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

[Waiman Long]

On 5/5/20 4:35 PM, Andrew Morton wrote:
> On Tue, 07 Apr 2020 21:21:57 +0100 David Howells <dhowells@xxxxxxxxxx> wrote:
>
> > David Howells <dhowells@xxxxxxxxxx> 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