Re: [PATCH 2/2] x86/KASLR: Use old ident map page table if physical randomization failed

[Kees Cook]

On Wed, Apr 26, 2017 at 3:39 AM, Baoquan He <bhe@xxxxxxxxxx> wrote:
> Dave found when kdump kernel will reset to bios immediately if kaslr
> is enabled and physical randomization failed to faind a new position
> for kernel. But nokaslr works in this case.
>
> The reason is kaslr will install a new page table for ident mapping,
> while it missed to consider building ident mapping for original area
> of kernel if kaslr failed on physical randomization.
>
> In fact bootloaders including kexec/kdump have built ident mapping
> for original place of kernel. We can only install new ident mapping
> page table when physical kaslr succeeds. Otherwise we just keep the
> old page table unchanged just like nokaslr does.
>
> Signed-off-by: Baoquan He <bhe@xxxxxxxxxx>
> Signed-off-by: Dave Young <dyoung@xxxxxxxxxx>
> Cc: "H. Peter Anvin" <hpa@xxxxxxxxx>
> Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
> Cc: Ingo Molnar <mingo@xxxxxxxxxx>
> Cc: x86@xxxxxxxxxx
> Cc: Kees Cook <keescook@xxxxxxxxxxxx>
> Cc: Yinghai Lu <yinghai@xxxxxxxxxx>
> Cc: Borislav Petkov <bp@xxxxxxx>
> Cc: Dave Jiang <dave.jiang@xxxxxxxxx>
> Cc: Thomas Garnier <thgarnie@xxxxxxxxxx>

Nice catch!

Acked-by: Kees Cook <keescook@xxxxxxxxxxxx>

-Kees

> ---
>  arch/x86/boot/compressed/kaslr.c | 10 ++++++++--
>  1 file changed, 8 insertions(+), 2 deletions(-)
>
> diff --git a/arch/x86/boot/compressed/kaslr.c b/arch/x86/boot/compressed/kaslr.c
> index e5eb0c3..7a8b443 100644
> --- a/arch/x86/boot/compressed/kaslr.c
> +++ b/arch/x86/boot/compressed/kaslr.c
> @@ -650,10 +650,16 @@ void choose_random_location(unsigned long input,
>                         add_identity_map(random_addr, output_size);
>                         *output = random_addr;
>                 }
> +
> +               /*
> +                * This actually loads the identity pagetable on x86_64.
> +                * And this should only be done only if a new position
> +                * is found. Otherwise we should keep the old page table
> +                * to make it be like nokaslr case.
> +                */
> +               finalize_identity_maps();
>         }
>
> -       /* This actually loads the identity pagetable on x86_64. */
> -       finalize_identity_maps();
>
>         /* Pick random virtual address starting from LOAD_PHYSICAL_ADDR. */
>         if (IS_ENABLED(CONFIG_X86_64))
> --
> 2.5.5
>



-- 
Kees Cook
Pixel Security