linux-kernel - Re: [PATCH] usb: serial: Perform verification for FTDI FT232R devices

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Message-ID: <5449355C.2060301@marcansoft.com>
Date:	Fri, 24 Oct 2014 02:05:32 +0900
From:	Hector Martin <hector@...cansoft.com>
To:	Russ Dill <russ.dill@...il.com>
CC:	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] usb: serial: Perform verification for FTDI FT232R devices

On 23/10/14 23:14, Russ Dill wrote:
> On Thu, Oct 23, 2014 at 5:44 AM, Hector Martin <hector@...cansoft.com> wrote:
>> +       write_eeprom(port, 0, eeprom_data[0]);
>> +       write_eeprom(port, 1, 0);
>> +       write_eeprom(port, eeprom_size - 2, eeprom_data[eeprom_size - 2]);
>> +       write_eeprom(port, eeprom_size - 1, checksum);
> 
> Damned off by one errors. Yes, it should be the product ID, not the
> vendor ID. These write u16's though, writing to wIndex 2 writes to
> bytes 4 and 5. the correct code is:
> 
> write_eeprom(port, 2, 0);
> write_eeprom(port, eeprom_size - 2, checksum);
> 
> And yes, the checksum code needs to be modified to create a specially
> crafted value that allows the existing checksum to pass.

Oh, I thought/assumed that the switch to VID and the real checksum was a
deliberate attempt at perversing the original code into something that
bricks even real FTDIs :-) (In which case all you'd need to add is the
two extra dummy writes, to load the proper data into the buffer register
on a real FT232RL to prevent it from corrupting the adjacent EEPROM words).

-- 
Hector Martin (hector@...cansoft.com)
Public Key: http://www.marcansoft.com/marcan.asc

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