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

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Message-ID: <19cbf3b1-2c3f-dd0f-a5c6-69ca3f77dd68@redhat.com>
Date:   Mon, 6 Apr 2020 13:58:54 -0400
From:   Waiman Long <longman@...hat.com>
To:     Joe Perches <joe@...ches.com>, David Howells <dhowells@...hat.com>
Cc:     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 4/6/20 12:10 PM, Joe Perches wrote:
> 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.
>
>
I have actually been thinking about that. I saw a couple of cases in the
crypto code where a memzero_explicit() is followed by kfree(). Those can
be replaced by kfree_sensitive.

Cheers,
Longman