brcmfmac: firmware: Fix crash in brcm_alt_fw_path

The call to brcm_alt_fw_path in brcmf_fw_get_firmwares is not protected
by a check to the validity of the fwctx->req->board_type pointer. This
results in a crash in strlcat when, for example, the WLAN chip is found
in a USB dongle.

Prevent the crash by adding the necessary check.

See: https://github.com/raspberrypi/linux/issues/4833

Fixes: 5ff01391 ("brcmfmac: firmware: Allow per-board firmware binaries")
Signed-off-by: Phil Elwell <phil@raspberrypi.com>
Signed-off-by: Kalle Valo <kvalo@kernel.org>
Link: https://lore.kernel.org/r/20220118154514.3245524-1-phil@raspberrypi.com

drivers/net/wireless/broadcom/brcm80211/brcmfmac/firmware.c

@@ -693,7 +693,7 @@ int brcmf_fw_get_firmwares(struct device *dev, struct brcmf_fw_request *req,
 {
 	struct brcmf_fw_item *first = &req->items[0];
 	struct brcmf_fw *fwctx;
-	char *alt_path;
+	char *alt_path = NULL;
 	int ret;
 
 	brcmf_dbg(TRACE, "enter: dev=%s\n", dev_name(dev));
@@ -712,7 +712,9 @@ int brcmf_fw_get_firmwares(struct device *dev, struct brcmf_fw_request *req,
 	fwctx->done = fw_cb;
 
 	/* First try alternative board-specific path if any */
-	alt_path = brcm_alt_fw_path(first->path, fwctx->req->board_type);
+	if (fwctx->req->board_type)
+		alt_path = brcm_alt_fw_path(first->path,
+					    fwctx->req->board_type);
 	if (alt_path) {
 		ret = request_firmware_nowait(THIS_MODULE, true, alt_path,
 					      fwctx->dev, GFP_KERNEL, fwctx,