[net-next,v4,0/6] tls: implement key updates for TLS1.3

Message

Sabrina Dubroca Nov. 14, 2024, 3:50 p.m. UTC

This adds support for receiving KeyUpdate messages (RFC 8446, 4.6.3
[1]). A sender transmits a KeyUpdate message and then changes its TX
key. The receiver should react by updating its RX key before
processing the next message.

This patchset implements key updates by:
 1. pausing decryption when a KeyUpdate message is received, to avoid
    attempting to use the old key to decrypt a record encrypted with
    the new key
 2. returning -EKEYEXPIRED to syscalls that cannot receive the
    KeyUpdate message, until the rekey has been performed by userspace
 3. passing the KeyUpdate message to userspace as a control message
 4. allowing updates of the crypto_info via the TLS_TX/TLS_RX
    setsockopts

This API has been tested with gnutls to make sure that it allows
userspace libraries to implement key updates [2]. Thanks to Frantisek
Krenzelok <[e-mail]> for providing the implementation in
gnutls and testing the kernel patches.


=======================================================================
Discussions around v2 of this patchset focused on how HW offload would
interact with rekey.

RX
 - The existing SW path will handle all records between the KeyUpdate
   message signaling the change of key and the new key becoming known
   to the kernel -- those will be queued encrypted, and decrypted in
   SW as they are read by userspace (once the key is provided, ie same
   as this patchset)
 - Call ->tls_dev_del + ->tls_dev_add immediately during
   setsockopt(TLS_RX)

TX
 - After setsockopt(TLS_TX), switch to the existing SW path (not the
   current device_fallback) until we're able to re-enable HW offload
   - tls_device_sendmsg will call into tls_sw_sendmsg under lock_sock
     to avoid changing socket ops during the rekey while another
     thread might be waiting on the lock
 - We only re-enable HW offload (call ->tls_dev_add to install the new
   key in HW) once all records sent with the old key have been
   ACKed. At this point, all unacked records are SW-encrypted with the
   new key, and the old key is unused by both HW and retransmissions.
   - If there are no unacked records when userspace does
     setsockopt(TLS_TX), we can (try to) install the new key in HW
     immediately.
   - If yet another key has been provided via setsockopt(TLS_TX), we
     don't install intermediate keys, only the latest.
   - TCP notifies ktls of ACKs via the icsk_clean_acked callback. In
     case of a rekey, tls_icsk_clean_acked will record when all data
     sent with the most recent past key has been sent. The next call
     to sendmsg will install the new key in HW.
   - We close and push the current SW record before reenabling
     offload.

If ->tls_dev_add fails to install the new key in HW, we stay in SW
mode. We can add a counter to keep track of this.


In addition:

Because we can't change socket ops during a rekey, we'll also have to
modify do_tls_setsockopt_conf to check ctx->tx_conf and only call
either tls_set_device_offload or tls_set_sw_offload. RX already uses
the same ops for both TLS_HW and TLS_SW, so we could switch between HW
and SW mode on rekey.

An alternative would be to have a common sendmsg which locks
the socket and then calls the correct implementation. We'll need that
anyway for the offload under rekey case, so that would only add a test
to the SW path's ops (compared to the current code). That should allow
us to simplify build_protos a bit, but might have a performance
impact - we'll need to check it if we want to go that route.
=======================================================================

Changes since v3:
 - rebase on top of net-next
 - rework tls_check_pending_rekey according to Jakub's feedback
 - add statistics for rekey: {RX,TX}REKEY{OK,ERROR}
 - some coding style clean ups

Link: https://lore.kernel.org/netdev/[e-mail]/ [v3]
Link: https://lore.kernel.org/netdev/[e-mail]/ [v2]
Link: https://lore.kernel.org/netdev/[e-mail]/ [v1]

Link: https://www.rfc-editor.org/rfc/rfc8446#section-4.6.3 [1]
Link: https://gitlab.com/gnutls/gnutls/-/merge_requests/1625 [2]

Sabrina Dubroca (6):
  tls: block decryption when a rekey is pending
  tls: implement rekey for TLS1.3
  tls: add counters for rekey
  docs: tls: document TLS1.3 key updates
  selftests: tls: add key_generation argument to tls_crypto_info_init
  selftests: tls: add rekey tests

 Documentation/networking/tls.rst  |  31 ++
 include/net/tls.h                 |   3 +
 include/uapi/linux/snmp.h         |   4 +
 net/tls/tls.h                     |   3 +-
 net/tls/tls_device.c              |   2 +-
 net/tls/tls_main.c                |  71 ++++-
 net/tls/tls_proc.c                |   4 +
 net/tls/tls_sw.c                  | 138 +++++++--
 tools/testing/selftests/net/tls.c | 480 +++++++++++++++++++++++++++++-
 9 files changed, 676 insertions(+), 60 deletions(-)