linux-kernel - Re: [PATCH] cdc-acm: prevent infinite loop when parsing CDC headers.

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Date:	Tue, 14 Apr 2015 14:01:57 +0300
From:	Sergei Shtylyov <sergei.shtylyov@...entembedded.com>
To:	Quentin Casasnovas <quentin.casasnovas@...cle.com>,
	lkml <linux-kernel@...r.kernel.org>
CC:	linux-usb <linux-usb@...r.kernel.org>,
	Phil Turnbull <phil.turnbull@...cle.com>,
	Oliver Neukum <oneukum@...e.de>, Adam Lee <adam8157@...il.com>
Subject: Re: [PATCH] cdc-acm: prevent infinite loop when parsing CDC headers.

Hello.

On 4/13/2015 7:20 PM, Sergei Shtylyov wrote:

>> Phil and I found out a problem with commit:

>>    7e860a6e ("cdc-acm: add sanity checks")

>> It added some sanity checks to ignore potential garbage in CDC headers but
>> also introduced a potential infinite loop.  This can happen at the first
>> loop iteration (elength = 0 in that case) if the description isn't a
>> DT_CS_INTERFACE or later if 'buffer[0]' is zero.
>
>> It should also be noted that the wrong length was being added to 'buffer'
>> in case 'buffer[1]' was not a DT_CS_INTERFACE descriptor, since elength was
>> assigned after that check in the loop.

>> A specially crafted USB device could be used to trigger this infinite loop.

>> Fixes: 7e860a6e ("cdc-acm: add sanity checks")

>     12-digit SHA1 hash is required here.

>> Signed-off-by: Phil Turnbull <phil.turnbull@...cle.com>
>> Signed-off-by: Quentin Casasnovas <quentin.casasnovas@...cle.com>
>> CC: Oliver Neukum <oneukum@...e.de>
>> CC: Adam Lee <adam8157@...il.com>
>> ---
>>   drivers/usb/class/cdc-acm.c | 3 ++-
>>   1 file changed, 2 insertions(+), 1 deletion(-)

>> diff --git a/drivers/usb/class/cdc-acm.c b/drivers/usb/class/cdc-acm.c
>> index 6836177..1ac4587 100644
>> --- a/drivers/usb/class/cdc-acm.c
>> +++ b/drivers/usb/class/cdc-acm.c
>> @@ -1133,11 +1133,12 @@ static int acm_probe(struct usb_interface *intf,
>>       }
>>
>>       while (buflen > 0) {
>> +        if ((elength = buffer[0]) == 0)

>     Please run your patches thru scripts/checkpatch.pl. Assignments in the
> *if* operator are not allowed.

    s/operator/statement/, of course. :-)

> [...]

WBR, Sergei

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