vt_ioctl: fix array_index_nospec in vt_setactivate

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>

diff --git a/drivers/tty/vt/vt_ioctl.c b/drivers/tty/vt/vt_ioctl.c
index e1c1627a3356b3265df8fbd02c9c9aea11a7a903..954e5c40f473ee0f419ae4bc07ddb78851b490df 100644 (file)
--- 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) {