Messages in this thread | | | Subject | Re: [PATCH 01/11] media: tm6000: fix potential Spectre variant 1 | From | "Gustavo A. R. Silva" <> | Date | Mon, 23 Apr 2018 14:22:52 -0500 |
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On 04/23/2018 02:17 PM, Mauro Carvalho Chehab wrote: > Em Mon, 23 Apr 2018 14:11:02 -0500 > "Gustavo A. R. Silva" <gustavo@embeddedor.com> 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@embeddedor.com> 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@vger.kernel.org >>>> Reported-by: Dan Carpenter <dan.carpenter@oracle.com> >>>> Signed-off-by: Gustavo A. R. Silva <gustavo@embeddedor.com> >>>> --- >>>> 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. >
Yeah. I got it.
>> Sorry for the noise and thanks for the feedback. > > Anytime. >
Much appreciated. :)
Thanks -- Gustavo
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