linux-kernel - Re: [PATCH] security: keys: Use kvfree

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Date:   Fri, 11 Sep 2020 08:04:24 -0700
From:   James Bottomley <James.Bottomley@...senPartnership.com>
To:     Alex Dewar <alex.dewar90@...il.com>
Cc:     David Howells <dhowells@...hat.com>,
        Jarkko Sakkinen <jarkko.sakkinen@...ux.intel.com>,
        James Morris <jmorris@...ei.org>,
        "Serge E. Hallyn" <serge@...lyn.com>, keyrings@...r.kernel.org,
        linux-security-module@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] security: keys: Use kvfree_sensitive in a few places

On Fri, 2020-09-11 at 12:44 +0100, Alex Dewar wrote:
> In big_key.c, there are a few places where memzero_explicit + kvfree
> is used. It is better to use kvfree_sensitive instead, which is more
> readable and also prevents the compiler from eliding the call to
> memzero_explicit. Fix this.

That last bit is untrue: the compiler can't elide memzero_explicit ...
that's why it has the explicit suffix.

The original problem was a lot of people do memset(.., 0, ..); kfree()
which the compiler can elide if it understands the memory is going out
of scope.  Or the even more problematic memset(..., 0, ...) on a stack
variable before it goes out of scope.

We can argue about readability but there's no secret leak here.

James