linux-hardening - Re: [PATCH v9 03/15] kallsyms: Hide layout

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Date:   Mon,  3 Jan 2022 16:40:23 +0100
From:   Alexander Lobakin <alexandr.lobakin@...el.com>
To:     Borislav Petkov <bp@...en8.de>
Cc:     Alexander Lobakin <alexandr.lobakin@...el.com>,
        linux-hardening@...r.kernel.org, x86@...nel.org,
        Jesse Brandeburg <jesse.brandeburg@...el.com>,
        Kristen Carlson Accardi <kristen@...ux.intel.com>,
        Kees Cook <keescook@...omium.org>,
        Miklos Szeredi <miklos@...redi.hu>,
        Ard Biesheuvel <ardb@...nel.org>,
        Tony Luck <tony.luck@...el.com>,
        Bruce Schlobohm <bruce.schlobohm@...el.com>,
        Jessica Yu <jeyu@...nel.org>,
        kernel test robot <lkp@...el.com>,
        Miroslav Benes <mbenes@...e.cz>,
        Evgenii Shatokhin <eshatokhin@...tuozzo.com>,
        Jonathan Corbet <corbet@....net>,
        Masahiro Yamada <masahiroy@...nel.org>,
        Michal Marek <michal.lkml@...kovi.net>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        Herbert Xu <herbert@...dor.apana.org.au>,
        "David S. Miller" <davem@...emloft.net>,
        Thomas Gleixner <tglx@...utronix.de>,
        Will Deacon <will@...nel.org>, Ingo Molnar <mingo@...hat.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        "H. Peter Anvin" <hpa@...or.com>,
        Andy Lutomirski <luto@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Arnd Bergmann <arnd@...db.de>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        Nathan Chancellor <nathan@...nel.org>,
        Masami Hiramatsu <mhiramat@...nel.org>,
        Marios Pomonis <pomonis@...gle.com>,
        Sami Tolvanen <samitolvanen@...gle.com>,
        "H.J. Lu" <hjl.tools@...il.com>, Nicolas Pitre <nico@...xnic.net>,
        linux-kernel@...r.kernel.org, linux-kbuild@...r.kernel.org,
        linux-arch@...r.kernel.org, live-patching@...r.kernel.org,
        llvm@...ts.linux.dev
Subject: Re: [PATCH v9 03/15] kallsyms: Hide layout

From: Borislav Petkov <bp@...en8.de>
Date: Thu, 30 Dec 2021 23:36:00 +0100

> On Thu, Dec 23, 2021 at 01:21:57AM +0100, Alexander Lobakin wrote:
> > Subject: Re: [PATCH v9 03/15] kallsyms: Hide layout
> 
> That title is kinda laconic...

"kallsyms: randomize /proc/kallsyms output order"?

> 
> > From: Kristen Carlson Accardi <kristen@...ux.intel.com>
> > 
> > This patch makes /proc/kallsyms display in a random order, rather
> 
> Avoid having "This patch" or "This commit" in the commit message. It is
> tautologically useless.
> 
> Also, do
> 
> $ git grep 'This patch' Documentation/process
> 
> for more details.

Goes straight from the original series. Worth changing anyways.

> 
> > than sorted by address in order to hide the newly randomized address
> > layout.
> 
> Sorted by address?
> 
> My /proc/kallsyms says
> 
> $ awk '{ print $1 }' /proc/kallsyms | uniq -c
>  119086 0000000000000000
> 
> so all the addresses are 0. Aha, and when I list them as root, only then
> I see non-null addresses.
> 
> So why do we that patch at all?

It displays zeros for non-roots, but the symbols are still sorted by
their addresses. As a result, if you leak one address, you could
determine some others.
This is especially critical with FG-KASLR as its text layout is
random each time and sorted /proc/kallsyms would make the entire
feature useless.

> 
> > alobakin:
> > Don't depend FG-KASLR and always do that for unpriviledged accesses
> 
> Unknown word [unpriviledged] in commit message, suggestions:
>         ['unprivileged', 'underprivileged', 'privileged']

I either have some problems with checkpatch + codespell, or they
missed all that typos you're noticing. Thanks, and apologies =\

> 
> > as suggested by several folks.
> > Also, introduce and use a shuffle_array() macro which shuffles an
> > array using Fisher-Yates.
> 
> Fisher-Yates what?
> 
> /me goes and looks at the wikipedia article.
> 
> Aha, a Fisher-Yates shuffle algoithm.
> 
> Don't be afraid to explain more in your commit messages and make them
> more reader-friendly.

Sure.
This patch initially was at the tail of the set, after the commits
where this algo is mentioned several times in a more detailed
manner, but I moved it to the head then as the requests for doing
this unconditionally converted it to a pre-requisite.

> 
> > We'll make use of it several more times
> > later on.
> 
> Not important for this commit.
> 
> ...
> 
> -- 
> Regards/Gruss,
>     Boris.
> 
> https://people.kernel.org/tglx/notes-about-netiquette

Thanks!
Al