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

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Date:   Thu, 24 Feb 2022 16:35:51 -0800
From:   Kees Cook <keescook@...omium.org>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     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,
        Nathan Chancellor <nathan@...nel.org>, llvm@...ts.linux.dev
Subject: Re: [PATCH v2 05/39] x86: Base IBT bits

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?) :)

> [...]
> +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