fix(auth): properly validate X.509 extensions

fix(auth): properly validate X.509 extensions

get_ext() does not check the return value of the various mbedtls_*
functions, as cert_parse() is assumed to have guaranteed that they will
always succeed.  However, it passes the end of an extension as the end
pointer to these functions, whereas cert_parse() passes the end of the
TBSCertificate.  Furthermore, cert_parse() does *not* check that the
contents of the extension have the same length as the extension itself.
Before fd37982a19a4a291 ("fix(auth): forbid junk after extensions"),
cert_parse() also does not check that the extension block extends to the
end of the TBSCertificate.

This is a problem, as mbedtls_asn1_get_tag() leaves *p and *len
undefined on failure.  In practice, this results in get_ext() continuing
to parse at different offsets than were used (and validated) by
cert_parse(), which means that the in-bounds guarantee provided by
cert_parse() no longer holds.

This patch fixes the remaining flaw by enforcing that the contents of an
extension are the same length as the extension itself.

Change-Id: Id4570f911402e34d5d6c799ae01a01f184c68d7c
Signed-off-by: Demi Marie Obenour <demiobenour@gmail.com>
Signed-off-by: Sandrine Bailleux <sandrine.bailleux@arm.com>