vt_ioctl: add array_index_nospec to VT_ACTIVATE

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 <[email protected]>
Signed-off-by: Brian Johannesmeyer <[email protected]>
Signed-off-by: Jakob Koschel <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Cc: stable <[email protected]>
Signed-off-by: Greg Kroah-Hartman <[email protected]>

Author: Jakob-Koschel

drivers/tty/vt/vt_ioctl.c

@@ -845,6 +845,7 @@ int vt_ioctl(struct tty_struct *tty,
 			return -ENXIO;
 
 		arg--;
+		arg = array_index_nospec(arg, MAX_NR_CONSOLES);
 		console_lock();
 		ret = vc_allocate(arg);
 		console_unlock();