Hi Greg,

On 05/17/2018 01:51 AM, Greg Kroah-Hartman wrote:
On Wed, May 16, 2018 at 05:22:00PM -0500, Gustavo A. R. Silva wrote:
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'

Nit, no need to line-wrap long error messages from tools :)

Got it.

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: sta...@vger.kernel.org
Signed-off-by: Gustavo A. R. Silva <gust...@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);

Shouldn't we just do this in one place, in the valid_port() function?

That way it keeps the range checking logic in one place (now it is in 3
places in the function), which should make maintenance much simpler.

Yep, I thought about that, the thing is: what happens if the hardware is "trained" to predict that valid_port always evaluates to false, and then malicious values are stored in pdev_nr and nhport?

It seems to me that under this scenario we need to serialize instructions in this place.

What do you think?


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