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

[Yinghai Lu]

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.

Thanks

Yinghai