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Date: Mon, 13 Feb 2017 10:09:52 -0800
From: Kees Cook <keescook@...omium.org>
To: Eddie Kovsky <ewk@...ovsky.org>
Cc: Jessica Yu <jeyu@...hat.com>, Rusty Russell <rusty@...tcorp.com.au>, 
	"kernel-hardening@...ts.openwall.com" <kernel-hardening@...ts.openwall.com>
Subject: Re: [RFC] [PATCH 2/2] extable: verify address is read-only

On Sun, Feb 12, 2017 at 3:31 PM, Eddie Kovsky <ewk@...ovsky.org> wrote:
> Provide a mechanism to check if the address of a variable is
> const or ro_after_init. It mimics the existing functions that test if an
> address is inside the kernel's text section.
>
> Signed-off-by: Eddie Kovsky <ewk@...ovsky.org>
> ---
>  include/linux/kernel.h |  2 ++
>  kernel/extable.c       | 31 +++++++++++++++++++++++++++++++
>  2 files changed, 33 insertions(+)
>
> diff --git a/include/linux/kernel.h b/include/linux/kernel.h
> index 4c26dc3a8295..51beea39e6c4 100644
> --- a/include/linux/kernel.h
> +++ b/include/linux/kernel.h
> @@ -444,6 +444,8 @@ extern int core_kernel_data(unsigned long addr);
>  extern int __kernel_text_address(unsigned long addr);
>  extern int kernel_text_address(unsigned long addr);
>  extern int func_ptr_is_kernel_text(void *ptr);
> +extern int core_kernel_ro_data(unsigned long addr);
> +extern int kernel_ro_address(unsigned long addr);
>
>  unsigned long int_sqrt(unsigned long);
>
> diff --git a/kernel/extable.c b/kernel/extable.c
> index 6b0d09051efb..f5a29c4ae391 100644
> --- a/kernel/extable.c
> +++ b/kernel/extable.c
> @@ -149,3 +149,34 @@ int func_ptr_is_kernel_text(void *ptr)
>                 return 1;
>         return is_module_text_address(addr);
>  }
> +
> +/**
> + * core_kernel_ro_data - Verify address points to read-only section
> + * @addr: address to test
> + *
> + */
> +int core_kernel_ro_data(unsigned long addr)
> +{
> +       if (addr >= (unsigned long)__start_rodata &&
> +           addr < (unsigned long)__end_rodata)
> +               return 1;
> +
> +       if (addr >= (unsigned long)__start_data_ro_after_init &&
> +           addr < (unsigned long)__end_data_ro_after_init)
> +               return 1;
> +
> +       return 0;
> +}
> +
> +/* Verify that address is const or ro_after_init. */
> +int kernel_ro_address(unsigned long addr)
> +{
> +       if (core_kernel_ro_data(addr))
> +               return 1;
> +       if (is_module_ro_address(addr))
> +               return 1;
> +       if (is_ftrace_trampoline(addr))
> +               return 1;

Why the trampoline test here?

> +
> +       return 0;
> +}
> --
> 2.11.1

Otherwise looks exactly like what I had in mind; nice!

-Kees

-- 
Kees Cook
Pixel Security

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