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

From: Jia-Ju Bai
Date: Sun Aug 30 2020 - 03:24:48 EST




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