4.18-stable review patch.  If anyone has any objections, please let me know.

------------------

From: H. Peter Anvin <hpa@zytor.com>

commit 991a25194097006ec1e0d2e0814ff920e59e3465 upstream.

On architectures with CBAUDEX == 0 (Alpha and PowerPC), the code in tty_baudrate.c does
not do any limit checking on the tty_baudrate[] array, and in fact a
buffer overrun is possible on both architectures. Add a limit check to
prevent that situation.

This will be followed by a much bigger cleanup/simplification patch.

Signed-off-by: H. Peter Anvin (Intel) <hpa@zytor.com>
Requested-by: Cc: Johan Hovold [off-list ref]
Cc: Jiri Slaby <redacted>
Cc: Al Viro <viro@zeniv.linux.org.uk>
Cc: Richard Henderson <redacted>
Cc: Ivan Kokshaysky <redacted>
Cc: Matt Turner <mattst88@gmail.com>
Cc: Thomas Gleixner <redacted>
Cc: Kate Stewart <redacted>
Cc: Philippe Ombredanne <redacted>
Cc: Eugene Syromiatnikov <redacted>
Cc: Alan Cox <redacted>
Cc: stable <redacted>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

---
 drivers/tty/tty_baudrate.c |    4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

--- a/drivers/tty/tty_baudrate.c
+++ b/drivers/tty/tty_baudrate.c

@@ -77,7 +77,7 @@ speed_t tty_termios_baud_rate(struct kte
 		else
 			cbaud += 15;
 	}
-	return baud_table[cbaud];
+	return cbaud >= n_baud_table ? 0 : baud_table[cbaud];
 }
 EXPORT_SYMBOL(tty_termios_baud_rate);
 

@@ -113,7 +113,7 @@ speed_t tty_termios_input_baud_rate(stru
 		else
 			cbaud += 15;
 	}
-	return baud_table[cbaud];
+	return cbaud >= n_baud_table ? 0 : baud_table[cbaud];
 #else
 	return tty_termios_baud_rate(termios);
 #endif