gpg: Prevent an invalid memory read using a garbled keyring.
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Description

gpg: Prevent an invalid memory read using a garbled keyring.

* g10/keyring.c (keyring_get_keyblock): Whitelist allowed packet
types.
* g10/keydb.c (parse_keyblock_image): Ditto.

The keyring DB code did not reject packets which don't belong into a
keyring. If for example the keyblock contains a literal data packet
it is expected that the processing code stops at the data packet and
reads from the input stream which is referenced from the data packets.
Obviously the keyring processing code does not and cannot do that.
However, when exporting this messes up the IOBUF and leads to an
invalid read of sizeof (int).

We now skip all packets which are not allowed in a keyring.

Test data:

gpg2 --no-default-keyring --keyring FILE --export >/dev/null

With this unpacked data for FILE:

-----BEGIN PGP ARMORED FILE-----

mI0EVNP2zQEEALvETPVDCJDBXkegF4esiV1fqlne40yJnCmJeDEJYocwFPXfFA86
sSGjInzgDbpbC9gQPwq91Qe9x3Vy81CkyVonPOejhINlzfpzqAAa3A6viJccZTwt
DJ8E/I9jg53sbYW8q+VgfLn1hlggH/XQRT0HkXMP5y9ClURYnTsNwJhXABEBAAGs
CXRlc3QgdGVzdIi5BBMBCgAjBQJU0/bNAhsDBwsJCAcDAgEGFQgCCQoLBBYCAwEC
HgECF4AACgkQlsmuCapsqYLvtQP/byY0tM0Lc3moftbHQZ2eHj9ykLjsCjeMDfPx
kZUUtUS3HQaqgZLZOeqPjM7XgGh5hJsd9pfhmRWJ0x+iGB47XQNpRTtdLBV/WMCS
l5z3uW7e9Md7QVUVuSlJnBgQHTS6EgP8JQadPkAiF+jgpJZXP+gFs2j3gobS0qUF
eyTtxs+wAgAD