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

[Jia-Ju Bai]

On 2020/8/31 6:25, Laurent Pinchart wrote:
> Hi Jia-Ju,
>
> 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.


Best wishes,
Jia-Ju Bai