[PATCH v2 0/2] smb: client: Spec-compliance fixes for Kerberos key derivation

[Piyush Sachdeva <s.piyush1024@gmail.com>]

This series fixes two MS-SMB2 section 3.2.5.3 spec violations in the
Kerberos session key handling path of fs/smb/client.

Patch 1/2 (resend of v1 with a small cleanup folded in) fixes the
AES-256 mount failure with sec=krb5: encryption and decryption key
derivation must use Session.FullSessionKey (the full Kerberos session
key, typically 32 bytes for the aes256-cts-hmac-sha1-96 enctype)
instead of Session.SessionKey (the first 16 bytes).

Patch 2/2 closes the related corner case in the same section of the
spec: when the GSS protocol returns a session key shorter than 16
bytes, the buffer must be right-padded with zero bytes. The current
code copies the GSS key verbatim, which causes generate_key() to read
past the end of the allocated buffer and derive keys that do not match
the server. The trigger is deprecated short-key Kerberos enctypes
(e.g. single-DES, 8-byte session key); modern KDCs disable these by
default, so this is a latent issue rather than a reachable one, but it
is still a kernel slab over-read and a literal spec violation.

Verified against Azure Files (AES-256-GCM + Kerberos aes256-cts) which
previously failed to mount with EAGAIN; the dmesg "Session Key" trace
under CONFIG_CIFS_DEBUG_DUMP_KEYS now shows the full 32-byte session
key being used for encryption/decryption KDF input.

Link: https://lore.kernel.org/linux-cifs/20260409161538.3618-1-s.piyush1024@gmail.com/

Changes since v1:
  - Patch 1/2: initialize full_key_size at declaration to silence
    -Wmaybe-uninitialized on some toolchains, and drop the now-
    redundant else branch (self-review).
  - Patch 1/2: tighten the FullSessionKey condition to also require
    Connection.Dialect == "3.1.1", matching MS-SMB2 3.2.5.3.1 verbatim.
  - New patch 2/2: zero-pad short GSS session keys per MS-SMB2 3.2.5.3,
    eliminating a latent slab over-read in generate_key().

Signed-off-by: Piyush Sachdeva <psachdeva@microsoft.com>
Signed-off-by: Piyush Sachdeva <s.piyush1024@gmail.com>
Bcc: vaibsharma@microsoft.com
---
Piyush Sachdeva (2):
      smb: client: Use FullSessionKey for AES-256 encryption key derivation
      smb: client: Zero-pad short GSS session keys per MS-SMB2

 fs/smb/client/ioctl.c         |  2 +-
 fs/smb/client/smb2pdu.c       | 23 ++++++++++++++++++-----
 fs/smb/client/smb2transport.c | 35 ++++++++++++++++++++++++++---------
 3 files changed, 45 insertions(+), 15 deletions(-)
---
base-commit: 0cbc300257d9b399491909806777f504ec687c1d
change-id: 20260429-kerbmi-dc0853cd29fc

Best regards,
--  
Piyush Sachdeva <s.piyush1024@gmail.com>