Re: [RFC][PATCH 08/10] x86/mm: do not forbid _PAGE_RW before init for __ro_after_init

[Kees Cook]

On Thu, Feb 22, 2018 at 12:37 PM, Dave Hansen
<dave.hansen@xxxxxxxxxxxxxxx> wrote:
>
> From: Dave Hansen <dave.hansen@xxxxxxxxxxxxxxx>
>
> __ro_after_init data gets stuck in the .rodata section.  That's normally
> fine because the kernel itself manages the R/W properties.
>
> But, if we run __change_page_attr() on an area which is __ro_after_init,
> the .rodata checks will trigger and force the area to be immediately
> read-only, even if it is early-ish in boot.  This caused problems when
> trying to clear the _PAGE_GLOBAL bit for these area in the PTI code:
> it cleared _PAGE_GLOBAL like I asked, but also took it up on itself
> to clear _PAGE_RW.  The kernel then oopses the next time it wrote to
> a __ro_after_init data structure.
>
> To fix this, add the kernel_set_to_readonly check, just like we have
> for kernel text, just a few lines below in this function.

Yup, looks sensible. Thanks!

>
> Signed-off-by: Dave Hansen <dave.hansen@xxxxxxxxxxxxxxx>
> Cc: Andrea Arcangeli <aarcange@xxxxxxxxxx>
> Cc: Andy Lutomirski <luto@xxxxxxxxxx>
> Cc: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
> Cc: Kees Cook <keescook@xxxxxxxxxx>
> Cc: Hugh Dickins <hughd@xxxxxxxxxx>
> Cc: Juergen Gross <jgross@xxxxxxxx>
> Cc: x86@xxxxxxxxxx
> Cc: Nadav Amit <namit@xxxxxxxxxx>

Acked-by: Kees Cook <keescook@xxxxxxxxxxxx>

-Kees

> ---
>
>  b/arch/x86/mm/pageattr.c |    6 ++++--
>  1 file changed, 4 insertions(+), 2 deletions(-)
>
> diff -puN arch/x86/mm/pageattr.c~check-kernel_set_to_readonly arch/x86/mm/pageattr.c
> --- a/arch/x86/mm/pageattr.c~check-kernel_set_to_readonly       2018-02-22 12:36:21.531036546 -0800
> +++ b/arch/x86/mm/pageattr.c    2018-02-22 12:36:21.535036546 -0800
> @@ -298,9 +298,11 @@ static inline pgprot_t static_protection
>
>         /*
>          * The .rodata section needs to be read-only. Using the pfn
> -        * catches all aliases.
> +        * catches all aliases.  This also includes __ro_after_init,
> +        * so do not enforce until kernel_set_to_readonly is true.
>          */
> -       if (within(pfn, __pa_symbol(__start_rodata) >> PAGE_SHIFT,
> +       if (kernel_set_to_readonly &&
> +           within(pfn, __pa_symbol(__start_rodata) >> PAGE_SHIFT,
>                    __pa_symbol(__end_rodata) >> PAGE_SHIFT))
>                 pgprot_val(forbidden) |= _PAGE_RW;
>
> _



-- 
Kees Cook
Pixel Security