RFR 8163251 : Hard coded loop limit prevents reading of smart card data greater than 8k

Michael StJohns mstjohns at comcast.net
Thu Feb 13 05:43:23 UTC 2020

Hi -

I needed to go take a quick look at 7816-3 to figure out what's what.

Basically, this code is a bit problematic.

1) Since this code doesn't support extended length APDUs for T=0, you 
should never have to do multiple calls to get the responses - e.g. the 
max response handled here should be 256 bytes.  But this is the only 
case in which the GetResponse code should actually be active???  T=1 
uses low level mapping of I-Blocks rather than an APDU mapping.  And 
looking at pcsc.c - you've got 8192 as the maximum size for a per-call 
return from SCardTransmit, and I'm not sure what SCardTransmit will do - 
I'm wondering if its returning the 6Cxx error code.

2) The iteration count should probably be set from the Le value in the 
transmitted CommandAPDU rather than set to a fixed value. Or even 
better, don't use an iteration count, but simply count down from the 
specified Le and stop when negative.

I'm very confused now.

What protocol is the customer using?   What's the actual APDU they're 
sending to get the data? (Mostly curious about Lc and Le here).

If you changed the iteration loop to parse the Le field and use that to 
count down received data, would that break things?


On 2/12/2020 11:23 PM, Valerie Peng wrote:
> Hi, Ivan,
> I share your thought/confusion on the current impl as I am also not 
> familiar with ISO/IEC 7816-4.
> Based on my reading of this standard and the CardImpl code, it looks 
> like the while-true loop is for retrieving additional response data. 
> Per the standard and javadoc, max length of the response data is 
> 65536. Given that SW2 is only one byte, it would probably only return 
> at most 256 byte response data at a time. Thus, the iteration count 
> would be at most 256.
> So far we are on the same page.
> With the latest webrev (webrev.01), it seems that the loop will only 
> be run 255 instead of 256 times as k is incremented before comparison. 
> Thus, I think we should fix the check.
> Valerie
> On 2/11/2020 12:18 PM, Ivan Gerasimov wrote:
>> Hi Valerie!
>> To be honest, the all these limitations are not quite clear to me.
>> If the command is using an extended Le word to specify the expected 
>> length of the response data, then this length can be at most 65536.
>> If a short Le was used, then the length can be at most 256.
>> However, if we received 0x61 in the SW1, that means that more data 
>> bytes are available and they can be retrieved by issuing another 
>> transmit call with a short Le.  This next call can again potentially 
>> result in 0x61 in SW1, and so on.
>> In the standard, I cannot see any explicit limitations on the number 
>> of retries.  So, I see it as it might be possible to retrieve more 
>> data than 65536 bytes.
>> On the other hand, in the specification for CommandAPDU [1] we have 
>> hardcoded limit for the maximum response length, which is 65536.  So, 
>> even if it were possible to retrieve larger data, there's no point to 
>> try, as the current API prohibits it.
>> Assuming that the 0x61 response can only be received when a short Le 
>> is used, the maximum RESPONSE_ITERATOR should be set to 256, and an 
>> exception should be thrown once that number is exceeded.
>> I've updated the webrev in-place accordingly.
>> [0] http://cr.openjdk.java.net/~igerasim/8163251/01/webrev/
>> [1] 
>> https://docs.oracle.com/en/java/javase/13/docs/api/java.smartcardio/javax/smartcardio/CommandAPDU.html#%3Cinit%3E(int,int,int,int,int)
>> With kind regards,
>> Ivan
>> On 2/10/20 6:40 PM, Valerie Peng wrote:
>>> Hi Ivan,
>>> You removed the "=", so the actual iteration count is reduced by one.
>>> Should the iteration count be 256 or 257? If the actual count should 
>>> be 257, then may be the check needs to be changed to k++ from ++k?
>>> Valerie
>>> On 2/10/2020 5:07 PM, Ivan Gerasimov wrote:
>>>> Thank you Michael!
>>>> It's a good point about maximum length.
>>>> Here's the updated webrev with the new System property dropped and 
>>>> the increased number of iterations:
>>>> http://cr.openjdk.java.net/~igerasim/8163251/01/webrev/
>>>> With kind regards,
>>>> Ivan
>>>> On 2/10/20 4:18 PM, Michael StJohns wrote:
>>>>> On 2/10/2020 6:49 PM, Ivan Gerasimov wrote:
>>>>>> Hello!
>>>>>> Current implementation of the method 
>>>>>> javax.smartcardio.CardChannel.transmit() has an internal 
>>>>>> limitation on the maximum allowed amount of the transmitted data.
>>>>>> This limitation is dictated by the maximum number of iterations 
>>>>>> to transmit data from a card:  Each iteration can transmit up to 
>>>>>> 256 bytes of data, and we have a hardcoded limit of 32 iterations.
>>>>>> Over time, we've received requests to increase this limit, as 
>>>>>> there are occasions when the effective limit of 8k is not enough.
>>>>>> Would you please help review a proposal:  First, it is proposed 
>>>>>> to increase the default limit of iteration to 128 (so that up to 
>>>>>> 32k of data can be transmitted); Second, the limit of iterations 
>>>>>> is made configurable via a System property. This limit can be 
>>>>>> increased up to 4096 (so that the total amount of transmitted 
>>>>>> data can be made up to 1m).
>>>>>> BUGURL: https://bugs.openjdk.java.net/browse/JDK-8163251
>>>>>> WEBREV: http://cr.openjdk.java.net/~igerasim/8163251/00/webrev/
>>>>>> If there is an agreement on the proposal, I'll file a CSR to 
>>>>>> document a new System property.
>>>>>> Thanks in advance!
>>>>> Given that the maximum length for an extended APDU is 64K (65536) 
>>>>> (hmm +7 for the header and +2 for LE), and for its return is 64K + 
>>>>> 2 bytes,  I'm not quite sure why you'd up the number to 4096/1M - 
>>>>> I'd set the default and fixed value to 257 (64K) and leave it at 
>>>>> that.
>>>>> Mike

-------------- next part --------------
An HTML attachment was scrubbed...
URL: <https://mail.openjdk.java.net/pipermail/security-dev/attachments/20200213/b13ef4fb/attachment-0001.htm>

More information about the security-dev mailing list