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

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Date:   Tue, 7 Apr 2020 15:12:55 -0700
From:   Matthew Wilcox <willy@...radead.org>
To:     Waiman Long <longman@...hat.com>
Cc:     Joe Perches <joe@...ches.com>,
        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, 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.