Re: [PATCH v7 1/9] x86/KASLR: Initialize mapping_info every time

[Kees Cook]

On Mon, May 9, 2016 at 3:01 PM, Yinghai Lu <yinghai@xxxxxxxxxx> wrote:
> On Mon, May 9, 2016 at 1:22 PM, Kees Cook <keescook@xxxxxxxxxxxx> wrote:
>> As it turns out, mapping_info DOES need to be initialized every
>> time. Without this, page tables were not being corrected updated, which
>> could cause reboots when a physical address beyond 2G was chosen.
>>
>> Signed-off-by: Kees Cook <keescook@xxxxxxxxxxxx>
>> ---
>>  arch/x86/boot/compressed/pagetable.c | 16 +++++-----------
>>  1 file changed, 5 insertions(+), 11 deletions(-)
>>
>> diff --git a/arch/x86/boot/compressed/pagetable.c b/arch/x86/boot/compressed/pagetable.c
>> index 3c99051566a9..34b95df14e69 100644
>> --- a/arch/x86/boot/compressed/pagetable.c
>> +++ b/arch/x86/boot/compressed/pagetable.c
>> @@ -90,23 +90,17 @@ static void prepare_level4(void)
>>  }
>>
>>  /*
>> - * Mapping information structure passed to kernel_ident_mapping_init().
>> - * Since this never changes, there's no reason to repeatedly fill it
>> - * in on the stack when calling add_identity_map().
>> - */
>> -static struct x86_mapping_info mapping_info = {
>> -       .alloc_pgt_page = alloc_pgt_page,
>> -       .context        = &pgt_data,
>> -       .pmd_flag       = __PAGE_KERNEL_LARGE_EXEC,
>> -};
>> -
>> -/*
>>   * Adds the specified range to what will become the new identity mappings.
>>   * Once all ranges have been added, the new mapping is activated by calling
>>   * finalize_identity_maps() below.
>>   */
>>  void add_identity_map(unsigned long start, unsigned long size)
>>  {
>> +       struct x86_mapping_info mapping_info = {
>> +               .alloc_pgt_page = alloc_pgt_page,
>> +               .context        = &pgt_data,
>> +               .pmd_flag       = __PAGE_KERNEL_LARGE_EXEC,
>> +       };
>>         unsigned long end = start + size;
>
> Still should be assigned once, and should be done in
> prepare_level4(). --- by may need different name for that ?
>
> The exact reason to have assigning in functions.
>
> pgt_data address could be changed during kernel relocation.
> so can not assigned during compiling time.

Ah-ha, that explains why I had to keep it on the stack. Thanks for the
clarification!

-Kees

-- 
Kees Cook
Chrome OS & Brillo Security