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

From: Joe Perches <joe@perches.com>
To: David Howells <dhowells@redhat.com>
Cc: Waiman Long <longman@redhat.com>,
	Andrew Morton <akpm@linux-foundation.org>,
	 Jarkko Sakkinen <jarkko.sakkinen@linux.intel.com>,
	James Morris <jmorris@namei.org>,
	"Serge E. Hallyn" <serge@hallyn.com>,
	linux-mm@kvack.org, keyrings@vger.kernel.org,
	 linux-kernel@vger.kernel.org,
	Linus Torvalds <torvalds@linux-foundation.org>
Subject: Re: [PATCH] mm: Add kvfree_sensitive() for freeing sensitive data objects
Date: Mon, 06 Apr 2020 09:10:02 -0700	[thread overview]
Message-ID: <d509771b7e08fff0d18654b746e413e93ed62fe8.camel@perches.com> (raw)
In-Reply-To: <319765.1586188840@warthog.procyon.org.uk>

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.