vt_ioctl: fix array_index_nospec in vt_setactivate

array_index_nospec ensures that an out-of-bounds value is set to zero
on the transient path. Decreasing the value by one afterwards causes
a transient integer underflow. vsa.console should be decreased first
and then sanitized with array_index_nospec.

Kasper Acknowledgements: Jakob Koschel, Brian Johannesmeyer, Kaveh
Razavi, Herbert Bos, Cristiano Giuffrida from the VUSec group at VU
Amsterdam.
Co-developed-by: NBrian Johannesmeyer <bjohannesmeyer@gmail.com>
Signed-off-by: NBrian Johannesmeyer <bjohannesmeyer@gmail.com>
Signed-off-by: NJakob Koschel <jakobkoschel@gmail.com>
Link: https://lore.kernel.org/r/20220127144406.3589293-1-jakobkoschel@gmail.com
Cc: stable <stable@vger.kernel.org>
Signed-off-by: NGreg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/tty/vt/vt_ioctl.c

@@ -599,8 +599,8 @@ static int vt_setactivate(struct vt_setactivate __user *sa)
 	if (vsa.console == 0 || vsa.console > MAX_NR_CONSOLES)
 		return -ENXIO;
 
-	vsa.console = array_index_nospec(vsa.console, MAX_NR_CONSOLES + 1);
 	vsa.console--;
+	vsa.console = array_index_nospec(vsa.console, MAX_NR_CONSOLES);
 	console_lock();
 	ret = vc_allocate(vsa.console);
 	if (ret) {