Re: [PATCH] arm64: make CONFIG_DEBUG_RODATA non-optional

[Ard Biesheuvel]

On 28 January 2016 at 01:14, Kees Cook <keescook@xxxxxxxxxxxx> wrote:
> On Wed, Jan 27, 2016 at 4:09 PM, David Brown <david.brown@xxxxxxxxxx> wrote:
>> From 2efef8aa0f8f7f6277ffebe4ea6744fc93d54644 Mon Sep 17 00:00:00 2001
>> From: David Brown <david.brown@xxxxxxxxxx>
>> Date: Wed, 27 Jan 2016 13:58:44 -0800
>>
>> This removes the CONFIG_DEBUG_RODATA option and makes it always
>> enabled.
>>
>> Signed-off-by: David Brown <david.brown@xxxxxxxxxx>
>
> I'm all for this!
>

I agree that this is probably a good idea, but please note that Mark
Rutland's pagetable rework patches targeted for v4.6 make significant
changes in this area, so you're probably better off building on top of
those.

-- 
Ard.


> Reviewed-by: Kees Cook <keescook@xxxxxxxxxxxx>
>
> -Kees
>
>> ---
>> v1: This is in the same spirit as the x86 patch, removing allowing
>> this option to be config selected.  The associated patch series adds a
>> runtime option for the same thing.  However, it does affect the way
>> some things are mapped, and could possibly result in either increased
>> memory usage, or a performance hit (due to TLB misses from 4K pages).
>>
>> I've tested this on a Hikey 96board (hi6220-hikey.dtb), both with and
>> without 'rodata=off' on the command line.
>>
>> arch/arm64/Kconfig              |  3 +++
>> arch/arm64/Kconfig.debug        | 10 ----------
>> arch/arm64/kernel/insn.c        |  2 +-
>> arch/arm64/kernel/vmlinux.lds.S |  5 +----
>> arch/arm64/mm/mmu.c             | 12 ------------
>> 5 files changed, 5 insertions(+), 27 deletions(-)
>>
>> diff --git a/arch/arm64/Kconfig b/arch/arm64/Kconfig
>> index 8cc6228..ffa617a 100644
>> --- a/arch/arm64/Kconfig
>> +++ b/arch/arm64/Kconfig
>> @@ -201,6 +201,9 @@ config KERNEL_MODE_NEON
>> config FIX_EARLYCON_MEM
>>         def_bool y
>>
>> +config DEBUG_RODATA
>> +       def_bool y
>> +
>> config PGTABLE_LEVELS
>>         int
>>         default 2 if ARM64_16K_PAGES && ARM64_VA_BITS_36
>> diff --git a/arch/arm64/Kconfig.debug b/arch/arm64/Kconfig.debug
>> index e13c4bf..db994ec 100644
>> --- a/arch/arm64/Kconfig.debug
>> +++ b/arch/arm64/Kconfig.debug
>> @@ -48,16 +48,6 @@ config DEBUG_SET_MODULE_RONX
>>           against certain classes of kernel exploits.
>>           If in doubt, say "N".
>>
>> -config DEBUG_RODATA
>> -       bool "Make kernel text and rodata read-only"
>> -       help
>> -         If this is set, kernel text and rodata will be made read-only.
>> This
>> -         is to help catch accidental or malicious attempts to change the
>> -         kernel's executable code. Additionally splits rodata from kernel
>> -         text so it can be made explicitly non-executable.
>> -
>> -          If in doubt, say Y
>> -
>> config DEBUG_ALIGN_RODATA
>>         depends on DEBUG_RODATA && ARM64_4K_PAGES
>>         bool "Align linker sections up to SECTION_SIZE"
>> diff --git a/arch/arm64/kernel/insn.c b/arch/arm64/kernel/insn.c
