Re: [RFC PATCH 0/2] crypto: caam - fix cts(cbc(aes)) with CAAM driver

From: Horia Geantă
Date: Fri Jun 16 2017 - 17:01:55 EST

On 6/16/2017 11:00 AM, Herbert Xu wrote:
> On Fri, Jun 16, 2017 at 07:57:00AM +0000, Horia Geantă wrote:
>> Commit 0605c41cc53ca ("crypto: cts - Convert to skcipher") appends
>> CRYPTO_TFM_REQ_MAY_BACKLOG to the original crypto request flags for the
>> last block - when calling cts_cbc_encrypt().
>> Is it really needed?
> Yes, because at this point we cannot tell the sender to back off.
>> For cts(cbc(aes)) with cbc(aes) offloaded in HW, i.e. running in async
>> mode, we get the below stack for CAAM driver.
>> Driver is told that it can sleep (CRYPTO_TFM_REQ_MAY_BACKLOG flag), so
>> it uses GFP_KERNEL to allocate memory. However, this is incorrect, since
>> driver runs in atomic context (softirq).
> This is wrong. Whether you can sleep or not is determined by
> MAY_SLEEP, not MAY_BACKLOG. MAY_BACKLOG only indicates that this
> request must be queued, even if the queue is full.
Indeed, CAAM driver incorrectly decides to use GFP_KERNEL for allocation
when MAY_BACKLOG flag is set. This seems to be a long-standing issue, I
will send a fix (separately).

Still I think we have a problem.
David reported that the user is fscrypt. Looking into fscrypt code, I
see that besides MAY_BACKLOG, MAY_SLEEP flag is also set. So we end up
in the situation I described earlier: the last block is encrypted in
atomic context and with MAY_SLEEP set.