serial: 8250_mid: Fix NULL function pointer dereference on DNV/ICX-D/SNR platforms

commit 7fb13fd upstream.

Commit b1b4efe ("serial: 8250_mid: Disable DMA for selected
platforms") replaced the dnv_board setup and exit callbacks with
PTR_IF(false, ...), which evaluates to NULL. However, the three call
sites in mid8250_probe() and mid8250_remove() unconditionally
dereference these function pointers without NULL checks, causing a NULL
pointer dereference (kernel oops) on any Denverton (DNV), Ice Lake Xeon
D (ICX-D/CDF), or Snowridge (SNR) platform.

Fix this by adding the missing NULL checks before calling the setup and
exit callbacks.

Fixes: b1b4efe ("serial: 8250_mid: Disable DMA for selected platforms")
Cc: stable <stable@kernel.org>
Reviewed-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: Jiangshan Yi <yijiangshan@kylinos.cn>
Link: https://patch.msgid.link/20260715073546.1875083-1-yijiangshan@kylinos.cn
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Author: Jiangshan Yi

drivers/tty/serial/8250/8250_mid.c

@@ -318,9 +318,11 @@ static int mid8250_probe(struct pci_dev *pdev, const struct pci_device_id *id)
 	if (!uart.port.membase)
 		return -ENOMEM;
 
-	ret = mid->board->setup(mid, &uart.port);
-	if (ret)
-		return ret;
+	if (mid->board->setup) {
+		ret = mid->board->setup(mid, &uart.port);
+		if (ret)
+			return ret;
+	}
 
 	ret = mid8250_dma_setup(mid, &uart);
 	if (ret)
@@ -336,7 +338,8 @@ static int mid8250_probe(struct pci_dev *pdev, const struct pci_device_id *id)
 	return 0;
 
 err:
-	mid->board->exit(mid);
+	if (mid->board->exit)
+		mid->board->exit(mid);
 	return ret;
 }
 
@@ -346,7 +349,8 @@ static void mid8250_remove(struct pci_dev *pdev)
 
 	serial8250_unregister_port(mid->line);
 
-	mid->board->exit(mid);
+	if (mid->board->exit)
+		mid->board->exit(mid);
 }
 
 static const struct mid8250_board pnw_board = {