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

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Date:   Mon, 06 Apr 2020 09:10:02 -0700
From:   Joe Perches <joe@...ches.com>
To:     David Howells <dhowells@...hat.com>
Cc:     Waiman Long <longman@...hat.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Jarkko Sakkinen <jarkko.sakkinen@...ux.intel.com>,
        James Morris <jmorris@...ei.org>,
        "Serge E. Hallyn" <serge@...lyn.com>, linux-mm@...ck.org,
        keyrings@...r.kernel.org, linux-kernel@...r.kernel.org,
        Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: [PATCH] mm: Add kvfree_sensitive() for freeing sensitive data
 objects

On Mon, 2020-04-06 at 17:00 +0100, David Howells wrote:
> Joe Perches <joe@...ches.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.