Re: [PATCHv3 15/27] x86/vdso: Allocate timens vdso

[Jann Horn <jannh@google.com>]

On Thu, Apr 25, 2019 at 6:14 PM Dmitry Safonov <dima@arista.com> wrote:
>
> As it has been discussed on timens RFC, adding a new conditional branch
> `if (inside_time_ns)` on VDSO for all processes is undesirable.
> It will add a penalty for everybody as branch predictor may mispredict
> the jump. Also there are instruction cache lines wasted on cmp/jmp.
>
> Those effects of introducing time namespace are very much unwanted
> having in mind how much work have been spent on micro-optimisation
> vdso code.
>
> The propose is to allocate a second vdso code with dynamically
> patched out (disabled by static_branch) timens code on boot time.
>
> Allocate another vdso and copy original code.
[...]
> diff --git a/arch/x86/entry/vdso/vma.c b/arch/x86/entry/vdso/vma.c
> index 80cbb2167eba..6aae9c0d400d 100644
> --- a/arch/x86/entry/vdso/vma.c
> +++ b/arch/x86/entry/vdso/vma.c
[...]
>  static vm_fault_t vdso_fault(const struct vm_special_mapping *sm,
>                       struct vm_area_struct *vma, struct vm_fault *vmf)
>  {
>         const struct vdso_image *image = vma->vm_mm->context.vdso_image;
> +       unsigned long offset = vmf->pgoff << PAGE_SHIFT;
>
>         if (!image || (vmf->pgoff << PAGE_SHIFT) >= image->size)
>                 return VM_FAULT_SIGBUS;
>
> -       vmf->page = virt_to_page(image->text + (vmf->pgoff << PAGE_SHIFT));
> +       if (current_timens_offsets() && image->text_timens)

I'm pretty sure that accessing `current` in here is wrong. AFAIK this
fault handler can be invoked on remote processes, through interfaces
like /proc/$pid/mem and process_vm_readv(); in that case, the kernel
should install a page based on the time namespace of the target
process, not based on the time namespace of the caller.

> +               vmf->page = vmalloc_to_page(image->text_timens + offset);
> +       else
> +               vmf->page = virt_to_page(image->text + offset);
> +
>         get_page(vmf->page);
>         return 0;
>  }
[...]