linux-security-module.vger.kernel.org archive mirror
 help / color / mirror / Atom feed
From: Jirka Hladky <jhladky@redhat.com>
To: Kees Cook <keescook@chromium.org>
Cc: Alexander Potapenko <glider@google.com>,
	kernel-hardening@lists.openwall.com, linux-mm@kvack.org,
	linux-security-module@vger.kernel.org
Subject: Re: init_on_alloc/init_on_free boot options
Date: Thu, 20 Aug 2020 02:35:04 +0200	[thread overview]
Message-ID: <CAE4VaGAGHNj0rWmTN4r5xJuN-ty2xYAsFxWwKEAod6tvMqjpUA@mail.gmail.com> (raw)
In-Reply-To: <202008191626.1420C63231@keescook>

Thanks a lot for the clarification! I was scratching my head if it
makes sense to enable both options simultaneously.


On Thu, Aug 20, 2020 at 1:36 AM Kees Cook <keescook@chromium.org> wrote:
>
> On Thu, Aug 20, 2020 at 12:18:33AM +0200, Jirka Hladky wrote:
> > Could you please help me to clarify the purpose of init_on_alloc=1
> > when init_on_free is enabled?
>
> It's to zero memory at allocation time. :) (They are independent
> options.)
>
> > If I get it right, init_on_free=1 alone guarantees that the memory
> > returned by the page allocator and SL[AU]B is initialized with zeroes.
>
> No, it's guarantees memory freed by the page/slab allocators are zeroed.
>
> > What is the purpose of init_on_alloc=1 in that case? We are zeroing
> > memory twice, or am I missing something?
>
> If you have both enabled, yes, you will zero twice. (In theory, if you
> have any kind of Use-After-Free/dangling pointers that get written
> through after free and before alloc, those contents wouldn't strictly be
> zero at alloc time without init_on_alloc. But that's pretty rare.
>
> I wouldn't expect many people to run with both options enabled;
> init_on_alloc is more performance-friendly (i.e. cache-friendly), and
> init_on_free minimizes the lifetime of stale data in memory.
>
> It appears that the shipping kernel defaults for several distros (Ubuntu,
> Arch, Debian, others?) and devices (Android, Chrome OS, others?) are using
> init_on_alloc=1. Will Fedora and/or RedHat be joining this trend?  :)
>
> --
> Kees Cook
>


-- 
-Jirka


      reply	other threads:[~2020-08-20  0:35 UTC|newest]

Thread overview: 3+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-08-19 22:18 init_on_alloc/init_on_free boot options Jirka Hladky
2020-08-19 23:36 ` Kees Cook
2020-08-20  0:35   ` Jirka Hladky [this message]

Reply instructions:

You may reply publicly to this message via plain-text email
using any one of the following methods:

* Save the following mbox file, import it into your mail client,
  and reply-to-all from there: mbox

  Avoid top-posting and favor interleaved quoting:
  https://en.wikipedia.org/wiki/Posting_style#Interleaved_style

* Reply using the --to, --cc, and --in-reply-to
  switches of git-send-email(1):

  git send-email \
    --in-reply-to=CAE4VaGAGHNj0rWmTN4r5xJuN-ty2xYAsFxWwKEAod6tvMqjpUA@mail.gmail.com \
    --to=jhladky@redhat.com \
    --cc=glider@google.com \
    --cc=keescook@chromium.org \
    --cc=kernel-hardening@lists.openwall.com \
    --cc=linux-mm@kvack.org \
    --cc=linux-security-module@vger.kernel.org \
    /path/to/YOUR_REPLY

  https://kernel.org/pub/software/scm/git/docs/git-send-email.html

* If your mail client supports setting the In-Reply-To header
  via mailto: links, try the mailto: link
Be sure your reply has a Subject: header at the top and a blank line before the message body. 
This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox;
as well as URLs for NNTP newsgroup(s).