[PATCH v3 6/7] arm64: map linear region as non-executable

[Ard Biesheuvel]

On 7 December 2015 at 17:19, Catalin Marinas <catalin.marinas at arm.com> wrote:
> On Mon, Nov 16, 2015 at 12:23:17PM +0100, Ard Biesheuvel wrote:
>> diff --git a/arch/arm64/mm/mmu.c b/arch/arm64/mm/mmu.c
>> index c7ba171951c8..526eeb7e1e97 100644
>> --- a/arch/arm64/mm/mmu.c
>> +++ b/arch/arm64/mm/mmu.c
>> @@ -357,47 +357,10 @@ 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)
>>  {
>> -     /*
>> -      * Set up the executable regions using the existing section mappings
>> -      * for now. This will get more fine grained later once all memory
>> -      * is mapped
>> -      */
>> -     unsigned long kernel_x_start = round_down(__pa(_stext), SWAPPER_BLOCK_SIZE);
>> -     unsigned long kernel_x_end = round_up(__pa(__init_end), SWAPPER_BLOCK_SIZE);
>> -
>> -     if (end < kernel_x_start) {
>> -             create_mapping(start, __phys_to_virt(start),
>> -                     end - start, PAGE_KERNEL);
>> -     } else if (start >= kernel_x_end) {
>> -             create_mapping(start, __phys_to_virt(start),
>> -                     end - start, PAGE_KERNEL);
>> -     } else {
>> -             if (start < kernel_x_start)
>> -                     create_mapping(start, __phys_to_virt(start),
>> -                             kernel_x_start - start,
>> -                             PAGE_KERNEL);
>> -             create_mapping(kernel_x_start,
>> -                             __phys_to_virt(kernel_x_start),
>> -                             kernel_x_end - kernel_x_start,
>> -                             PAGE_KERNEL_EXEC);
>> -             if (kernel_x_end < end)
>> -                     create_mapping(kernel_x_end,
>> -                             __phys_to_virt(kernel_x_end),
>> -                             end - kernel_x_end,
>> -                             PAGE_KERNEL);
>> -     }
>> -
>> -}
>> -#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);
>> +     create_mapping(start, __phys_to_virt(start), end - start, PAGE_KERNEL);
>>  }
>> -#endif
>>
>>  struct bootstrap_pgtables {
>>       pte_t   pte[PTRS_PER_PTE];
>> @@ -471,7 +434,7 @@ static unsigned long __init bootstrap_region(struct bootstrap_pgtables *reg,
>>                                     SWAPPER_BLOCK_SIZE));
>>
>>               create_mapping(__pa(vstart - va_offset), vstart, vend - vstart,
>> -                            PAGE_KERNEL_EXEC);
>> +                            PAGE_KERNEL);
>>
>>               return vend;
>>       }
>
> These make sense. However, shall we go a step further and unmap the
> kernel image completely from the linear mapping, maybe based on
> CONFIG_DEBUG_RODATA? The mark_rodata_ro() function changes the text to
> read-only but you can still get writable access to it via
> __va(__pa(_stext)).
>

If we can tolerate the fragmentation, then yes, let's unmap it
completely. As long as we don't unmap the.pgdir section, since that
will be referenced via the linear mapping