linux-kernel - Re: [PATCH] usbip: vhci_sysfs: fix potential Spectre v1

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Date:   Thu, 17 May 2018 12:57:49 -0500
From:   "Gustavo A. R. Silva" <gustavo@...eddedor.com>
To:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Cc:     Valentina Manea <valentina.manea.m@...il.com>,
        Shuah Khan <shuah@...nel.org>, linux-usb@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] usbip: vhci_sysfs: fix potential Spectre v1

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@...r.kernel.org
>> Signed-off-by: Gustavo A. R. Silva <gustavo@...eddedor.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