Mbed TLS -- Local side channel attack on classical CBC decryption in (D)TLS

Manuel Pégourié-Gonnard reports:

When decrypting/authenticating (D)TLS record in a connection using
a CBC ciphersuite without the Encrypt-then-Mac extension RFC 7366,
Mbed TLS used dummy rounds of the compression function associated
with the hash used for HMAC in order to hide the length of the
padding to remote attackers, as recommended in the original Lucky
Thirteen paper.
A local attacker who is able to observe the state of the cache
could monitor the presence of mbedtls_md_process() in the cache in
order to determine when the actual computation ends and when the
dummy rounds start. This is a reliable target as it’s always called
at least once, in response to a previous attack. The attacker can
then continue with one of many well-documented Lucky 13
variants.