Re: [PATCH v2 3/4] crypto: inside-secure - only update the result buffer when provided

[Antoine Tenart <antoine.tenart@free-electrons.com>]

Hi Herbert,

On Fri, Dec 01, 2017 at 11:31:09AM +1100, Herbert Xu wrote:
> On Thu, Nov 30, 2017 at 10:19:26AM +0100, Kamil Konieczny wrote:
> >
> > can the driver get request for final/finup/digest with null req->result ?
> > If yes (?), such checks can be done before any hardware processing, saving time,
> > for example:
> 
> This should not be possible through any user-space facing API.
> 
> If a kernel API user did this then they're just shooting themselves
> in the foot.
> 
> So unless there is a valida call path that leads to this then I
> would say that there is nothing to fix.

I agree this should not be the case.

But:
- Other drivers are doing this check (grep "if (!req->result)" or
  "if (req->result)" to see some of them).
- I see at least one commit fixing the exact same issue I'm facing here,
  393897c5156a415533ff85aa381458840417b032:

    crypto: ccp - Check for caller result area before using it

    For a hash operation, the caller doesn't have to supply a result
    area on every call so don't use it / update it if it hasn't
    been supplied.

I'm not entirely sure what was the code path that leads to this, I'll
reproduce the issue and try to understand what is going on (I clearly
recall having this crash though).

The crypto API does not enforce this somehow, and this should probably
be fixed. That might break some users. But it was seen as a valid use
for some, so we should probably fix this in previous versions of the
driver anyway.

Thanks!
Antoine

-- 
Antoine Ténart, Free Electrons
Embedded Linux and Kernel engineering
http://free-electrons.com