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Sabrina Dubroca says:
====================
tls: implement key updates for TLS1.3
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 <fkrenzel@redhat.com> 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 v4:
- add counter for received KeyUpdate messages
- improve wording in the documentation
- improve handling of bogus messages when looking for KeyUpdate's
- some coding style clean ups
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
====================
Signed-off-by: David S. Miller <davem@davemloft.net>
Linux kernel
============
There are several guides for kernel developers and users. These guides can
be rendered in a number of formats, like HTML and PDF. Please read
Documentation/admin-guide/README.rst first.
In order to build the documentation, use ``make htmldocs`` or
``make pdfdocs``. The formatted documentation can also be read online at:
https://www.kernel.org/doc/html/latest/
There are various text files in the Documentation/ subdirectory,
several of them using the reStructuredText markup notation.
Please read the Documentation/process/changes.rst file, as it contains the
requirements for building and running the kernel, and information about
the problems which may result by upgrading your kernel.