Re: [PATCH AUTOSEL 4.19 08/38] media: pci: ttpci: av7110: fix possible buffer overflow caused by bad DMA value in debiirq()

Laurent Pinchart
2020-08-31T13:55:55Z

Hi Jia-Ju,

On Mon, Aug 31, 2020 at 09:45:14PM +0800, Jia-Ju Bai wrote:
> On 2020/8/31 6:25, Laurent Pinchart wrote:
> > On Sun, Aug 30, 2020 at 03:33:11PM +0800, Jia-Ju Bai wrote:
> >> On 2020/8/30 1:16, Laurent Pinchart wrote:
> >>> On Sat, Aug 29, 2020 at 02:10:20PM +0200, Pavel Machek wrote:
> >>>> Hi!
> >>>>
> >>>>> The value av7110->debi_virt is stored in DMA memory, and it is assigned
> >>>>> to data, and thus data[0] can be modified at any time by malicious
> >>>>> hardware. In this case, "if (data[0] < 2)" can be passed, but then
> >>>>> data[0] can be changed into a large number, which may cause buffer
> >>>>> overflow when the code "av7110->ci_slot[data[0]]" is used.
> >>>>>
> >>>>> To fix this possible bug, data[0] is assigned to a local variable, which
> >>>>> replaces the use of data[0].
> >>>>
> >>>> I'm pretty sure hardware capable of manipulating memory can work
> >>>> around any such checks, but...
> >>>>
> >>>>> +++ b/drivers/media/pci/ttpci/av7110.c
> >>>>> @@ -424,14 +424,15 @@ static void debiirq(unsigned long cookie)
> >>>>> case DATA_CI_GET:
> >>>>> {
> >>>>> u8 *data = av7110->debi_virt;
> >>>>> + u8 data_0 = data[0];
> >>>>>
> >>>>> - if ((data[0] < 2) && data[2] == 0xff) {
> >>>>> + if (data_0 < 2 && data[2] == 0xff) {
> >>>>> int flags = 0;
> >>>>> if (data[5] > 0)
> >>>>> flags |= CA_CI_MODULE_PRESENT;
> >>>>> if (data[5] > 5)
> >>>>> flags |= CA_CI_MODULE_READY;
> >>>>> - av7110->ci_slot[data[0]].flags = flags;
> >>>>> + av7110->ci_slot[data_0].flags = flags;
> >>>>
> >>>> This does not even do what it says. Compiler is still free to access
> >>>> data[0] multiple times. It needs READ_ONCE() to be effective.
> >>>
> >>> Yes, it seems quite dubious to me. If we *really* want to guard against
> >>> rogue hardware here, the whole DMA buffer should be copied. I don't
> >>> think it's worth it, a rogue PCI device can do much more harm.
> >>
> >> From the original driver code, data[0] is considered to be bad and thus
> >> it should be checked, because the content of the DMA buffer may be
> >> problematic.
> >>
> >> Based on this consideration, data[0] can be also modified to bypass the
> >> check, and thus its value should be copied to a local variable for the
> >> check and use.
> >
> > What makes you think the hardware would do that ?
>
> Several recent papers show that the bad values from malicious or
> problematic hardware can cause security problems:
> [NDSS'19] PeriScope: An Effective Probing and Fuzzing Framework for the
> Hardware-OS Boundary
> [NDSS'19] Thunderclap: Exploring Vulnerabilities in Operating System
> IOMMU Protection via DMA from Untrustworthy Peripherals
> [USENIX Security'20] USBFuzz: A Framework for Fuzzing USB Drivers by
> Device Emulation
>
> In this case, the values from DMA can be bad, and the driver should
> carefully check these values to avoid security problems.
> IOMMU is an effective method to prevent the hardware from accessing
> arbitrary memory address via DMA, but it does not check whether the
> values from DMA are safe.
>
> I find that some drivers (including the av7110 driver) check (or try to
> check) the values from DMA, and thus I think these drivers have
> considered such security problems.
> However, some of these checks are not rigorous, so that they can be
> bypassed in some cases. The problem that I reported is such an example.

The AV7110 is an old chip, I'm not even sure if it can be used with a
modern system that supports IOMMUs for PCI devices. Without that, it's
game over anyway. Before trying to address the issue of a malicious
AV7110 playing with DMA and CPU races, I would ensure that it's worth
it.

--
Regards,

Laurent Pinchart