Re: [PATCH] mm: Add kvfree_sensitive() for freeing sensitive data objects

Joe Perches
2020-04-06T16:12:04Z

On Mon, 2020-04-06 at 17:00 +0100, David Howells wrote:
> Joe Perches <joe@perches.com> wrote:
>
> > > This patch introduces a new kvfree_sensitive() for freeing those
> > > sensitive data objects allocated by kvmalloc(). The relevnat places
> > > where kvfree_sensitive() can be used are modified to use it.
> >
> > Why isn't this called kvzfree like the existing kzfree?
>
> To quote Linus:
>
> We have a function for clearing sensitive information: it's called
> "memclear_explicit()", and it's about forced (explicit) clearing even
> if the data might look dead afterwards.
>
> The other problem with that function is the name: "__kvzfree()" is not
> a useful name for this function. We use the "__" format for internal
> low-level helpers, and it generally means that it does *less* than the
> full function. This does more, not less, and "__" is not following any
> sane naming model.
>
> So the name should probably be something like "kvfree_sensitive()" or
> similar. Or maybe it could go even further, and talk about _why_ it's
> sensitive, and call it "kvfree_cleartext()" or something like that.
>
> Because the clearing is really not what even matters. It might choose
> other patterns to overwrite things with, but it might do other things
> too, like putting special barriers for data leakage (or flags to tell
> return-to-user-mode to do so).
>
> And yes, kzfree() isn't a good name either, and had that same
> memset(), but at least it doesn't do the dual-underscore mistake.
>
> Including some kzfree()/crypto people explicitly - I hope we can get
> away from this incorrect and actively wrong pattern of thinking that
> "sensitive data should be memset(), and then we should add a random
> 'z' in the name somewhere to 'document' that".

Thanks.

While I agree with Linus about the __ prefix,
the z is pretty common and symmetric to all
the <foo>zalloc uses.

And if _sensitive is actually used, it'd be
good to do a s/kzfree/kfree_sensitive/ one day
sooner than later.