linux-kernel - Re: [PATCH 1/8] Replace memset() with memzero

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Message-ID: <CACXcFm=bPdoLqYHEUpeZEQEULVGW6ej4ESHX+vMdeGfvjc51tg@mail.gmail.com>
Date:   Wed, 17 Nov 2021 11:08:45 +0800
From:   Sandy Harris <sandyinchina@...il.com>
To:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Cc:     LKML <linux-kernel@...r.kernel.org>,
        Linux Crypto Mailing List <linux-crypto@...r.kernel.org>,
        "David S. Miller" <davem@...emloft.net>,
        Herbert Xu <herbert@...dor.apana.org.au>
Subject: Re: [PATCH 1/8] Replace memset() with memzero_explicit()

On Tue, Nov 16, 2021 at 7:51 PM Greg Kroah-Hartman
<gregkh@...uxfoundation.org> wrote:

> Have you looked at the output of the compiler to see if this really is
> needed or not?

No. To do that right you'd need to look at (at least) gcc & clang,
multiple architectures (cross-compiled & native) & various levels
of optimisation. I just looked at the C code.

> And what exactly are you zeroing out that could be read afterward
> somehow?

Whatever it is, the person who wrote the code thought it was
worth zeroing out with memset(). The only question is whether
it is safer to use memzero_explicit().

Granted in many cases this will not matter unless the kernel
is compiled at some optimisation level that does cross-function
analysis so it might be "smart" enough to optimise out the
memset(). Also granted it does not matter unless an attacker
can look inside the running kernel & if  he or she has that
level of privilege, then you have much else to worry about.

Still, it seemed safer to me to use memzero_explicit() in
these cases.