linux-kernel - Re: [PATCH v2 05/39] x86: Base IBT bits

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Message-ID: <YhgnA8O4Bp19hfse@dev-arch.archlinux-ax161>
Date:   Thu, 24 Feb 2022 17:46:59 -0700
From:   Nathan Chancellor <nathan@...nel.org>
To:     Kees Cook <keescook@...omium.org>
Cc:     Peter Zijlstra <peterz@...radead.org>, x86@...nel.org,
        joao@...rdrivepizza.com, hjl.tools@...il.com, jpoimboe@...hat.com,
        andrew.cooper3@...rix.com, linux-kernel@...r.kernel.org,
        ndesaulniers@...gle.com, samitolvanen@...gle.com,
        mark.rutland@....com, alyssa.milburn@...el.com, mbenes@...e.cz,
        rostedt@...dmis.org, mhiramat@...nel.org,
        alexei.starovoitov@...il.com, llvm@...ts.linux.dev
Subject: Re: [PATCH v2 05/39] x86: Base IBT bits

On Thu, Feb 24, 2022 at 04:35:51PM -0800, Kees Cook wrote:
> On Thu, Feb 24, 2022 at 03:51:43PM +0100, Peter Zijlstra wrote:
> > Add Kconfig, Makefile and basic instruction support for x86 IBT.
> > 
> > XXX clang is not playing ball, probably lld being 'funny', I'm having
> > problems with .plt entries appearing all over after linking.
> 
> I'll try to look into this; I know you've been chatting with Nathan
> about it. Is there an open bug for it? (And any kind of reproducer
> smaller than a 39 patch series we can show the linker folks?) :)

I should be able to create a reproducer with cvise and file a bug on
GitHub around this tomorrow, I should have done it after Peter's
comments on IRC.

Cheers,
Nathan

> > [...]
> > +config X86_KERNEL_IBT
> > +	prompt "Indirect Branch Tracking"
> > +	bool
> > +	depends on X86_64 && CC_HAS_IBT
> > +	help
> > +	  Build the kernel with support for Indirect Branch Tracking, a
> > +	  hardware supported CFI scheme. Any indirect call must land on
> 
> 	  hardware support course-grain forward-edge Control Flow Integrity
> 	  protection. It enforces that all indirect calls must land on
> 
> > +	  an ENDBR instruction, as such, the compiler will litter the
> > +	  code with them to make this happen.
> 
> "litter the code" -> "instrument the machine code".
> 
> 
> > +
> >  config X86_INTEL_MEMORY_PROTECTION_KEYS
> >  	prompt "Memory Protection Keys"
> >  	def_bool y
> > --- a/arch/x86/Makefile
> > +++ b/arch/x86/Makefile
> > @@ -36,7 +36,7 @@ endif
> >  
> >  # How to compile the 16-bit code.  Note we always compile for -march=i386;
> >  # that way we can complain to the user if the CPU is insufficient.
> > -REALMODE_CFLAGS	:= -m16 -g -Os -DDISABLE_BRANCH_PROFILING \
> > +REALMODE_CFLAGS	:= -m16 -g -Os -DDISABLE_BRANCH_PROFILING -D__DISABLE_EXPORTS \
> >  		   -Wall -Wstrict-prototypes -march=i386 -mregparm=3 \
> >  		   -fno-strict-aliasing -fomit-frame-pointer -fno-pic \
> >  		   -mno-mmx -mno-sse $(call cc-option,-fcf-protection=none)
> 
> This change seems important separately from this patch, yes? (Or at
> least a specific call-out in the commit log.)
> 
> Otherwise, looks good.
> 
> -- 
> Kees Cook