[PATCH 4.9 22/34] vt_ioctl: fix array_index_nospec in vt_setactivate

[Greg Kroah-Hartman <gregkh@linuxfoundation.org>]

From: Jakob Koschel <jakobkoschel@gmail.com>

commit 61cc70d9e8ef5b042d4ed87994d20100ec8896d9 upstream.

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: 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/20220127144406.3589293-1-jakobkoschel@gmail.com
Cc: stable <stable@vger.kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
---
 drivers/tty/vt/vt_ioctl.c |    4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

--- a/drivers/tty/vt/vt_ioctl.c
+++ b/drivers/tty/vt/vt_ioctl.c
@@ -718,9 +718,9 @@ int vt_ioctl(struct tty_struct *tty,
 		if (vsa.console == 0 || vsa.console > MAX_NR_CONSOLES)
 			ret = -ENXIO;
 		else {
-			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 == 0) {