linux-kernel - Re: [PATCH v2] mm: Add kvfree_sensitive() for freeing sensitive data objects

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Message-ID: <07e49a285eff9a22476c6b1c396485f6d5d39002.camel@perches.com>
Date:   Mon, 06 Apr 2020 23:41:23 -0700
From:   Joe Perches <joe@...ches.com>
To:     Waiman Long <longman@...hat.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>
Cc:     linux-mm@...ck.org, keyrings@...r.kernel.org,
        linux-kernel@...r.kernel.org,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Matthew Wilcox <willy@...radead.org>,
        David Rientjes <rientjes@...gle.com>
Subject: Re: [PATCH v2] mm: Add kvfree_sensitive() for freeing sensitive
 data objects

On Mon, 2020-04-06 at 22:16 -0400, Waiman Long wrote:
> On 4/6/20 3:38 PM, Joe Perches wrote:
> > On Mon, 2020-04-06 at 14:58 -0400, Waiman Long wrote:
> > > For kvmalloc'ed data object that contains sensitive information like
> > > cryptographic key, we need to make sure that the buffer is always
> > > cleared before freeing it. Using memset() alone for buffer clearing may
> > > not provide certainty as the compiler may compile it away. To be sure,
> > > the special memzero_explicit() has to be used.
> > [] 
> > >  extern void kvfree(const void *addr);
> > > +extern void kvfree_sensitive(const void *addr, size_t len);
> > Question: why should this be const?
> > 
> > 2.1.44 changed kfree(void *) to kfree(const void *) but
> > I didn't find a particular reason why.
> 
> I am just following the function prototype used by kvfree(). Even
> kzfree(const void *) use const. I can remove "const" if others agree.

No worries.  Nevermind me...

Lots of warnings if allocated pointers are const, so const is necessary
in the definition and declaration.

struct foo {
	...
};

struct bar {
	const struct foo *baz;
	...
};

some_func(void)
{
	bar.baz = kvalloc(...);
}

kvfree can't free bar.baz if it's defined with void * without warning,
so it must be const void *.

Apologies for the noise.