Re: libkcapi tests are failing on kernels 5.5+

[Stephan Mueller <smueller@chronox.de>]

Am Dienstag, 21. April 2020, 10:08:14 CEST schrieb Ondrej Mosnáček:

Hi Ondrej,

> Hi all,
> 
> the libkcapi [1] tests are failing on kernels 5.5-rc1 and above [2].
> All encryption/decryption tests that use 'ctr(aes)' and a message size
> that is not a multiple of 16 fail due to kcapi-enc returning different
> output than expected.

Confirmed.

On the recent kernels, the data generated by kcapi-enc contains trailing zero 
bytes for data that is a fraction of the block size.

I think the issue is in the following kernel code in _skcipher_recvmsg:

        unsigned int bs = crypto_skcipher_chunksize(tfm);

        /*
         * If more buffers are to be expected to be processed, process only
         * full block size buffers.
         */
        if (ctx->more || len < ctx->used)
                len -= len % bs;


The kernel truncates the size to be processed to the chunk size. As the 
chunksize returns the block size of the underlying cipher (e.g. AES -> 16), 
the kernel code will not process non-aligned data.

Herbert, could you help me identifying what exactly was the root cause for the 
patch 5b0fe9552336338acb52756daf65dd7a4eeca73f ? I.e. it seems that stream 
ciphers made out of a block cipher would not generate the data part that is a 
fraction of the block size (e.g. CTR, CTS).

Ciao
Stephan