[PATCH v2] ARM: mm: flip priority of CONFIG_DEBUG_RODATA

[Laura Abbott]

On 12/02/2015 12:27 PM, Kees Cook wrote:
> The use of CONFIG_DEBUG_RODATA is generally seen as an essential part of
> kernel self-protection:
> http://www.openwall.com/lists/kernel-hardening/2015/11/30/13
> Additionally, its name has grown to mean things beyond just rodata. To
> get ARM closer to this, we ought to rearrange the names of the configs
> that control how the kernel protects its memory. What was called
> CONFIG_ARM_KERNMEM_PERMS is really doing the work that other architectures
> call CONFIG_DEBUG_RODATA.
>
> This redefines CONFIG_DEBUG_RODATA to actually do the bulk of the
> ROing (and NXing). In the place of the old CONFIG_DEBUG_RODATA, use
> CONFIG_DEBUG_ALIGN_RODATA, since that's what the option does: adds
> section alignment for making rodata explicitly NX, as arm does not split
> the page tables like arm64 does without _ALIGN_RODATA.
>
> Also adds human readable names to the sections so I could more easily
> debug my typos, and makes CONFIG_DEBUG_RODATA default "y" for CPU_V7.
>
> Results in /sys/kernel/debug/kernel_page_tables for each config state:
>
>   # CONFIG_DEBUG_RODATA is not set
>   # CONFIG_DEBUG_ALIGN_RODATA is not set
>
> ---[ Kernel Mapping ]---
> 0x80000000-0x80900000           9M     RW x  SHD
> 0x80900000-0xa0000000         503M     RW NX SHD
>
>   CONFIG_DEBUG_RODATA=y
>   CONFIG_DEBUG_ALIGN_RODATA=y
>
> ---[ Kernel Mapping ]---
> 0x80000000-0x80100000           1M     RW NX SHD
> 0x80100000-0x80700000           6M     ro x  SHD
> 0x80700000-0x80a00000           3M     ro NX SHD
> 0x80a00000-0xa0000000         502M     RW NX SHD
>
>   CONFIG_DEBUG_RODATA=y
>   # CONFIG_DEBUG_ALIGN_RODATA is not set
>
> ---[ Kernel Mapping ]---
> 0x80000000-0x80100000           1M     RW NX SHD
> 0x80100000-0x80a00000           9M     ro x  SHD
> 0x80a00000-0xa0000000         502M     RW NX SHD
>
> Signed-off-by: Kees Cook <keescook at chromium.org>
> ---

Reviewed-by: Laura Abbott <labbott at fedoraproject.org>