linux-kernel - Re: [PATCH v3] mm: Add kvfree_sensitive() for freeing sensitive data objects

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Date:   Tue, 5 May 2020 21:29:58 -0400
From:   Waiman Long <longman@...hat.com>
To:     Andrew Morton <akpm@...ux-foundation.org>,
        David Howells <dhowells@...hat.com>
Cc:     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>,
        Joe Perches <joe@...ches.com>,
        Matthew Wilcox <willy@...radead.org>,
        David Rientjes <rientjes@...gle.com>
Subject: Re: [PATCH v3] mm: Add kvfree_sensitive() for freeing sensitive data
 objects

On 5/5/20 4:35 PM, Andrew Morton wrote:
> On Tue, 07 Apr 2020 21:21:57 +0100 David Howells <dhowells@...hat.com> wrote:
>
>> David Howells <dhowells@...hat.com> wrote:
>>
>>>>   			if (unlikely(key_data))
>>>> -				__kvzfree(key_data, key_data_len);
>>>> +				kvfree_sensitive(key_data, key_data_len);
>>> I think the if-statement is redundant.
>> Ah - I see that you explicitly wanted to keep it.
> Why's that?

There is a comment above it:

                 /*
                  * The key may change (unlikely) in between 2 consecutive
                  * __keyctl_read_key() calls. In this case, we reallocate
                  * a larger buffer and redo the key read when
                  * key_data_len < ret <= buflen.
                  */
                 if (ret > key_data_len) {
                         if (unlikely(key_data))
                                 __kvzfree(key_data, key_data_len);

key_data will be defined only if the unlikely case that the key increase 
in length between the 2 consecutive __keyctl_read_key() call and we have 
to enlarge the buffer and read the key again. I want to keep the 
unlikely() macro to emphasize the fact that this condition should not 
happen.

>> There's a good chance it'll get janitored at some point.
> Indeed.  Perhaps add a few little comments to explain the reasoning and
> to keep the janitorial fingers away?
>
I can reword the comment to make it more explicit and send a v4 if you 
think the current comment is not clear enough.

Cheers,
Longman