Re: [PATCH v2 1/6] crypto: Use memzero_explicit() for clearing state

[Eric Biggers]

On Fri, Oct 23, 2020 at 11:39:27AM -0400, Arvind Sankar wrote:
> On Wed, Oct 21, 2020 at 09:36:33PM -0700, Eric Biggers wrote:
> > On Tue, Oct 20, 2020 at 04:39:52PM -0400, Arvind Sankar wrote:
> > > Without the barrier_data() inside memzero_explicit(), the compiler may
> > > optimize away the state-clearing if it can tell that the state is not
> > > used afterwards. At least in lib/crypto/sha256.c:__sha256_final(), the
> > > function can get inlined into sha256(), in which case the memset is
> > > optimized away.
> > > 
> > > Signed-off-by: Arvind Sankar <nivedita@xxxxxxxxxxxx>
> > 
> > Reviewed-by: Eric Biggers <ebiggers@xxxxxxxxxx>
> > 
> > Maybe get the one in arch/arm64/crypto/sha3-ce-glue.c too?
> > 
> > - Eric
> 
> Hm, there are a few more as well like that. But now I'm thinking it's
> only the generic sha256.c that may be problematic. The rest of them are
> in _final() functions which will be stored as function pointers in a
> structure, so there should be no risk of them getting optimized away?

When clearing memory because "it may be sensitive" rather than "it's needed for
the code to behave correctly", I think it's best to use memzero_explicit() to
make the intent clear, even if it seems that memset() is sufficient.  Also keep
in mind that support for compiling the kernel with LTO (link-time optimization)
is being worked on (and some people already do it), which results in more code
being optimized out.

- Eric