KEYS: encrypted: fix buffer overread in valid_master_desc()

With the 'encrypted' key type it was possible for userspace to provide a
data blob ending with a master key description shorter than expected,
e.g. 'keyctl add encrypted desc "new x" @s'.  When validating such a
master key description, validate_master_desc() could read beyond the end
of the buffer.  Fix this by using strncmp() instead of memcmp().  [Also
clean up the code to deduplicate some logic.]

Change-Id: Ib88f30f89c1854a1650ea2b58c2871a515297d82
Cc: Mimi Zohar <zohar@linux.vnet.ibm.com>
Signed-off-by: Eric Biggers <ebiggers@google.com>
Signed-off-by: David Howells <dhowells@redhat.com>
Signed-off-by: James Morris <james.l.morris@oracle.com>
Git-commit: 794b4bc292f5d31739d89c0202c54e7dc9bc3add
Git-repo: https://android.googlesource.com/kernel/common


Signed-off-by: Srinivasa Rao Kuppala <srkupp@codeaurora.org>