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

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Date:   Tue, 07 Apr 2020 17:35:24 -0700
From:   Joe Perches <joe@...ches.com>
To:     Matthew Wilcox <willy@...radead.org>,
        Waiman Long <longman@...hat.com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        David Howells <dhowells@...hat.com>,
        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>,
        David Rientjes <rientjes@...gle.com>
Subject: Re: [PATCH v3] mm: Add kvfree_sensitive() for freeing sensitive
 data objects

On Tue, 2020-04-07 at 15:12 -0700, Matthew Wilcox wrote:
> On Tue, Apr 07, 2020 at 04:45:45PM -0400, Waiman Long wrote:
> > On 4/7/20 4:31 PM, Joe Perches wrote:
> > > On Tue, 2020-04-07 at 16:03 -0400, Waiman Long wrote:
> > > > +extern void kvfree_sensitive(const void *addr, size_t len);
> > > Why should size_t len be required?
> > > 
> > > Why not do what kzfree does and memset
> > > the entire allocation? (area->size)
> > 
> > If the memory is really virtually mapped, the only way to find out the
> > size of the object is to use find_vm_area() which can be relatively high
> > cost and no simple helper function is available. On the other hand, the
> > length is readily available in the callers. So passing the length
> > directly to the kvfree_sensitive is simpler.
> 
> Also it lets us zero only the first N bytes of the allocation.  That might
> be good for performance, if only the first N bytes of an M byte allocation
> are actually sensitive.  I don't know if we have any such cases, but
> they could exist.

I would really doubt it as the allocation of
sensitive data should generally be separate.

Also, a similar argument could apply to
kzfree/kfree_sensitive.