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
