iconv uses strlen directly on user supplied memory

iconv_sysctl_add from sys/libkern/iconv.c incorrectly limits the
size of user strings, such that several out of bounds reads could have
been possible.

static int
iconv_sysctl_add(SYSCTL_HANDLER_ARGS)
{
struct iconv_converter_class *dcp;
struct iconv_cspair *csp;
struct iconv_add_in din;
struct iconv_add_out dout;
int error;

error = SYSCTL_IN(req, &din, sizeof(din));
if (error)

		return error;

if (din.ia_version != ICONV_ADD_VER)

		return EINVAL;

if (din.ia_datalen > ICONV_CSMAXDATALEN)

		return EINVAL;

if (strlen(din.ia_from) >= ICONV_CSNMAXLEN)

		return EINVAL;

if (strlen(din.ia_to) >= ICONV_CSNMAXLEN)

		return EINVAL;

if (strlen(din.ia_converter) >= ICONV_CNVNMAXLEN)

		return EINVAL;

...

Since the din struct is directly copied from userland, there is no
guarantee that the strings supplied will be NULL terminated. The
strlen calls could continue reading past the designated buffer
sizes.

Declaration of struct iconv_add_in is found in sys/sys/iconv.h:

struct iconv_add_in {
int ia_version;
char ia_converter[ICONV_CNVNMAXLEN];
char ia_to[ICONV_CSNMAXLEN];
char ia_from[ICONV_CSNMAXLEN];
int ia_datalen;
const void *ia_data;
};

Our strings are followed by the ia_datalen member, which is checked
before the strlen calls:

if (din.ia_datalen > ICONV_CSMAXDATALEN)

Since ICONV_CSMAXDATALEN has value 0x41000 (and is unsigned),
this ensures that din.ia_datalen contains at least 1 byte of 0, so
it is not possible to trigger a read out of bounds of the struct
however, this code is fragile and could introduce subtle bugs in the
future if the struct is ever modified.

PR: 207302
Submitted by: CTurt <cturt@hardenedbsd.org>
Reported by: CTurt <cturt@hardenedbsd.org>
Reviewed by: jhb, vangyzen
MFC after: 1 week
Sponsored by: Dell EMC
Differential Revision: https://reviews.freebsd.org/D14521