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Subject: Re: [PATCH AUTOSEL 4.19 08/38] media: pci: ttpci: av7110: fix
 possible buffer overflow caused by bad DMA value in debiirq()
To:     Pavel Machek <pavel@ucw.cz>, Sasha Levin <sashal@kernel.org>
Cc:     linux-kernel@vger.kernel.org, stable@vger.kernel.org,
        Sean Young <sean@mess.org>,
        Mauro Carvalho Chehab <mchehab+huawei@kernel.org>,
        linux-media@vger.kernel.org
References: <20200821161807.348600-1-sashal@kernel.org>
 <20200821161807.348600-8-sashal@kernel.org>
 <20200829121020.GA20944@duo.ucw.cz>
From:   Jia-Ju Bai <baijiaju@tsinghua.edu.cn>
Message-ID: <a9c96d50-a4d1-29bf-3bb0-68c4f7cd4c10@tsinghua.edu.cn>
Date:   Sun, 30 Aug 2020 15:24:24 +0800
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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