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
2020-08-30T07:24:40Z

On 2020/8/29 20:10, 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.
>
>

Thanks for this advice, I will submit a v2 patch soon.

Best wishes,
Jia-Ju Bai