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 ?
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.