net/tls: add kernel-driven TLS RX resync

TLS offload device may lose sync with the TCP stream if packets
arrive out of order.  Drivers can currently request a resync at
a specific TCP sequence number.  When a record is found starting
at that sequence number kernel will inform the device of the
corresponding record number.

This requires the device to constantly scan the stream for a
known pattern (constant bytes of the header) after sync is lost.

This patch adds an alternative approach which is entirely under
the control of the kernel.  Kernel tracks records it had to fully
decrypt, even though TLS socket is in TLS_HW mode.  If multiple
records did not have any decrypted parts - it's a pretty strong
indication that the device is out of sync.

We choose the min number of fully encrypted records to be 2,
which should hopefully be more than will get retransmitted at
a time.

After kernel decides the device is out of sync it schedules a
resync request.  If the TCP socket is empty the resync gets
performed immediately.  If socket is not empty we leave the
record parser to resync when next record comes.

Before resync in message parser we peek at the TCP socket and
don't attempt the sync if the socket already has some of the
next record queued.

On resync failure (encrypted data continues to flow in) we
retry with exponential backoff, up to once every 128 records
(with a 16k record thats at most once every 2M of data).

Signed-off-by: Jakub Kicinski <jakub.kicinski@netronome.com>
Reviewed-by: Dirk van der Merwe <dirk.vandermerwe@netronome.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

Documentation/networking/tls-offload.rst

@@ -268,6 +268,9 @@ Device can only detect that segment 4 also contains a TLS header
 if it knows the length of the previous record from segment 2. In this case
 the device will lose synchronization with the stream.
 
+Stream scan resynchronization
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
 When the device gets out of sync and the stream reaches TCP sequence
 numbers more than a max size record past the expected TCP sequence number,
 the device starts scanning for a known header pattern. For example
@@ -298,6 +301,22 @@ Special care has to be taken if the confirmation request is passed
 asynchronously to the packet stream and record may get processed
 by the kernel before the confirmation request.
 
+Stack-driven resynchronization
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+The driver may also request the stack to perform resynchronization
+whenever it sees the records are no longer getting decrypted.
+If the connection is configured in this mode the stack automatically
+schedules resynchronization after it has received two completely encrypted
+records.
+
+The stack waits for the socket to drain and informs the device about
+the next expected record number and its TCP sequence number. If the
+records continue to be received fully encrypted stack retries the
+synchronization with an exponential back off (first after 2 encrypted
+records, then after 4 records, after 8, after 16... up until every
+128 records).
+
 Error handling
 ==============