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SubjectRe: [PATCH] usbip: vhci_sysfs: fix potential Spectre v1
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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: 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);
>
> 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?

Thanks
--
Gustavo


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