Re: [PATCH 01/11] media: tm6000: fix potential Spectre variant 1

[Mauro Carvalho Chehab]

Em Mon, 23 Apr 2018 14:11:02 -0500
"Gustavo A. R. Silva" <gustavo@xxxxxxxxxxxxxx> escreveu:

> On 04/23/2018 01:24 PM, Mauro Carvalho Chehab wrote:
> > Em Mon, 23 Apr 2018 12:38:03 -0500
> > "Gustavo A. R. Silva" <gustavo@xxxxxxxxxxxxxx> escreveu:
> >   
> >> f->index can be controlled by user-space, hence leading to a
> >> potential exploitation of the Spectre variant 1 vulnerability.
> >>
> >> Smatch warning:
> >> drivers/media/usb/tm6000/tm6000-video.c:879 vidioc_enum_fmt_vid_cap() warn: potential spectre issue 'format'
> >>
> >> Fix this by sanitizing f->index before using it to index
> >> array _format_
> >>
> >> 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
> >> Reported-by: Dan Carpenter <dan.carpenter@xxxxxxxxxx>
> >> Signed-off-by: Gustavo A. R. Silva <gustavo@xxxxxxxxxxxxxx>
> >> ---
> >>   drivers/media/usb/tm6000/tm6000-video.c | 2 ++
> >>   1 file changed, 2 insertions(+)
> >>
> >> diff --git a/drivers/media/usb/tm6000/tm6000-video.c b/drivers/media/usb/tm6000/tm6000-video.c
> >> index b2399d4..d701027 100644
> >> --- a/drivers/media/usb/tm6000/tm6000-video.c
> >> +++ b/drivers/media/usb/tm6000/tm6000-video.c
> >> @@ -26,6 +26,7 @@
> >>   #include <linux/kthread.h>
> >>   #include <linux/highmem.h>
> >>   #include <linux/freezer.h>
> >> +#include <linux/nospec.h>
> >>   
> >>   #include "tm6000-regs.h"
> >>   #include "tm6000.h"
> >> @@ -875,6 +876,7 @@ static int vidioc_enum_fmt_vid_cap(struct file *file, void  *priv,
> >>   	if (f->index >= ARRAY_SIZE(format))
> >>   		return -EINVAL;
> >>   
> >> +	f->index = array_index_nospec(f->index, ARRAY_SIZE(format));  
> > 
> > Please enlighten me: how do you think this could be exploited?
> > 
> > When an application calls VIDIOC_ENUM_FMT from a /dev/video0 device,
> > it will just enumerate a hardware functionality, with is constant
> > for a given hardware piece.
> > 
> > The way it works is that userspace do something like:
> > 
> > 	int ret = 0;
> > 
> > 	for (i = 0; ret == 0; i++) {
> > 		ret = ioctl(VIDIOC_ENUM_FMT, ...);
> > 	}
> > 
> > in order to read an entire const table.
> > 
> > Usually, it doesn't require any special privilege to call this ioctl,
> > but, even if someone changes its permission to 0x400, a simple lsusb
> > output is enough to know what hardware model is there. A lsmod
> > or cat /proc/modules) also tells that the tm6000 module was loaded,
> > with is a very good hint that the tm6000 is there or was there in the
> > past.
> > 
> > In the specific case of tm6000, all hardware supports exactly the
> > same formats, as this is usually defined per-driver. So, a quick look
> > at the driver is enough to know exactly what the ioctl would answer.
> > Also, the net is full of other resources that would allow anyone
> > to get the supported formats for a piece of hardware.
> > 
> > Even assuming that the OS doesn't have lsusb, that /proc is not
> > mounted, that /dev/video0 require special permissions, that the
> > potential attacker doesn't have physical access to the equipment (in
> > order to see if an USB board is plugged), etc... What possible harm
> > he could do by identifying a hardware feature?
> > 
> > Similar notes for the other patches to drivers/media in this
> > series: let's not just start adding bloatware where not needed.
> > 
> > Please notice that I'm fine if you want to submit potential
> > Spectre variant 1 fixups, but if you're willing to do so,
> > please provide an explanation about the potential threat scenarios
> > that you're identifying at the code.
> > 
> > Dan,
> > 
> > It probably makes sense to have somewhere at smatch a place where
> > we could explicitly mark the false-positives, in order to avoid
> > use to receive patches that would just add an extra delay where
> > it is not needed.
> >   
> I see I've missed some obvious things that you've pointed out here. I'll 
> mark these warnings as False Positives and take your points into account 
> for the analysis of the rest of the Spectre issues reported by Smatch.

Thanks, I 'll mark this series as rejected at patchwork.linuxtv.org. 
Please feel free to resubmit any patch if they represent a real
threat, adding a corresponding description about the threat scenario
at the body of the e-mail.

> Sorry for the noise and thanks for the feedback.

Anytime.

Thanks,
Mauro