Hi Mimi and Elaine, Apologies for my delayed reply as I was busy with other high priority work. On Wed, 9 Dec 2020 at 22:14, Mimi Zohar <zohar@xxxxxxxxxxxxx> wrote: > > From: Elaine Palmer <erpalmer@xxxxxxxxxx> > > Update trusted key documentation with additional comparisons between > discrete TPMs and TEE. Isn't this additional comparison limited to a particular type of TPM (discrete TPMs) and ignored other TPM implementations (virtual TPM, firmware TPM etc.)? I think your later comment about on-chip versus off-chip points at these missing pieces as well. I would rather suggest comparing TPM and TEE on the basis of interfaces and implementation guidelines provided by corresponding standards as I think this is the most relevant part to the kernel. > > Signed-off-by: Elaine Palmer <erpalmer@xxxxxxxxxx> > --- > .../security/keys/trusted-encrypted.rst | 73 +++++++++++++++++-- > 1 file changed, 65 insertions(+), 8 deletions(-) > > diff --git a/Documentation/security/keys/trusted-encrypted.rst b/Documentation/security/keys/trusted-encrypted.rst > index 16042c8ff8ae..90c02105ab89 100644 > --- a/Documentation/security/keys/trusted-encrypted.rst > +++ b/Documentation/security/keys/trusted-encrypted.rst > @@ -14,12 +14,14 @@ convenience, and are integrity verified. > Trust Source > ============ > > -Trust Source provides the source of security for the Trusted Keys, on which > -basis Trusted Keys establishes a Trust model with its user. A Trust Source could > -differ from one system to another depending on its security requirements. It > -could be either an off-chip device or an on-chip device. Following section > -demostrates a list of supported devices along with their security properties/ > -guarantees: > +A trust source provides the source of security for Trusted Keys. This > +section lists currently supported trust sources, along with their security > +considerations. Whether or not a trust source is sufficiently safe depends > +on the strength and correctness of its implementation, as well as the threat > +environment for a specific use case. Since the kernel doesn't know what the > +environment is, and there is no metric of trust, it is dependent on the > +consumer of the Trusted Keys to determine if the trust source is sufficiently > +safe. > > * Root of trust for storage > > @@ -116,6 +118,59 @@ guarantees: > Provides no protection by itself, relies on the underlying platform for > features such as tamper resistance. > > + * Provisioning - the trust source's unique and verifiable cryptographic > + identity is provisioned during manufacturing > + > + (1) TPM > + > + The unique and verifiable cryptographic identity is the endorsement > + key (EK) or its primary seed. A review of the generation of the EK > + and its accompanying certificate is part of the Common Criteria > + evaluation of the product's lifecycle processes (ALC_*). See "TCG > + Protection Profile for PC Client Specific TPM 2" > + > + (2) TEE > + > + A protection profile for TEEs does not yet exist. Really? Have a look here [1]. [1] https://globalplatform.org/specs-library/tee-protection-profile-v1-3/# > Therefore, the > + provisioning process that generates the Hardware Unique Key is not > + evaluated by an independent third party and is highly dependent on > + the manufacturing environment. > + > + > + * Cryptography > + > + (1) TPM > + > + As part of the TPM's mandatory Common Criteria evaluation, the > + correctness of the TPM's implementation of cryptographic algorithms, > + the protection of keys, and the generation of random numbers, and other > + security-relevant functions must be documented, reviewed, and tested by > + an independent third party evaluation agency. It must meet the > + requirements of FIPS 140-2, FIPS 140-3, or ISO/IEC 19790:2012. > + > + (2) TEE > + > + Evaluations of cryptographic modules within TEEs are not required, but > + some are available for specific implementations within TEEs. > + > + > + * Interfaces and APIs > + > + (1) TPM > + > + TPMs have well-documented, standardized interfaces and APIs. > + > + (2) TEE > + > + Unless TEEs implement functionality such as a virtual TPM, they have > + custom interfaces and APIs. > + Kernel interface to TEE is based on the standardized TEE client API specification from GlobalPlatform [2]. [2] https://globalplatform.org/specs-library/tee-client-api-specification/ -Sumit > + > + * Threat model > + > + The strength and appropriateness of a particular TPM or TEE for a given > + purpose must be assessed when using them to protect security-relevant data. > + > > Key Generation > ============== > @@ -123,8 +178,10 @@ Key Generation > Trusted Keys > ------------ > > -New keys are created from trust source generated random numbers, and are > -encrypted/decrypted using trust source storage root key. > +New keys are created from random numbers generated in the trust source. They > +are encrypted/decrypted using a child key in the storage key hierarchy. > +Encryption and decryption of the child key must be protected by a strong > +access control policy within the trust source. > > * TPM (hardware device) based RNG > > -- > 2.18.4 >
