high-level use-cases this would be need for, compared to
the existing mode where plaintext keys are accessible to the kernel? In which
cases would you use each mode?

Regards,
Jan
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