Re: [Qemu-devel] [PATCH for-2.8] virtio-crypto: zeroize the key material before free

["Michael S. Tsirkin" <mst@redhat.com>]

On Tue, Dec 06, 2016 at 05:33:37PM +0000, Stefan Hajnoczi wrote:
> On Tue, Dec 06, 2016 at 03:40:49PM +0200, Michael S. Tsirkin wrote:
> > On Tue, Dec 06, 2016 at 05:29:13PM +0800, Gonglei wrote:
> > > Zeroize the memory of CryptoDevBackendSymOpInfo structure pointed
> > > for key material security.
> > > 
> > > Signed-off-by: Gonglei <arei.gonglei@huawei.com>
> > > ---
> > >  hw/virtio/virtio-crypto.c | 13 ++++++++++++-
> > >  1 file changed, 12 insertions(+), 1 deletion(-)
> > > 
> > > diff --git a/hw/virtio/virtio-crypto.c b/hw/virtio/virtio-crypto.c
> > > index 2f2467e..ecb19b6 100644
> > > --- a/hw/virtio/virtio-crypto.c
> > > +++ b/hw/virtio/virtio-crypto.c
> > > @@ -337,7 +337,18 @@ static void virtio_crypto_free_request(VirtIOCryptoReq *req)
> > >  {
> > >      if (req) {
> > >          if (req->flags == CRYPTODEV_BACKEND_ALG_SYM) {
> > > -            g_free(req->u.sym_op_info);
> > > +            size_t max_len;
> > > +            CryptoDevBackendSymOpInfo *op_info = req->u.sym_op_info;
> > > +
> > > +            max_len = op_info->iv_len +
> > > +                      op_info->aad_len +
> > > +                      op_info->src_len +
> > > +                      op_info->dst_len +
> > > +                      op_info->digest_result_len;
> > > +
> > > +            /* Zeroize and free request data structure */
> > > +            memset(op_info, 0, sizeof(*op_info) + max_len);
> > > +            g_free(op_info);
> > 
> > Write into memory, then free it?  This looks rather strange. Why are we
> > doing this?
> 
> Common practice with sensitive information (key material, passwords,
> etc).

Some kind of explanation about what makes this one
more sensitive than others would be nice.

Also, what makes it 2.8 material? Considering the pointer math
involved, it's not risk-free.

> coredumps, memory disclosure bugs when heap memory is reused, etc.
>
> Sensitive information is sometimes also held in mlocked pages to prevent
> it being swapped to disk but that's not being done here.
> 
> Perhaps the comment should be more explicit but this patch seems
> reasonable.

Right.  One can see memset and free at a glance.
The comment and the commit log should explain the why,
not the what.

> Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>