x86/boot: Validate console=uart8250 baud rate to fix early boot hang

When the baud rate is empty, 0, invalid, or overflows to 0 when stored
as an int, the system will hang during early boot because of a division
by zero in early_serial_init().

Fall back to DEFAULT_BAUD when the resulting baud rate is 0 to prevent
an early system hang.

Fixes: ce0aa5d ("x86, setup: Make the setup code also accept console=uart8250")
Signed-off-by: Thorsten Blum <thorsten.blum@linux.dev>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Cc: "H. Peter Anvin" <hpa@zytor.com>
Cc: stable@vger.kernel.org
Link: https://patch.msgid.link/20260713194924.126472-3-thorsten.blum@linux.dev

Author: toblux

arch/x86/boot/early_serial_console.c

@@ -117,7 +117,7 @@ static unsigned int probe_baud(int port)
 static void parse_console_uart8250(void)
 {
 	char optstr[64], *options;
-	int baud = DEFAULT_BAUD;
+	int baud;
 	int port = 0;
 
 	/*
@@ -136,10 +136,13 @@ static void parse_console_uart8250(void)
 	else
 		return;
 
-	if (options && (options[0] == ','))
-		baud = simple_strtoull(options + 1, &options, 0);
-	else
+	if (options && (options[0] == ',')) {
+		baud = simple_strtoull(options + 1, NULL, 0);
+		if (!baud)
+			baud = DEFAULT_BAUD;
+	} else {
 		baud = probe_baud(port);
+	}
 
 	if (port)
 		early_serial_init(port, baud);