Re: [PATCH] crypto: sha512: remove imaginary and mystifying clearing of variables

From: Sandy Harris
Date: Sat Aug 28 2021 - 03:47:08 EST

On Fri, Aug 27, 2021 at 4:40 PM Herbert Xu <herbert@xxxxxxxxxxxxxxxxxxx> wrote:
> On Sun, Aug 22, 2021 at 12:31:07PM +0200, Lukas Bulwahn wrote:
> > The function sha512_transform() assigns all local variables to 0 before
> > returning to its caller with the intent to erase sensitive data.
> > ....
> >
> > The assignments to clear a through h and t1/t2 are optimized out by the
> > compiler because they are unused after the assignments.

Just no.

You are right, there is a problem here. I thank you for pointing it
out & I've already fixed it in some of my own code.

However, I think your solution is dead wrong. You are correct that
these assignments are useless because the compiler will optimise them
out, and that's a problem. However, it is not at all "mistiifying";
they are there for an obvious reason, to avoid leaving state that
might be useful to an enemy. That is quite a small risk, but then it
is a small mitigation, so worth doing.

The correct solution is not to just remove the assignments, but rather
to replace them with code that will not be optimised away, force the
compiler to do what we need. We already do that for operations that
clear various arrays and structures, using memzero_explicit() rather
than memset(). Similarly, we should replace the assignments with calls
to this macro:

clear a variable
in a way the compiler will not optimise out
#define clear(x) memzero_explicit( &x, sizeof(x) )