Re: crypto: zeroization of sensitive data in af_alg

[Stephan Mueller <smueller@chronox.de>]

Am Montag, 10. November 2014, 22:05:18 schrieb Herbert Xu:

Hi Herbert,

> On Sun, Nov 09, 2014 at 11:33:52PM +0100, Stephan Mueller wrote:
> > while working on the AF_ALG interface, I saw no active zeroizations of
> > memory that may hold sensitive data that is maintained outside the kernel
> > crypto API cipher handles. I think the following memory segments fall
> > under that
> > category:
> Are you talking about temporary data that we generate as part of
> the processing? If so they should be zeroed by the entity that
> generates them.

I currently see that the IV buffer (owned by skcipher) and the message digest 
buffer (owned by hash) are not memset(0) before freeing them. I agree that 
both are not really sensitive data. But wouldn't it be prudent to memset(0) 
them nonetheless in the skcipher_sock_destruct and hash_sock_destruct 
functions, respectively? 
> 
> > However, I am failing to find the right spot to add a zeroization for the
> > latter one, i.e. the code that handles the pages send in by the user or
> > the
> > pages that are returned by the crypto API. Initially I thought
> > skcipher_pull_sgl is a good spot for the symmetric ciphers as it evicts
> > the
> > used pages out of the scope of the kernel crypto API. I added a
> > clear_page(sg_page(sg+1)) as well as memset(sg_page(sg+1), 0, plen) right
> > before the put_page call. All that I got in return was a BUG() from the
> > memory management layer.
> 
> I don't think I understand what exactly you're trying to zero.
> Can you give an example?

Apologies, my bad as I did not check get_user_pages_fast well enough. I see 
now that we operate on the pages in user space directly without copy_from_user 
that would imply a kernel-internal copy. Please disregard my comment.
> 
> Thanks,


-- 
Ciao
Stephan