Re: [PATCH v2] nvme-auth: Hash DH shared secret to create session key

[Eric Biggers <ebiggers@kernel.org>]

On Wed, Mar 11, 2026 at 04:16:43PM -0700, Chris Leech wrote:
> The NVMe Base Specification 8.3.5.5.9 states that the session key Ks
> shall be computed from the ephemeral DH key by applying the hash
> function selected by the HashID parameter.
> 
> The current implementation stores the raw DH shared secret as the
> session key without hashing it. This causes redundant hash operations:
> 
> 1. Augmented challenge computation (section 8.3.5.5.4) requires
>    Ca = HMAC(H(g^xy mod p), C). The code compensates by hashing the
>    unhashed session key in nvme_auth_augmented_challenge() to produce
>    the correct result.
> 
> 2. PSK generation (section 8.3.5.5.9) requires PSK = HMAC(Ks, C1 || C2)
>    where Ks should already be H(g^xy mod p). As the DH shared secret
>    is always larger than the HMAC block size, HMAC internally hashes
>    it before use, accidentally producing the correct result.
> 
> When using secure channel concatenation with bidirectional
> authentication, this results in hashing the DH value three times: twice
> for augmented challenge calculations and once during PSK generation.
> 
> Fix this by:
> - Modifying nvme_auth_gen_shared_secret() to hash the DH shared secret
>   once after computation: Ks = H(g^xy mod p)
> - Removing the hash operation from nvme_auth_augmented_challenge()
>   as the session key is now already hashed
> - Updating session key buffer size from DH key size to hash output size
> - Adding specification references in comments
> 
> This avoid storing the raw DH shared secret and reduces the number of
> hash operations from three to one when using secure channel
> concatenation.
> 
> Signed-off-by: Chris Leech <cleech@redhat.com>
> ---
> v2: 
> - Renamed nvme_auth_gen_session_key() arguments from ctrl_key to
>   public_key, as it is the peers public DH key and this code is also
>   used on the target/controller side when the peer is the host.
> - Changed argument validation from erroring on sess_key_len < hash_len
>   to sess_key_len != hash_len to be more precise in calling requirements
> 
> created on the nvme-7.1 branch
> this depends on Eric Biggers "nvme-auth use crypto library" patches 
> from <20260302075959.338638-1-ebiggers@kernel.org>
> 
>  drivers/nvme/common/auth.c | 88 ++++++++++++++++++++++++++++++--------
>  drivers/nvme/host/auth.c   | 13 +++---
>  drivers/nvme/target/auth.c | 15 +++---
>  include/linux/nvme-auth.h  |  6 +--
>  4 files changed, 92 insertions(+), 36 deletions(-)

Reviewed-by: Eric Biggers <ebiggers@kernel.org>

- Eric