linux-hardening - Re: [PATCH v9 02/15] livepatch: use `-z unique-symbol` if available to nuke pos-based search

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Date:   Mon,  3 Jan 2022 17:06:15 +0100
From:   Alexander Lobakin <alexandr.lobakin@...el.com>
To:     Miroslav Benes <mbenes@...e.cz>
Cc:     Alexander Lobakin <alexandr.lobakin@...el.com>,
        Fāng-ruì Sòng <maskray@...gle.com>,
        Borislav Petkov <bp@...en8.de>,
        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>,
        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 02/15] livepatch: use `-z unique-symbol` if available to nuke pos-based search

From: Miroslav Benes <mbenes@...e.cz>
Date: Mon, 3 Jan 2022 14:55:42 +0100 (CET)

> On Thu, 30 Dec 2021, Fāng-ruì Sòng wrote:
> 
> > On Thu, Dec 30, 2021 at 3:11 AM Borislav Petkov <bp@...en8.de> wrote:
> > >
> > > On Thu, Dec 23, 2021 at 01:21:56AM +0100, Alexander Lobakin wrote:
> > > > [PATCH v9 02/15] livepatch: use `-z unique-symbol` if available to nuke pos-based search
> 
> ...
> 
> > Apologies since I haven't read the patch series.
> > 
> > The option does not exist in ld.lld and I am a bit concerning about
> > its semantics: https://maskray.me/blog/2020-11-15-explain-gnu-linker-options#z-unique-symbol
> > 
> > I thought that someone forwarded my comments (originally posted months
> > on a feature request ago) here but seems not.
> > (I am a ld.lld maintainer.)
> 
> Do you mean 
> https://lore.kernel.org/all/20210123225928.z5hkmaw6qjs2gu5g@google.com/T/#u 
> ?
> 
> Unfortunately, it did not lead anywhere. I think that '-z unique-symbol' 
> option should work fine as long as the live patching is concerned. Maybe I 
> misunderstood but your concerns mentioned at the blog do not apply. The 
> stability is not an issue for us since we (KLP) always work with already 
> built and fixed kernel. And(at least) GCC already uses number suffices for 
> IPA clones and it has not been a problem anywhere.

LLD doesn't have such an option, so FG-KASLR + livepatching builds
wouldn't be available for LLVM with the current approach (or we'd
still need a stub that prints "FG-KASLR is not compatible with
sympos != 0").
Unfortunately, I discovered this a bit late, just after sending this
revision.

OTOH, there's no easy alternative. <file + function> pair looks
appealing, but is it even possible for now to implement in the
kernel without much refactoring?

>
> Am I wrong?
> 
> Thanks
> 
> Miroslav 

Thanks,
Al