Re: [PATCH 07/12] crypto: qat - set to zero DH parameters before free

[Giovanni Cabiddu <giovanni.cabiddu@intel.com>]

On Sat, May 07, 2022 at 11:52:08AM -0700, Eric Biggers wrote:
> On Fri, May 06, 2022 at 04:41:52PM +0200, Greg KH wrote:
> > On Fri, May 06, 2022 at 11:01:17AM +0100, Giovanni Cabiddu wrote:
> > > On Fri, May 06, 2022 at 11:23:50AM +0200, Greg KH wrote:
> > > > On Fri, May 06, 2022 at 09:23:22AM +0100, Giovanni Cabiddu wrote:
> > > > > Set to zero the DH context buffers containing the DH key before they are
> > > > > freed.
> > > > 
> > > > That says what, but not why.
> > > > 
> > > > > Cc: stable@vger.kernel.org
> > > > > Fixes: c9839143ebbf ("crypto: qat - Add DH support")
> > > > > Signed-off-by: Giovanni Cabiddu <giovanni.cabiddu@intel.com>
> > > > > Reviewed-by: Adam Guerin <adam.guerin@intel.com>
> > > > > Reviewed-by: Wojciech Ziemba <wojciech.ziemba@intel.com>
> > > > > ---
> > > > >  drivers/crypto/qat/qat_common/qat_asym_algs.c | 3 +++
> > > > >  1 file changed, 3 insertions(+)
> > > > > 
> > > > > diff --git a/drivers/crypto/qat/qat_common/qat_asym_algs.c b/drivers/crypto/qat/qat_common/qat_asym_algs.c
> > > > > index d75eb77c9fb9..2fec89b8a188 100644
> > > > > --- a/drivers/crypto/qat/qat_common/qat_asym_algs.c
> > > > > +++ b/drivers/crypto/qat/qat_common/qat_asym_algs.c
> > > > > @@ -421,14 +421,17 @@ static int qat_dh_set_params(struct qat_dh_ctx *ctx, struct dh *params)
> > > > >  static void qat_dh_clear_ctx(struct device *dev, struct qat_dh_ctx *ctx)
> > > > >  {
> > > > >  	if (ctx->g) {
> > > > > +		memzero_explicit(ctx->g, ctx->p_size);
> > > > >  		dma_free_coherent(dev, ctx->p_size, ctx->g, ctx->dma_g);
> > > > 
> > > > Why is a memset() not sufficient here?
> > > Based on the previous conversation a memset() should be sufficient.
> > > 
> > > > And what is this solving?  Who would get this stale data?
> > > This is to make sure the buffer containing sensitive data (i.e. a key)
> > > is not leaked out by a subsequent allocation.
> > 
> > But as all sane distros have CONFIG_INIT_ON_ALLOC_DEFAULT_ON enabled,
> > right?  That should handle any worries you have with secrets being on
> > the heap.  But even then, are you trying to protect yourself against
> > other kernel modules?  Think this through...
> > 
> 
> This patch looks fine to me; it's always recommended to zero out crypto keys at
> the end of their lifetime so that they can't be recovered from free memory if
> system memory is compromised before the memory happens to be allocated and
> overwritten again.  See the hundreds of existing callers of kfree_sensitive(),
> which exist for exactly this reason.
> 
> Note that preventing the key from being "leaked out by a subsequent allocation"
> is *not* the point, and thus CONFIG_INIT_ON_ALLOC_DEFAULT_ON is irrelevant.
I'm going to clarify the commit message and re-send it detached from the
set.

Regards,

-- 
Giovanni