tty/vt/keyboard: fix OOB access in do_compute_shiftstate()

The size of individual keymap in drivers/tty/vt/keyboard.c is NR_KEYS,
which is currently 256, whereas number of keys/buttons in input device (and
therefor in key_down) is much larger - KEY_CNT - 768, and that can cause
out-of-bound access when we do

	sym = U(key_maps[0][k]);

with large 'k'.

To fix it we should not attempt iterating beyond smaller of NR_KEYS and
KEY_CNT.

Also while at it let's switch to for_each_set_bit() instead of open-coding
it.
Reported-by: Sasha Levin <sasha.levin@oracle.com>
Reviewed-by: Guenter Roeck <linux@roeck-us.net>
Cc: stable@vger.kernel.org
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>

drivers/tty/vt/keyboard.c

@@ -366,34 +366,22 @@ static void to_utf8(struct vc_data *vc, uint c)
 
 static void do_compute_shiftstate(void)
 {
-	unsigned int i, j, k, sym, val;
+	unsigned int k, sym, val;
 
 	shift_state = 0;
 	memset(shift_down, 0, sizeof(shift_down));
 
-	for (i = 0; i < ARRAY_SIZE(key_down); i++) {
-
-		if (!key_down[i])
+	for_each_set_bit(k, key_down, min(NR_KEYS, KEY_CNT)) {
+		sym = U(key_maps[0][k]);
+		if (KTYP(sym) != KT_SHIFT && KTYP(sym) != KT_SLOCK)
 			continue;
 
-		k = i * BITS_PER_LONG;
-
-		for (j = 0; j < BITS_PER_LONG; j++, k++) {
-
-			if (!test_bit(k, key_down))
-				continue;
+		val = KVAL(sym);
+		if (val == KVAL(K_CAPSSHIFT))
+			val = KVAL(K_SHIFT);
 
-			sym = U(key_maps[0][k]);
-			if (KTYP(sym) != KT_SHIFT && KTYP(sym) != KT_SLOCK)
-				continue;
-
-			val = KVAL(sym);
-			if (val == KVAL(K_CAPSSHIFT))
-				val = KVAL(K_SHIFT);
-
-			shift_down[val]++;
-			shift_state |= (1 << val);
-		}
+		shift_down[val]++;
+		shift_state |= BIT(val);
 	}
 }