HID: hiddev: fix potential Spectre v1

["Gustavo A. R. Silva" <gustavo@embeddedor.com>]

uref->field_index, uref->usage_index, finfo.field_index and
cinfo.index can be 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/hid/usbhid/hiddev.c:473 hiddev_ioctl_usage() warn: potential
spectre issue 'report->field' (local cap)
drivers/hid/usbhid/hiddev.c:477 hiddev_ioctl_usage() warn: potential
spectre issue 'field->usage' (local cap)
drivers/hid/usbhid/hiddev.c:757 hiddev_ioctl() warn: potential spectre
issue 'report->field' (local cap)
drivers/hid/usbhid/hiddev.c:801 hiddev_ioctl() warn: potential spectre
issue 'hid->collection' (local cap)

Fix this by sanitizing such structure fields before using them to index
report->field, field->usage and hid->collection

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

Cc: stable@vger.kernel.org
Signed-off-by: Gustavo A. R. Silva <gustavo@embeddedor.com>
---
 drivers/hid/usbhid/hiddev.c | 11 +++++++++++
 1 file changed, 11 insertions(+)

diff --git a/drivers/hid/usbhid/hiddev.c b/drivers/hid/usbhid/hiddev.c
index e3ce233..23872d0 100644
--- a/drivers/hid/usbhid/hiddev.c
+++ b/drivers/hid/usbhid/hiddev.c
@@ -36,6 +36,7 @@
 #include <linux/hiddev.h>
 #include <linux/compat.h>
 #include <linux/vmalloc.h>
+#include <linux/nospec.h>
 #include "usbhid.h"
 
 #ifdef CONFIG_USB_DYNAMIC_MINORS
@@ -469,10 +470,14 @@ static noinline int hiddev_ioctl_usage(struct hiddev *hiddev, unsigned int cmd,
 
 		if (uref->field_index >= report->maxfield)
 			goto inval;
+		uref->field_index = array_index_nospec(uref->field_index,
+						       report->maxfield);
 
 		field = report->field[uref->field_index];
 		if (uref->usage_index >= field->maxusage)
 			goto inval;
+		uref->usage_index = array_index_nospec(uref->usage_index,
+						       field->maxusage);
 
 		uref->usage_code = field->usage[uref->usage_index].hid;
 
@@ -499,6 +504,8 @@ static noinline int hiddev_ioctl_usage(struct hiddev *hiddev, unsigned int cmd,
 
 			if (uref->field_index >= report->maxfield)
 				goto inval;
+			uref->field_index = array_index_nospec(uref->field_index,
+							       report->maxfield);
 
 			field = report->field[uref->field_index];
 
@@ -753,6 +760,8 @@ static long hiddev_ioctl(struct file *file, unsigned int cmd, unsigned long arg)
 
 		if (finfo.field_index >= report->maxfield)
 			break;
+		finfo.field_index = array_index_nospec(finfo.field_index,
+						       report->maxfield);
 
 		field = report->field[finfo.field_index];
 		memset(&finfo, 0, sizeof(finfo));
@@ -797,6 +806,8 @@ static long hiddev_ioctl(struct file *file, unsigned int cmd, unsigned long arg)
 
 		if (cinfo.index >= hid->maxcollection)
 			break;
+		cinfo.index = array_index_nospec(cinfo.index,
+						 hid->maxcollection);
 
 		cinfo.type = hid->collection[cinfo.index].type;
 		cinfo.usage = hid->collection[cinfo.index].usage;