vt: keyboard: avoid signed integer overflow in k_ascii

When k_ascii is invoked several times in a row there is a potential for
signed integer overflow:

UBSAN: Undefined behaviour in drivers/tty/vt/keyboard.c:888:19 signed integer overflow:
10 * 1111111111 cannot be represented in type 'int'
CPU: 0 PID: 0 Comm: swapper/0 Not tainted 5.6.11 #1
Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS Bochs 01/01/2011
Call Trace:
<IRQ>
__dump_stack lib/dump_stack.c:77 [inline]
dump_stack+0xce/0x128 lib/dump_stack.c:118
ubsan_epilogue+0xe/0x30 lib/ubsan.c:154
handle_overflow+0xdc/0xf0 lib/ubsan.c:184
__ubsan_handle_mul_overflow+0x2a/0x40 lib/ubsan.c:205
k_ascii+0xbf/0xd0 drivers/tty/vt/keyboard.c:888
kbd_keycode drivers/tty/vt/keyboard.c:1477 [inline]
kbd_event+0x888/0x3be0 drivers/tty/vt/keyboard.c:1495

While it can be worked around by using check_mul_overflow()/
check_add_overflow(), it is better to introduce a separate flag to
signal that number pad is being used to compose a symbol, and
change type of the accumulator from signed to unsigned, thus
avoiding undefined behavior when it overflows.

Reported-by: Kyungtae Kim <[e-mail]>
Signed-off-by: Dmitry Torokhov <[e-mail]>
Cc: stable <[e-mail]>
Link: https://lore.kernel.org/r/20200525232740.GA262061@dtor-ws
Signed-off-by: Greg Kroah-Hartman <[e-mail]>

drivers/tty/vt/keyboard.c

···
 static unsigned long key_down[BITS_TO_LONGS(KEY_CNT)];	/* keyboard key bitmap */
 static unsigned char shift_down[NR_SHIFT];		/* shift state counters.. */
 static bool dead_key_next;
-static int npadch = -1;					/* -1 or number assembled on pad */
+
+/* Handles a number being assembled on the number pad */
+static bool npadch_active;
+static unsigned int npadch_value;
+
 static unsigned int diacr;
 static char rep;					/* flag telling character repeat */
 
···
 		shift_state &= ~(1 << value);
 
 	/* kludge */
-	if (up_flag && shift_state != old_state && npadch != -1) {
+	if (up_flag && shift_state != old_state && npadch_active) {
 		if (kbd->kbdmode == VC_UNICODE)
-			to_utf8(vc, npadch);
+			to_utf8(vc, npadch_value);
 		else
-			put_queue(vc, npadch & 0xff);
-		npadch = -1;
+			put_queue(vc, npadch_value & 0xff);
+		npadch_active = false;
 	}
 }
 
···
 
 static void k_ascii(struct vc_data *vc, unsigned char value, char up_flag)
 {
-	int base;
+	unsigned int base;
 
 	if (up_flag)
 		return;
···
 		base = 16;
 	}
 
-	if (npadch == -1)
-		npadch = value;
-	else
-		npadch = npadch * base + value;
+	if (!npadch_active) {
+		npadch_value = 0;
+		npadch_active = true;
+	}
+
+	npadch_value = npadch_value * base + value;
 }
 
 static void k_lock(struct vc_data *vc, unsigned char value, char up_flag)