tty: keyboard, do not speculate on func_table index

It is very unlikely for processor to speculate on the func_table index.
The index is uchar and func_table is of size 256. So the compiler would
need to screw up and generate a really bad code.

But to stay on the safe side, forbid speculation on this user passed
index.

Signed-off-by: Jiri Slaby <jslaby@suse.cz>
Cc: Jiri Kosina <jikos@kernel.org>
Link: https://lore.kernel.org/r/20200730105546.24268-1-jslaby@suse.cz
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/tty/vt/keyboard.c

@@ -32,6 +32,7 @@
 #include <linux/tty.h>
 #include <linux/tty_flip.h>
 #include <linux/mm.h>
+#include <linux/nospec.h>
 #include <linux/string.h>
 #include <linux/init.h>
 #include <linux/slab.h>
@@ -2019,7 +2020,7 @@ int vt_do_kdgkb_ioctl(int cmd, struct kbsentry __user *user_kdgkb, int perm)
 		goto reterr;
 	}
 	kbs->kb_string[sizeof(kbs->kb_string)-1] = '\0';
-	i = kbs->kb_func;
+	i = array_index_nospec(kbs->kb_func, MAX_NR_FUNC);
 
 	switch (cmd) {
 	case KDGKBSENT: