mbedtls: arbitrary code execution

When the client creates its ClientHello message, due to insufficient
bounds checking it can overflow the heap-based buffer containing the
message while writing some extensions. Two extensions in particular
could be used by a remote attacker to trigger the overflow: the session
ticket extension and the server name indication (SNI) extension.

Starting with PolarSSL 1.3.0 which added support for session tickets,
any server the client connects to can send an overlong session ticket
which will cause a buffer overflow if and when the client attempts to
resume the connection with the server. Clients that disabled session
tickets or never attempt to reconnect to a server using a saved session
are not vulnerable to this attack vector.

Starting with PolarSSL 1.0.0, this overflow could also be triggered by
an attacker convincing a client to use an overlong hostname for the SNI
extension. The hostname needs to be almost as long at
SSL_MAX_CONTENT_LEN, which as 16KB by default, but could be smaller if a
custom configuration is used. Clients that do not accept hostnames from
untrusted parties are not vulnerable to this attack vector.