>> index 7371455..a04bdef 100644
>> --- a/arch/arm64/kernel/insn.c
>> +++ b/arch/arm64/kernel/insn.c
>> @@ -95,7 +95,7 @@ static void __kprobes *patch_map(void *addr, int fixmap)
>>
>>         if (module && IS_ENABLED(CONFIG_DEBUG_SET_MODULE_RONX))
>>                 page = vmalloc_to_page(addr);
>> -       else if (!module && IS_ENABLED(CONFIG_DEBUG_RODATA))
>> +       else if (!module)
>>                 page = virt_to_page(addr);
>>         else
>>                 return addr;
>> diff --git a/arch/arm64/kernel/vmlinux.lds.S
>> b/arch/arm64/kernel/vmlinux.lds.S
>> index e3928f5..f80903c 100644
>> --- a/arch/arm64/kernel/vmlinux.lds.S
>> +++ b/arch/arm64/kernel/vmlinux.lds.S
>> @@ -65,12 +65,9 @@ PECOFF_FILE_ALIGNMENT = 0x200;
>> #if defined(CONFIG_DEBUG_ALIGN_RODATA)
>> #define ALIGN_DEBUG_RO                  . = ALIGN(1<<SECTION_SHIFT);
>> #define ALIGN_DEBUG_RO_MIN(min)         ALIGN_DEBUG_RO
>> -#elif defined(CONFIG_DEBUG_RODATA)
>> +#else
>> #define ALIGN_DEBUG_RO                  . = ALIGN(1<<PAGE_SHIFT);
>> #define ALIGN_DEBUG_RO_MIN(min)         ALIGN_DEBUG_RO
>> -#else
>> -#define ALIGN_DEBUG_RO
>> -#define ALIGN_DEBUG_RO_MIN(min)                . = ALIGN(min);
>> #endif
>>
>> SECTIONS
>> diff --git a/arch/arm64/mm/mmu.c b/arch/arm64/mm/mmu.c
>> index 58faeaa..3b411b7 100644
>> --- a/arch/arm64/mm/mmu.c
>> +++ b/arch/arm64/mm/mmu.c
>> @@ -313,7 +313,6 @@ static void create_mapping_late(phys_addr_t phys,
>> unsigned long virt,
>>                                 phys, virt, size, prot, late_alloc);
>> }
>>
>> -#ifdef CONFIG_DEBUG_RODATA
>> static void __init __map_memblock(phys_addr_t start, phys_addr_t end)
>> {
>>         /*
>> @@ -347,13 +346,6 @@ static void __init __map_memblock(phys_addr_t start,
>> phys_addr_t end)
>>         }
>>
>> }
>> -#else
>> -static void __init __map_memblock(phys_addr_t start, phys_addr_t end)
>> -{
>> -       create_mapping(start, __phys_to_virt(start), end - start,
>> -                       PAGE_KERNEL_EXEC);
>> -}
>> -#endif
>>
>> static void __init map_mem(void)
>> {
>> @@ -410,7 +402,6 @@ static void __init map_mem(void)
>>
>> static void __init fixup_executable(void)
>> {
>> -#ifdef CONFIG_DEBUG_RODATA
>>         /* now that we are actually fully mapped, make the start/end more
>> fine grained */
>>         if (!IS_ALIGNED((unsigned long)_stext, SWAPPER_BLOCK_SIZE)) {
>>                 unsigned long aligned_start = round_down(__pa(_stext),
>> @@ -428,10 +419,8 @@ static void __init fixup_executable(void)
>>                                 aligned_end - __pa(__init_end),
>>                                 PAGE_KERNEL);
>>         }
>> -#endif
>> }
>>
>> -#ifdef CONFIG_DEBUG_RODATA
>> void mark_rodata_ro(void)
>> {
>>         create_mapping_late(__pa(_stext), (unsigned long)_stext,
>> @@ -439,7 +428,6 @@ void mark_rodata_ro(void)
>>                                 PAGE_KERNEL_ROX);
>>
>> }
>> -#endif
>>
>> void fixup_init(void)
>> {
>> --
>> 2.7.0
>>
>
>
>
> --
> Kees Cook
> Chrome OS & Brillo Security