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Subject: Re: [PATCH] mm: Add kvfree_sensitive() for freeing sensitive data
 objects
To:     Joe Perches <joe@perches.com>, David Howells <dhowells@redhat.com>
Cc:     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>
References: <a291cce3ff1ba978e7ad231a8e1b7d82f6164e86.camel@perches.com>
 <20200406023700.1367-1-longman@redhat.com>
 <319765.1586188840@warthog.procyon.org.uk>
 <d509771b7e08fff0d18654b746e413e93ed62fe8.camel@perches.com>
From:   Waiman Long <longman@redhat.com>
Organization: Red Hat
Message-ID: <19cbf3b1-2c3f-dd0f-a5c6-69ca3f77dd68@redhat.com>
Date:   Mon, 6 Apr 2020 13:58:54 -0400
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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@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.
>
>
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