vt_ioctl: add array_index_nospec to VT_ACTIVATE

commit 28cb138f

 upstream.

in vt_setactivate an almost identical code path has been patched
with array_index_nospec. In the VT_ACTIVATE path the user input
is from a system call argument instead of a usercopy.
For consistency both code paths should have the same mitigations
applied.

Kasper Acknowledgements: Jakob Koschel, Brian Johannesmeyer, Kaveh
Razavi, Herbert Bos, Cristiano Giuffrida from the VUSec group at VU
Amsterdam.
Co-developed-by: Brian Johannesmeyer <bjohannesmeyer@gmail.com>
Signed-off-by: Brian Johannesmeyer <bjohannesmeyer@gmail.com>
Signed-off-by: Jakob Koschel <jakobkoschel@gmail.com>
Link: https://lore.kernel.org/r/20220127144406.3589293-2-jakobkoschel@gmail.com


Cc: stable <stable@vger.kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/tty/vt/vt_ioctl.c

@@ -691,6 +691,7 @@ int vt_ioctl(struct tty_struct *tty,
 			ret =  -ENXIO;
 		else {
 			arg--;
+			arg = array_index_nospec(arg, MAX_NR_CONSOLES);
 			console_lock();
 			ret = vc_allocate(arg);
 			console_unlock();