Re: [RFC][PATCH] s390, postinit-readonly: implement post-init RO

[Kees Cook]

On Mon, Mar 7, 2016 at 4:20 PM, Kees Cook <keescook@xxxxxxxxxxxx> wrote:
> Since s390 already sets its .rodata section RO from the start, the generic
> .data..ro_after_init section is already RO before init runs. For s390,
> split the post-init read-only section off separately and handle that
> when the call to mark_rodata_ro() is made.
>
> Signed-off-by: Kees Cook <keescook@xxxxxxxxxxxx>
> ---
> This is totally untested...
> ---
>  arch/s390/Kconfig                |  3 +++
>  arch/s390/include/asm/cache.h    |  2 ++
>  arch/s390/include/asm/sections.h |  2 +-
>  arch/s390/kernel/vmlinux.lds.S   |  6 ++++++
>  arch/s390/mm/init.c              | 10 ++++++++++
>  5 files changed, 22 insertions(+), 1 deletion(-)
>
> diff --git a/arch/s390/Kconfig b/arch/s390/Kconfig
> index 3be9c832dec1..3f8b96f2cd2d 100644
> --- a/arch/s390/Kconfig
> +++ b/arch/s390/Kconfig
> @@ -59,6 +59,9 @@ config PCI_QUIRKS
>  config ARCH_SUPPORTS_UPROBES
>         def_bool y
>
> +config DEBUG_RODATA
> +       def_bool y
> +
>  config S390
>         def_bool y
>         select ARCH_HAS_ATOMIC64_DEC_IF_POSITIVE
> diff --git a/arch/s390/include/asm/cache.h b/arch/s390/include/asm/cache.h
> index 4d7ccac5fd1d..816c2964bbee 100644
> --- a/arch/s390/include/asm/cache.h
> +++ b/arch/s390/include/asm/cache.h
> @@ -15,4 +15,6 @@
>
>  #define __read_mostly __attribute__((__section__(".data..read_mostly")))
>
> +#define __ro_after_init __attribute__((__section__(".arch_ro_after_init")))
> +
>  #endif
> diff --git a/arch/s390/include/asm/sections.h b/arch/s390/include/asm/sections.h
> index fbd9116eb17b..6cc6acf87416 100644
> --- a/arch/s390/include/asm/sections.h
> +++ b/arch/s390/include/asm/sections.h
> @@ -3,6 +3,6 @@
>
>  #include <asm-generic/sections.h>
>
> -extern char _eshared[], _ehead[];
> +extern char _eshared[], _ehead[], __ro_after_init[];

0day points out this should be _ro_after_init

>
>  #endif
> diff --git a/arch/s390/kernel/vmlinux.lds.S b/arch/s390/kernel/vmlinux.lds.S
> index 445657fe658c..39a2c7e4cdd2 100644
> --- a/arch/s390/kernel/vmlinux.lds.S
> +++ b/arch/s390/kernel/vmlinux.lds.S
> @@ -52,6 +52,12 @@ SECTIONS
>
>         RW_DATA_SECTION(0x100, PAGE_SIZE, THREAD_SIZE)
>
> +       . = ALIGN(PAGE_SIZE)
> +       __ro_after_init = .;

and here

> +       .arch_ro_after_init : {
> +               *(.arch_ro_after_init)  /* Read only after init */
> +       }
> +
>         _edata = .;             /* End of data section */
>
>         /* will be freed after init */
> diff --git a/arch/s390/mm/init.c b/arch/s390/mm/init.c
> index 73e290337092..6033d396b96c 100644
> --- a/arch/s390/mm/init.c
> +++ b/arch/s390/mm/init.c
> @@ -136,6 +136,16 @@ void free_initmem(void)
>         free_initmem_default(POISON_FREE_INITMEM);
>  }
>
> +void mark_rodata_ro(void)
> +{
> +       unsigned long start = (unsigned long) &__ro_after_init;

and here. :)

> +       unsigned long end = (unsigned long) &_edata;
> +
> +       printk(KERN_INFO "Write protecting post-init read-only data: %luk\n",
> +              (end - start) >> 10);
> +       set_memory_ro(start, (end - start) >> PAGE_SHIFT);
> +}
> +
>  #ifdef CONFIG_BLK_DEV_INITRD
>  void __init free_initrd_mem(unsigned long start, unsigned long end)
>  {
> --
> 2.6.3
>
>
> --
> Kees Cook
> Chrome OS & Brillo Security

I will go try Andy's virtme next... ;)

-Kees


-- 
Kees Cook
Chrome OS & Brillo Security