mbedtls: fix --x509-track post-authentication remote DoS (CVE-2017-7522)

mbedtls: fix --x509-track post-authentication remote DoS (CVE-2017-7522)

asn1_buf_to_c_string() returned a literal string if the input ASN.1 string
contained a NUL character, while the caller expects a mutable string.
The caller will attempt to change this string, which allows a client to
crash a server by sending a certificate with an embedded NUL character.

(The other way around is not interesting, as servers are allowed to stop
a client by design.)

Impact analysis:
 * applies to mbedtls builds only
 * introduced in 2.4 (so 2.3 is not affected)
 * can only be exploited if the --x509-track option is used
 * requires the CA to sign a certificate with an embedded NUL in the
   certificate subject

This bug was discovered and reported to the OpenVPN security team by
Guido Vranken.

CVE: 2017-7522
Signed-off-by: Steffan Karger <steffan.karger@fox-it.com>
Acked-by: Gert Doering <gert@greenie.muc.de>
Message-Id: <1497864520-12219-2-git-send-email-steffan.karger@fox-it.com>
URL: https://www.mail-archive.com/search?l=mid&q=1497864520-12219-2-git-send-email-steffan.karger@fox-it.com
Signed-off-by: Gert Doering <gert@greenie.muc.de>
(cherry picked from commit 426392940c7060300a10077c389f5156c790c2f6)