[PATCH 3.16 246/328] tty: vt_ioctl: fix potential Spectre v1

Ben Hutchings
2018-12-09T22:21:45Z

From: "Gustavo A. R. Silva" <gustavo@embeddedor.com>

commit e97267cb4d1ee01ca0929638ec0fcbb0904f903d upstream.

vsa.console is indirectly controlled by user-space, hence leading to
a potential exploitation of the Spectre variant 1 vulnerability.

This issue was detected with the help of Smatch:

drivers/tty/vt/vt_ioctl.c:711 vt_ioctl() warn: potential spectre issue
'vc_cons' [r]

Fix this by sanitizing vsa.console before using it to index vc_cons

Notice that given that speculation windows are large, the policy is
to kill the speculation on the first load and not worry if it can be
completed with a dependent load/store [1].

[1] https://marc.info/?l=linux-kernel&m=152449131114778&w=2

Signed-off-by: Gustavo A. R. Silva <gustavo@embeddedor.com>
Reviewed-by: Alan Cox <alan@linux.intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>
---
drivers/tty/vt/vt_ioctl.c | 4 ++++
1 file changed, 4 insertions(+)

--- a/drivers/tty/vt/vt_ioctl.c
+++ b/drivers/tty/vt/vt_ioctl.c
@@ -31,6 +31,8 @@
#include <asm/io.h>
#include <asm/uaccess.h>

+#include <linux/nospec.h>
+
#include <linux/kbd_kern.h>
#include <linux/vt_kern.h>
#include <linux/kbd_diacr.h>
@@ -703,6 +705,8 @@ 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--;
console_lock();
ret = vc_allocate(vsa.console);