[PATCH] ARM: mm: mark section-aligned portion of rodata NX

[Ard Biesheuvel]

On 7 December 2015 at 22:56, Kees Cook <keescook at chromium.org> wrote:
> On Mon, Dec 7, 2015 at 1:52 PM, Ard Biesheuvel
> <ard.biesheuvel at linaro.org> wrote:
>> On 7 December 2015 at 21:54, Kees Cook <keescook at chromium.org> wrote:
>>> When rodata is large enough that it crosses a section boundary after the
>>> kernel text, mark the rest NX. This is as close to full NX of rodata as
>>> we can get without splitting page tables or doing section alignment via
>>> CONFIG_DEBUG_ALIGN_RODATA.
>>>
>>> Signed-off-by: Kees Cook <keescook at chromium.org>
>>> ---
>>> I am baffled why I can't put the ALIGN in the ".start = " initializer.
>>> GCC seems to think it's non-static, but only because of the "&" operator.
>>> Does anyone see a way to do this that doesn't require the runtime ALIGN?
>>
>> Yes, but you need to move the ALIGN() expression to the linker script
>> as the value of a new symbol
>
> I didn't see a way to do this without actually bumping the alignment
> (CONFIG_DEBUG_ALIGN_RODATA already does that, I want the other case).
> Do you have an example anywhere I could look at?
>

Something like this (untested)

diff --git a/arch/arm/kernel/vmlinux.lds.S b/arch/arm/kernel/vmlinux.lds.S
index 8b60fde5ce48..9fb98c5dd35b 100644
--- a/arch/arm/kernel/vmlinux.lds.S
+++ b/arch/arm/kernel/vmlinux.lds.S
@@ -336,6 +336,8 @@ SECTIONS
  STABS_DEBUG
 }

+__start_rodata_section_aligned = ALIGN(__start_rodata, 1 << SECTION_SHIFT);
+
 /*
  * These must never be empty
  * If you have to comment these two assert statements out, your
diff --git a/arch/arm/mm/init.c b/arch/arm/mm/init.c
index 8a63b4cdc0f2..ddacd3bd8f8b 100644
--- a/arch/arm/mm/init.c
+++ b/arch/arm/mm/init.c
@@ -592,15 +592,13 @@ static struct section_perm nx_perms[] = {
  .mask = ~PMD_SECT_XN,
  .prot = PMD_SECT_XN,
  },
-#ifdef CONFIG_DEBUG_RODATA
  /* Make rodata NX (set RO in ro_perms below). */
  {
- .start  = (unsigned long)__start_rodata,
+ .start  = (unsigned long)__start_rodata_section_aligned,
  .end    = (unsigned long)__init_begin,
  .mask   = ~PMD_SECT_XN,
  .prot   = PMD_SECT_XN,
  },
-#endif
 };

 #ifdef CONFIG_DEBUG_RODATA