docs: tls: document TLS1.3 key updates

Document the kernel's behavior and userspace expectations.

Suggested-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: Sabrina Dubroca <sd@queasysnail.net>
Signed-off-by: David S. Miller <davem@davemloft.net>

diff --git a/Documentation/networking/tls.rst b/Documentation/networking/tls.rst
index 658ed3a71e1b53f4b4688de345fe3dc7f802f837..c7904a1bc167410397fc8bb0b77773f634521b87 100644 (file)
--- a/Documentation/networking/tls.rst
+++ b/Documentation/networking/tls.rst
@@ -200,6 +200,32 @@ received without a cmsg buffer set.
 
 recv will never return data from mixed types of TLS records.
 
+TLS 1.3 Key Updates
+-------------------
+
+In TLS 1.3, KeyUpdate handshake messages signal that the sender is
+updating its TX key. Any message sent after a KeyUpdate will be
+encrypted using the new key. The userspace library can pass the new
+key to the kernel using the TLS_TX and TLS_RX socket options, as for
+the initial keys. TLS version and cipher cannot be changed.
+
+To prevent attempting to decrypt incoming records using the wrong key,
+decryption will be paused when a KeyUpdate message is received by the
+kernel, until the new key has been provided using the TLS_RX socket
+option. Any read occurring after the KeyUpdate has been read and
+before the new key is provided will fail with EKEYEXPIRED. poll() will
+not report any read events from the socket until the new key is
+provided. There is no pausing on the transmit side.
+
+Userspace should make sure that the crypto_info provided has been set
+properly. In particular, the kernel will not check for key/nonce
+reuse.
+
+The number of successful and failed key updates is tracked in the
+``TlsTxRekeyOk``, ``TlsRxRekeyOk``, ``TlsTxRekeyError``,
+``TlsRxRekeyError`` statistics. The ``TlsRxRekeyReceived`` statistic
+counts KeyUpdate handshake messages that have been received.
+
 Integrating in to userspace TLS library
 ---------------------------------------
 
@@ -286,3 +312,13 @@ TLS implementation exposes the following per-namespace statistics
 - ``TlsRxNoPadViolation`` -
   number of data RX records which had to be re-decrypted due to
   ``TLS_RX_EXPECT_NO_PAD`` mis-prediction.
+
+- ``TlsTxRekeyOk``, ``TlsRxRekeyOk`` -
+  number of successful rekeys on existing sessions for TX and RX
+
+- ``TlsTxRekeyError``, ``TlsRxRekeyError`` -
+  number of failed rekeys on existing sessions for TX and RX
+
+- ``TlsRxRekeyReceived`` -
+  number of received KeyUpdate handshake messages, requiring userspace
+  to provide a new RX key