[PATCH] usbip: vhci_sysfs: fix potential Spectre v1

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

pdev_nr and rhport can be 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/usb/usbip/vhci_sysfs.c:238 detach_store() warn: potential
spectre issue 'vhcis'
drivers/usb/usbip/vhci_sysfs.c:328 attach_store() warn: potential
spectre issue 'vhcis'
drivers/usb/usbip/vhci_sysfs.c:338 attach_store() warn: potential
spectre issue 'vhci->vhci_hcd_ss->vdev'
drivers/usb/usbip/vhci_sysfs.c:340 attach_store() warn: potential
spectre issue 'vhci->vhci_hcd_hs->vdev'

Fix this by sanitizing pdev_nr and rhport before using them to index
vhcis and vhci->vhci_hcd_ss->vdev respectively.

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/usb/usbip/vhci_sysfs.c | 6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/drivers/usb/usbip/vhci_sysfs.c b/drivers/usb/usbip/vhci_sysfs.c
index 4880838..9045888 100644
--- a/drivers/usb/usbip/vhci_sysfs.c
+++ b/drivers/usb/usbip/vhci_sysfs.c
@@ -10,6 +10,8 @@
 #include <linux/platform_device.h>
 #include <linux/slab.h>
 
+#include <linux/nospec.h>
+
 #include "usbip_common.h"
 #include "vhci.h"
 
@@ -235,6 +237,8 @@ static ssize_t detach_store(struct device *dev, struct device_attribute *attr,
 	if (!valid_port(pdev_nr, rhport))
 		return -EINVAL;
 
+	pdev_nr = array_index_nospec(pdev_nr, vhci_num_controllers);
+	rhport = array_index_nospec(rhport, VHCI_HC_PORTS);
 	hcd = platform_get_drvdata(vhcis[pdev_nr].pdev);
 	if (hcd == NULL) {
 		dev_err(dev, "port is not ready %u\n", port);
@@ -325,6 +329,8 @@ static ssize_t attach_store(struct device *dev, struct device_attribute *attr,
 	if (!valid_args(pdev_nr, rhport, speed))
 		return -EINVAL;
 
+	pdev_nr = array_index_nospec(pdev_nr, vhci_num_controllers);
+	rhport = array_index_nospec(rhport, VHCI_HC_PORTS);
 	hcd = platform_get_drvdata(vhcis[pdev_nr].pdev);
 	if (hcd == NULL) {
 		dev_err(dev, "port %d is not ready\n", port);
-- 
2.7.4