Re: kernel tainted while exporting shash context using af_alg interface

[Stephan Mueller <smueller@chronox.de>]

Am Mittwoch, 28. Oktober 2015, 16:24:34 schrieb Harsh Jain:

Hi Harsh,

>Hi Stephan,
>
>I tried your patch on my machine. Kernel is not crashing. The openssl
>break with this. Can you share HMAC program which you are suspecting
>it will not work or do you already have some test written in
>libkcapi/test.sh which will fail.

See comments above test/kcapi-main.c:cavs_hash

 * HMAC command line invocation:
 * $ ./kcapi -x 3 -c "hmac(sha1)" -k 6e77ebd479da794707bc6cde3694f552ea892dab 
-p  
31b62a797adbff6b8a358d2b5206e01fee079de8cdfc4695138bba163b4efbf30127343e7fd4fbc696c3d38d8f27f57c024b5056f726ceeb4c31d98e57751ec8cbe8904ee0f9b031ae6a0c55da5e062475b3d7832191d4057643ef5fa446801d59a04693e573a8159cd2416b7bd39c7f0fe63c599365e04d596c05736beaab58
 * 7f204ea665666f5bd2b370e546d1b408005e4d85

To do that, apply your patch and then

1. open lib/kcapi-kernel-if.c and change line 567 from

handle->opfd = accept(handle->tfmfd, NULL, 0);


to

handle->opfd = accept(handle->tfmfd, NULL, 0);
handle->opfd = accept(handle->opfd, NULL, 0);
handle->opfd = accept(handle->opfd, NULL, 0);
handle->opfd = accept(handle->opfd, NULL, 0);
handle->opfd = accept(handle->opfd, NULL, 0);

You will see that the hash commands will pass, the HMAC fails

Without your patch, the kernel crashes (same as with your OpenSSL code).

The reason is that setkey is applied on the TFM that is not conveyed to the 
subsequent TFMs generated with new accepts.
>
>
>Regards
>Harsh Jain
>
>On Wed, Oct 28, 2015 at 6:25 AM, Stephan Mueller <smueller@chronox.de> wrote:
>> Am Mittwoch, 28. Oktober 2015, 01:09:58 schrieb Stephan Mueller:
>> 
>> Hi Harsh,
>> 
>>> However, any error in user space should not crash the kernel. So, a fix
>>> should be done. But I think your code is not correct as it solidifies a
>>> broken user space code.
>> 
>> After thinking a bit again, I think your approach is correct after all. I
>> was able to reproduce the crash by simply adding more accept calls to my
>> test code. And I can confirm that your patch works, for hashes.
>> 
>> *BUT* it does NOT work for HMAC as the key is set on the TFM and the
>> subsequent accepts do not transport the key. Albeit your code prevents the
>> kernel from crashing, the HMAC calculation will be done with an empty key
>> as
>> the setkey operation does not reach the TFM handle in the subordinate
>> accept() call.
>> 
>> So, I would think that the second accept is simply broken, for HMAC at
>> least.
>> 
>> Herbert, what is the purpose of that subordinate accept that is implemented
>> with hash_accept? As this is broken for HMACs, should it be removed
>> entirely?
>> 
>> --
>> Ciao
>> Stephan
>
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Ciao
Stephan