linux-kernel - Re: [PATCH v10 10/15] FG-KASLR: use a scripted approach to handle .text.* sections

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Message-Id: <20220214113434.5256-1-alexandr.lobakin@intel.com>
Date:   Mon, 14 Feb 2022 12:34:34 +0100
From:   Alexander Lobakin <alexandr.lobakin@...el.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Alexander Lobakin <alexandr.lobakin@...el.com>,
        linux-hardening@...r.kernel.org, x86@...nel.org,
        Borislav Petkov <bp@...en8.de>,
        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>,
        Christoph Hellwig <hch@....de>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        "H. Peter Anvin" <hpa@...or.com>,
        Andy Lutomirski <luto@...nel.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 v10 10/15] FG-KASLR: use a scripted approach to handle .text.* sections

From: Peter Zijlstra <peterz@...radead.org>
Date: Fri, 11 Feb 2022 16:37:06 +0100

> On Wed, Feb 09, 2022 at 07:57:47PM +0100, Alexander Lobakin wrote:
> > +sub read_sections {
> > +	open(my $fh, "\"$readelf\" -SW \"$file\" 2>/dev/null |")
> > +		or die "$0: ERROR: failed to execute \"$readelf\": $!";
> > +
> > +	while (<$fh>) {
> > +		my $name;
> > +		my $align;
> > +		chomp;
> > +
> > +		($name, $align) = $_ =~ /^\s*\[[\s0-9]*\]\s*(\.\S*)\s*[A-Z]*\s*[0-9a-f]{16}\s*[0-9a-f]*\s*[0-9a-f]*\s*[0-9a-f]*\s*[0-9a-f]{2}\s*[A-Z]{2}\s*[0-9]\s*[0-9]\s*([0-9]*)$/;
> 
> Is there really no readable way to write this?

I'm no regexp master, so I'd be glad if someone could help improve
this :D I only tried to optimize it using online tools, and they
eventually didn't help.
It could probably be replaced with a pipe to `cut` or `tr`, I'll
take a look a bit later if there won't be any other proposals.

> 
> > +
> > +		if (!defined($name)) {
> > +			next;
> > +		}
> > +
> > +		## Clang 13 onwards emits __cfi_check_fail only on final
> > +		## linking, so it won't appear in .o files and will be
> > +		## missing in @sections. Add it manually to prevent
> > +		## spawning orphans.
> > +		if ($name eq ".text.__cfi_check_fail") {
> > +			$has_ccf = 1;
> > +		}
> 
> How is that relevant, x86-64 doesn't and won't do clang-cfi.

1. One of my test cases was based on Sami's ClangCFI x86 series from
   LKML (the one that was getting patched by objtool :P).
2. ClangCFI is present on ARM64 and it can receive FG-KASLR in a
   future, why not make it work together from the start?

Re "won't do" -- sorry for trying to hijack this thread a bit, but
did I miss something? The last comments I've read were that LLVM
tools need to change their approach for CFI on x86, and Sami went
redo it, but I can't recall any "life-time" nacks.

Thanks,
Al