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

Waiman Long
2020-04-06T17:59:06Z

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