Re: [RFC PATCH 3/3] Prevent user from writing to IBT bitmap.

[Andy Lutomirski]

On Fri, Jun 28, 2019 at 12:50 PM Yu-cheng Yu <yu-cheng.yu@xxxxxxxxx> wrote:
>
> The IBT bitmap is visiable from user-mode, but not writable.
>
> Signed-off-by: Yu-cheng Yu <yu-cheng.yu@xxxxxxxxx>
>
> ---
>  arch/x86/mm/fault.c | 7 +++++++
>  1 file changed, 7 insertions(+)
>
> diff --git a/arch/x86/mm/fault.c b/arch/x86/mm/fault.c
> index 59f4f66e4f2e..231196abb62e 100644
> --- a/arch/x86/mm/fault.c
> +++ b/arch/x86/mm/fault.c
> @@ -1454,6 +1454,13 @@ void do_user_addr_fault(struct pt_regs *regs,
>          * we can handle it..
>          */
>  good_area:
> +#define USER_MODE_WRITE (FAULT_FLAG_WRITE | FAULT_FLAG_USER)
> +       if (((flags & USER_MODE_WRITE)  == USER_MODE_WRITE) &&
> +           (vma->vm_flags & VM_IBT)) {
> +               bad_area_access_error(regs, hw_error_code, address, vma);
> +               return;
> +       }
> +

Just make the VMA have VM_WRITE and VM_MAYWRITE clear.  No new code
like this should be required.