linux-kernel - [PATCH 1/2] vt_ioctl: fix array_index_nospec in vt

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Date:   Thu, 27 Jan 2022 15:44:04 +0100
From:   Jakob Koschel <jakobkoschel@...il.com>
To:     linux-kernel@...r.kernel.org
Cc:     Brian Johannesmeyer <bjohannesmeyer@...il.com>,
        Cristiano Giuffrida <c.giuffrida@...nl>,
        "Gustavo A. R. Silva" <gustavo@...eddedor.com>,
        "Bos, H.J." <h.j.bos@...nl>, Thomas Gleixner <tglx@...utronix.de>,
        Jakob Koschel <jakobkoschel@...il.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Jiri Slaby <jirislaby@...nel.org>,
        "Maciej W. Rozycki" <macro@...am.me.uk>,
        "Ahmed S. Darwish" <a.darwish@...utronix.de>,
        Sebastian Andrzej Siewior <bigeasy@...utronix.de>
Subject: [PATCH 1/2] vt_ioctl: fix array_index_nospec in vt_setactivate

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@...il.com>
Signed-off-by: Brian Johannesmeyer <bjohannesmeyer@...il.com>
Signed-off-by: Jakob Koschel <jakobkoschel@...il.com>
---
 drivers/tty/vt/vt_ioctl.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/drivers/tty/vt/vt_ioctl.c b/drivers/tty/vt/vt_ioctl.c
index 3639bb6dc372..e0714a9c9fd7 100644
--- a/drivers/tty/vt/vt_ioctl.c
+++ b/drivers/tty/vt/vt_ioctl.c
@@ -599,8 +599,8 @@ static int vt_setactivate(struct vt_setactivate __user *sa)
 	if (vsa.console == 0 || vsa.console > MAX_NR_CONSOLES)
 		return -ENXIO;

-	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) {
--
2.25.1