From: Jakob Koschel <jakobkoschel@xxxxxxxxx> 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@xxxxxxxxx> Signed-off-by: Brian Johannesmeyer <bjohannesmeyer@xxxxxxxxx> Signed-off-by: Jakob Koschel <jakobkoschel@xxxxxxxxx> Link: https://lore.kernel.org/r/20220127144406.3589293-1-jakobkoschel@xxxxxxxxx Cc: stable <stable@xxxxxxxxxxxxxxx> Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx> --- 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 @@ -715,9 +715,9 @@ 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) {
