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 2020/8/30 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.

I agree with Pavel that the compiler optimization may drop the copying 
operation, and thus READ_ONCE() should be used here.
I will submit a v2 patch soon.


Best wishes,
Jia-Ju Bai