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

[Ben Hutchings]

3.16.62-rc1 review patch.  If anyone has any objections, please let me know.

------------------

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

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@xxxxxxxxxxxxxx>
Reviewed-by: Alan Cox <alan@xxxxxxxxxxxxxxx>
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
Signed-off-by: Ben Hutchings <ben@xxxxxxxxxxxxxxx>
---
 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);