linux-kernel - Re: [PATCH] lkdtm: Keep the rodata test from causing retbleed WARNINGS

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Message-ID: <202207202006.46F304D45@keescook>
Date:   Wed, 20 Jul 2022 20:06:27 -0700
From:   Kees Cook <keescook@...omium.org>
To:     Josh Poimboeuf <jpoimboe@...nel.org>
Cc:     Steven Rostedt <rostedt@...dmis.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Thomas Gleixner <tglx@...utronix.de>, x86@...nel.org,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Tim Chen <tim.c.chen@...ux.intel.com>,
        Andrew Cooper <Andrew.Cooper3@...rix.com>,
        Pawan Gupta <pawan.kumar.gupta@...ux.intel.com>,
        Johannes Wikner <kwikner@...z.ch>,
        Alyssa Milburn <alyssa.milburn@...ux.intel.com>,
        Jann Horn <jannh@...gle.com>, "H.J. Lu" <hjl.tools@...il.com>,
        Joao Moreira <joao.moreira@...el.com>,
        Joseph Nuzman <joseph.nuzman@...el.com>,
        Juergen Gross <jgross@...e.com>,
        "Peter Zijlstra (Intel)" <peterz@...radead.org>,
        Masami Hiramatsu <mhiramat@...nel.org>,
        Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>
Subject: Re: [PATCH] lkdtm: Keep the rodata test from causing retbleed
 WARNINGS

On Wed, Jul 20, 2022 at 03:58:09PM -0700, Josh Poimboeuf wrote:
> On Wed, Jul 20, 2022 at 03:56:18PM -0700, Josh Poimboeuf wrote:
> > On Wed, Jul 20, 2022 at 01:13:05PM -0700, Kees Cook wrote:
> > > On Wed, Jul 20, 2022 at 03:55:07PM -0400, Steven Rostedt wrote:
> > > > From: "Steven Rostedt (Google)" <rostedt@...dmis.org>
> > > > 
> > > > The rodata test checks if executable code in the .rodata section will not
> > > > execute. But this also means that fix ups that the return hook code does
> > > > will not be able to change this code at boot up, and this causes a
> > > > warning.
> > > > 
> > > > By removing the RETHOOK_CFLAGS from the compilation of the rodata function
> > > > makes it hidden from objtool and it will not add its return hook into the
> > > > .returns section. This keeps the fix up code from trying and failing on
> > > > modifying the rodata return call.
> > > > 
> > > > Link: https://lore.kernel.org/all/20220720125736.48164a14@gandalf.local.home/
> > > > 
> > > > Fixes: ee88d363d1561 ("x86,static_call: Use alternative RET encoding")
> > > > Signed-off-by: Steven Rostedt (Google) <rostedt@...dmis.org>
> > > 
> > > Yeah, this matches a portion of what was suggested:
> > > https://lore.kernel.org/lkml/20220713213133.455599-1-keescook@chromium.org/
> > > 
> > > But it still needed objtool fixes:
> > > https://lore.kernel.org/lkml/20220715032333.neywlazczbb35w3b@treble/
> > > 
> > > Perhaps objtool has already had patches landed for this? (Otherwise the
> > > patch you suggested would fail to build on x86 with rethunk support.)
> > 
> > I send this patch out a few days ago, nobody picked it up yet:
> > 
> > https://lkml.kernel.org/lkml/8ec0039712f252693049c70ed3891d39a2357112.1658155446.git.jpoimboe@kernel.org
> 
> Actually Peter said he grabbed it and is planning to push it to -tip
> tomorrow.

Ah perfect; thank you!

-- 
Kees Cook