firmware: cs_dsp: Fix overrun of unterminated control name string

For wmfw format v2 and later the coefficient name strings have a length
field and are NOT null-terminated. Use kasprintf() to convert the
unterminated string into a null-terminated string in an allocated buffer.

The previous code handled this duplication incorrectly using kmemdup()
and getting the length from a strlen() of the (unterminated) source string.
This resulted in creating a string that continued up to the next byte in
the firmware file that just happened to be 0x00.

Signed-off-by: Richard Fitzgerald <rf@opensource.cirrus.com>
Fixes: f6bc909 ("firmware: cs_dsp: add driver to support firmware loading on Cirrus Logic DSPs")
Link: https://lore.kernel.org/r/20220412163927.1303470-1-rf@opensource.cirrus.com
Signed-off-by: Mark Brown <broonie@kernel.org>

Author: rfvirgil

drivers/firmware/cirrus/cs_dsp.c

@@ -955,8 +955,7 @@ static int cs_dsp_create_control(struct cs_dsp *dsp,
 	ctl->alg_region = *alg_region;
 	if (subname && dsp->fw_ver >= 2) {
 		ctl->subname_len = subname_len;
-		ctl->subname = kmemdup(subname,
-				       strlen(subname) + 1, GFP_KERNEL);
+		ctl->subname = kasprintf(GFP_KERNEL, "%.*s", subname_len, subname);
 		if (!ctl->subname) {
 			ret = -ENOMEM;
 			goto err_ctl;