[PATCH] ARM: mm: flip priority of CONFIG_DEBUG_RODATA

[Kees Cook]

On Mon, Nov 30, 2015 at 5:20 PM, Laura Abbott <labbott at redhat.com> wrote:
> On 11/30/2015 05:08 PM, Kees Cook wrote:
>>
>> On Mon, Nov 30, 2015 at 5:03 PM, Laura Abbott <labbott at redhat.com> wrote:
>>>
>>> On 11/30/2015 03:38 PM, Kees Cook wrote:
>>>>
>>>>
>>>> Given the choice between making things NX or making things RO, we want
>>>> RO first. As such, redefine CONFIG_DEBUG_RODATA to actually do the bulk
>>>
>>>
>>>
>>> Can you give a citation for why? The thread that inspired it might be
>>> a good link.
>>
>>
>> This was inspired by my examining the existing architecture's
>> implementations of CONFIG_DEBUG_RODATA after Ingo suggested it be made
>> a common feature not a build-time config (or at least renamed):
>> http://www.openwall.com/lists/kernel-hardening/2015/11/30/13
>>
>
> Thanks. I read the thread and I think it would be good to put a link
> in the commit message to make it clearer why this is going in.

Okay, I will add this (and a summary, as Nicolas suggested).

>>>> index 41218867a9a6..b617084e9520 100644
>>>> --- a/arch/arm/mm/Kconfig
>>>> +++ b/arch/arm/mm/Kconfig
>>>> @@ -1039,24 +1039,26 @@ config ARCH_SUPPORTS_BIG_ENDIAN
>>>>            This option specifies the architecture can support big endian
>>>>            operation.
>>>>
>>>> -config ARM_KERNMEM_PERMS
>>>> -       bool "Restrict kernel memory permissions"
>>>> +config DEBUG_RODATA
>>>> +       bool "Make kernel text and rodata read-only"
>>>>          depends on MMU
>>>> +       default y if CPU_V7
>>>>          help
>>>> -         If this is set, kernel memory other than kernel text (and
>>>> rodata)
>>>> -         will be made non-executable. The tradeoff is that each region
>>>> is
>>>> -         padded to section-size (1MiB) boundaries (because their
>>>> permissions
>>>> -         are different and splitting the 1M pages into 4K ones causes
>>>> TLB
>>>> -         performance problems), wasting memory.
>>>> +         If this is set, kernel memory (text, rodata, etc) will be made
>>>> +         read-only, and non-text kernel memory will be made
>>>> non-executable.
>>>> +         The tradeoff is that each region is padded to section-size
>>>> (1MiB)
>>>> +         boundaries (because their permissions are different and
>>>> splitting
>>>> +         the 1M pages into 4K ones causes TLB performance problems),
>>>> which
>>>> +         can waste memory.
>>>>
>>>> -config DEBUG_RODATA
>>>> -       bool "Make kernel text and rodata read-only"
>>>> -       depends on ARM_KERNMEM_PERMS
>>>> +config DEBUG_ALIGN_RODATA
>>>> +       bool "Make rodata strictly non-executable"
>>>> +       depends on DEBUG_RODATA
>>>>          default y
>>>>          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. This creates
>>>> -         another section-size padded region, so it can waste more
>>>> memory
>>>> -         space while gaining the read-only protections.
>>>> +         If this is set, rodata will be made explicitly non-executable.
>>>> This
>>>> +         provides protection on the rare chance that attackers might
>>>> find
>>>> and
>>>> +         use ROP gadgets that exist in the rodata section. This adds an
>>>> +         additional section-aligned split of rodata from kernel text so
>>>> it
>>>> +         can be made explicitly non-executable. This padding may waste
>>>> memory
>>>> +         space to gain this additional protection.
>>>
>>>
>>>
>>> I get that you want to make this match arm64 but it's really not
>>> intuitive that
>>> something with ALIGN_RODATA in the name is actually for setting NX. The
>>> purpose
>>> of ALIGN_RODATA was also slightly different on arm64 since the RO/NX will
>>> still
>>> be there, the difference is if the sections are present versus broken
>>> down into
>>> pages.
>>
>>
>> Well, it seems to have the same effect: without the alignment, a
>> portion of rodata may remain executable on arm64. Unless I
>> misunderstand?
>>
>
> No, on arm64 everything should always be NX, the difference is part of the
> NX
> sections may be mapped as pages instead of sections so you take the TLB hit.
> It's a trade off of memory vs TLB pressure instead of just security vs TLB.

Ah! I understand now. I will clarify the commit and Kconfig text on
ARM. I'd still prefer to keep the name, since it's still doing
section-alignment of rodata, it's just that without ARM's
ALIGN_RODATA, you get an executable rodata section.

-Kees

-- 
Kees Cook
Chrome OS & Brillo Security