Re: [RFC PATCH v9 24/27] x86/cet/shstk: ELF header parsing for Shadow Stack

[Kees Cook <keescook@xxxxxxxxxxxx>]

On Wed, Feb 05, 2020 at 10:19:32AM -0800, Yu-cheng Yu wrote:
> Check an ELF file's .note.gnu.property, and setup Shadow Stack if the
> application supports it.
> 
> v9:
> - Change cpu_feature_enabled() to static_cpu_has().
> 
> Signed-off-by: Yu-cheng Yu <yu-cheng.yu@xxxxxxxxx>
> ---
>  arch/x86/Kconfig             |  2 ++
>  arch/x86/include/asm/elf.h   | 13 +++++++++++++
>  arch/x86/kernel/process_64.c | 31 +++++++++++++++++++++++++++++++
>  3 files changed, 46 insertions(+)
> 
> diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig
> index 6c34b701c588..d1447380e02e 100644
> --- a/arch/x86/Kconfig
> +++ b/arch/x86/Kconfig
> @@ -1987,6 +1987,8 @@ config X86_INTEL_SHADOW_STACK_USER
>  	select ARCH_USES_HIGH_VMA_FLAGS
>  	select X86_INTEL_CET
>  	select ARCH_HAS_SHSTK
> +	select ARCH_USE_GNU_PROPERTY
> +	select ARCH_BINFMT_ELF_STATE
>  	---help---
>  	  Shadow Stack (SHSTK) provides protection against program
>  	  stack corruption.  It is active when the kernel has this
> diff --git a/arch/x86/include/asm/elf.h b/arch/x86/include/asm/elf.h
> index 69c0f892e310..fac79b621e0a 100644
> --- a/arch/x86/include/asm/elf.h
> +++ b/arch/x86/include/asm/elf.h
> @@ -367,6 +367,19 @@ extern int compat_arch_setup_additional_pages(struct linux_binprm *bprm,
>  					      int uses_interp);
>  #define compat_arch_setup_additional_pages compat_arch_setup_additional_pages
>  
> +#ifdef CONFIG_ARCH_BINFMT_ELF_STATE
> +struct arch_elf_state {
> +	unsigned int gnu_property;
> +};
> +
> +#define INIT_ARCH_ELF_STATE {	\
> +	.gnu_property = 0,	\
> +}
> +
> +#define arch_elf_pt_proc(ehdr, phdr, elf, interp, state) (0)
> +#define arch_check_elf(ehdr, interp, interp_ehdr, state) (0)
> +#endif
> +
>  /* Do not change the values. See get_align_mask() */
>  enum align_flags {
>  	ALIGN_VA_32	= BIT(0),
> diff --git a/arch/x86/kernel/process_64.c b/arch/x86/kernel/process_64.c
> index 506d66830d4d..99548cde0cc6 100644
> --- a/arch/x86/kernel/process_64.c
> +++ b/arch/x86/kernel/process_64.c
> @@ -732,3 +732,34 @@ unsigned long KSTK_ESP(struct task_struct *task)
>  {
>  	return task_pt_regs(task)->sp;
>  }
> +
> +#ifdef CONFIG_ARCH_USE_GNU_PROPERTY
> +int arch_parse_elf_property(u32 type, const void *data, size_t datasz,
> +			     bool compat, struct arch_elf_state *state)
> +{
> +	if (type != GNU_PROPERTY_X86_FEATURE_1_AND)
> +		return 0;
> +
> +	if (datasz != sizeof(unsigned int))
> +		return -ENOEXEC;
> +
> +	state->gnu_property = *(unsigned int *)data;
> +	return 0;
> +}
> +
> +int arch_setup_elf_property(struct arch_elf_state *state)
> +{
> +	int r = 0;
> +
> +	memset(&current->thread.cet, 0, sizeof(struct cet_status));
> +
> +	if (static_cpu_has(X86_FEATURE_SHSTK)) {
> +		if (state->gnu_property & GNU_PROPERTY_X86_FEATURE_1_SHSTK)
> +			r = cet_setup_shstk();
> +		if (r < 0)
> +			return r;

This test is redundant; there's no loop. This can just fall through to
the final return.

-Kees

> +	}
> +
> +	return r;
> +}
> +#endif
> -- 
> 2.21.0
> 

-- 
Kees Cook