Re: [RFC][PATCH 2/6] x86: Base IBT bits

[Kees Cook <keescook@chromium.org>]

On Mon, Nov 22, 2021 at 06:03:03PM +0100, Peter Zijlstra wrote:
> Add Kconfig, Makefile and basic instruction support for x86 IBT.
> 
> Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
> ---
>  arch/x86/Kconfig           |   10 ++++++++++
>  arch/x86/Makefile          |    5 ++++-
>  arch/x86/include/asm/ibt.h |   40 ++++++++++++++++++++++++++++++++++++++++
>  3 files changed, 54 insertions(+), 1 deletion(-)
> 
> --- a/arch/x86/Kconfig
> +++ b/arch/x86/Kconfig
> @@ -1848,6 +1848,16 @@ config X86_UMIP
>  	  specific cases in protected and virtual-8086 modes. Emulated
>  	  results are dummy.
>  
> +config CC_HAS_IBT
> +	def_bool $(cc-option, -fcf-protection=branch)
> +
> +config X86_IBT
> +	prompt "Indirect Branch Tracking"
> +	bool
> +	depends on X86_64 && CC_HAS_IBT
> +	help
> +	  Increase kernel text size for giggles

How about:

	  For systems that support CET, enable Indirect Branch Tracking,
	  which blocks all JOP and indirect call pointer attacks that
	  are not pointing at function entry points (i.e. marked with
	  ENDBR). This also eliminates the use of all of the "misaligned"
	  gadgets that might be reachable in the middle of instructions.

> +
>  config X86_INTEL_MEMORY_PROTECTION_KEYS
>  	prompt "Memory Protection Keys"
>  	def_bool y
> --- a/arch/x86/Makefile
> +++ b/arch/x86/Makefile
> @@ -50,8 +50,11 @@ export BITS
>  #
>  KBUILD_CFLAGS += -mno-sse -mno-mmx -mno-sse2 -mno-3dnow -mno-avx
>  
> -# Intel CET isn't enabled in the kernel
> +ifeq ($(CONFIG_X86_IBT),y)
> +KBUILD_CFLAGS += $(call cc-option,-fcf-protection=branch)
> +else
>  KBUILD_CFLAGS += $(call cc-option,-fcf-protection=none)
> +endif
>  
>  ifeq ($(CONFIG_X86_32),y)
>          BITS := 32
> --- /dev/null
> +++ b/arch/x86/include/asm/ibt.h
> @@ -0,0 +1,40 @@
> +/* SPDX-License-Identifier: GPL-2.0 */
> +#ifndef _ASM_X86_IBT_H
> +#define _ASM_X86_IBT_H
> +
> +#ifdef CONFIG_X86_IBT
> +
> +#ifndef __ASSEMBLY__
> +
> +// XXX note about GAS version required
> +
> +#ifdef CONFIG_X86_64
> +#define ASM_ENDBR	".byte 0xf3, 0x0f, 0x1e, 0xfa\n\t"
> +#else
> +#define ASM_ENDBR	".byte 0xf3, 0x0f, 0x1e, 0xfb\n\t"
> +#endif
> +
> +#else /* __ASSEMBLY__ */
> +
> +#ifdef CONFIG_X86_64
> +#define ENDBR	.byte 0xf3, 0x0f, 0x1e, 0xfa
> +#else
> +#define ENDBR	.byte 0xf3, 0x0f, 0x1e, 0xfb
> +#endif
> +
> +#endif /* __ASSEMBLY__ */
> +
> +#else /* !IBT */
> +
> +#ifndef __ASSEMBLY__
> +
> +#define ASM_ENDBR
> +
> +#else /* __ASSEMBLY__ */
> +
> +#define ENDBR
> +
> +#endif /* __ASSEMBLY__ */
> +
> +#endif /* CONFIG_X86_IBT */
> +#endif /* _ASM_X86_IBT_H */
> 
> 

-- 
Kees Cook