Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 23.128.96.18 as permitted sender) client-ip=23.128.96.18;
Subject: Re: [PATCH] media: usb: ttusb-dec: avoid buffer overflow in
 ttusb_dec_handle_irq() when DMA failures/attacks occur
To:     Sean Young <sean@mess.org>
Cc:     mchehab@kernel.org, kstewart@linuxfoundation.org,
        tomasbortoli@gmail.com, gregkh@linuxfoundation.org,
        allison@lohutok.net, tglx@linutronix.de,
        linux-media@vger.kernel.org, linux-kernel@vger.kernel.org
References: <20200505142110.7620-1-baijiaju1990@gmail.com>
 <20200507084332.GA14459@gofer.mess.org>
From:   Jia-Ju Bai <baijiaju1990@gmail.com>
Message-ID: <4f95bc0d-0b75-dd75-642c-b2238feb9890@gmail.com>
Date:   Thu, 7 May 2020 18:11:06 +0800
User-Agent: Mozilla/5.0 (Windows NT 10.0; WOW64; rv:68.0) Gecko/20100101
 Thunderbird/68.4.1
MIME-Version: 1.0
In-Reply-To: <20200507084332.GA14459@gofer.mess.org>
Content-Type: text/plain; charset=utf-8; format=flowed
Content-Transfer-Encoding: 7bit
Content-Language: en-US
Sender: linux-kernel-owner@vger.kernel.org
Precedence: bulk

Thanks for the reply, Sean :)

On 2020/5/7 16:43, Sean Young wrote:
> On Tue, May 05, 2020 at 10:21:10PM +0800, Jia-Ju Bai wrote:
>> Signed-off-by: Jia-Ju Bai <baijiaju1990@gmail.com>
>> ---
>>   drivers/media/usb/ttusb-dec/ttusb_dec.c | 9 +++++----
>>   1 file changed, 5 insertions(+), 4 deletions(-)
>>
>> diff --git a/drivers/media/usb/ttusb-dec/ttusb_dec.c b/drivers/media/usb/ttusb-dec/ttusb_dec.c
>> index 3198f9624b7c..8543c552515b 100644
>> --- a/drivers/media/usb/ttusb-dec/ttusb_dec.c
>> +++ b/drivers/media/usb/ttusb-dec/ttusb_dec.c
>> @@ -250,6 +250,7 @@ static void ttusb_dec_handle_irq( struct urb *urb)
>>   	struct ttusb_dec *dec = urb->context;
>>   	char *buffer = dec->irq_buffer;
>>   	int retval;
>> +	u8 index = buffer[4];
>>   
>>   	switch(urb->status) {
>>   		case 0: /*success*/
>> @@ -281,11 +282,11 @@ static void ttusb_dec_handle_irq( struct urb *urb)
>>   		 * this should/could be added later ...
>>   		 * for now lets report each signal as a key down and up
>>   		 */
>> -		if (buffer[4] - 1 < ARRAY_SIZE(rc_keys)) {
> Here buffer[4] is signed char, so if buffer[4] == 0 then (buffer[4] - 1) = -1,
> this becomes "if (-1 < ARRAY_SIZE(rc_keys))", which evaluates to false,
> due to it becoming an unsigned compare. _I think_.

I think you are right.
Maybe I should use "int index = buffer[4]" here.

>> -			dprintk("%s:rc signal:%d\n", __func__, buffer[4]);
>> -			input_report_key(dec->rc_input_dev, rc_keys[buffer[4] - 1], 1);
>> +		if (index - 1 < ARRAY_SIZE(rc_keys)) {
>> +			dprintk("%s:rc signal:%d\n", __func__, index);
>> +			input_report_key(dec->rc_input_dev, rc_keys[index - 1], 1);
>>   			input_sync(dec->rc_input_dev);
>> -			input_report_key(dec->rc_input_dev, rc_keys[buffer[4] - 1], 0);
>> +			input_report_key(dec->rc_input_dev, rc_keys[index - 1], 0);
> Like Greg said, this patch reduces the number of dereferences and makes
> the code much cleaner, but the commit message is misleading.

Okay, I will change my log and send a new patch.


Best wishes,
Jia-Ju Bai