Re: [PATCH] usbip: vhci_sysfs: fix potential Spectre v1

[Gustavo A. R. Silva]

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@xxxxxxxxxxxxxxx
> > Signed-off-by: Gustavo A. R. Silva <gustavo@xxxxxxxxxxxxxx>
> > ---
> >   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