[PATCH 06/17] x86/alternative: use temporary mm for text poking

[Andy Lutomirski]

On Wed, Jan 16, 2019 at 4:33 PM Rick Edgecombe
<rick.p.edgecombe at intel.com> wrote:
>
> From: Nadav Amit <namit at vmware.com>
>
> text_poke() can potentially compromise the security as it sets temporary
> PTEs in the fixmap. These PTEs might be used to rewrite the kernel code
> from other cores accidentally or maliciously, if an attacker gains the
> ability to write onto kernel memory.

i think this may be sufficient, but barely.

> +       pte_clear(poking_mm, poking_addr, ptep);
> +
> +       /*
> +        * __flush_tlb_one_user() performs a redundant TLB flush when PTI is on,
> +        * as it also flushes the corresponding "user" address spaces, which
> +        * does not exist.
> +        *
> +        * Poking, however, is already very inefficient since it does not try to
> +        * batch updates, so we ignore this problem for the time being.
> +        *
> +        * Since the PTEs do not exist in other kernel address-spaces, we do
> +        * not use __flush_tlb_one_kernel(), which when PTI is on would cause
> +        * more unwarranted TLB flushes.
> +        *
> +        * There is a slight anomaly here: the PTE is a supervisor-only and
> +        * (potentially) global and we use __flush_tlb_one_user() but this
> +        * should be fine.
> +        */
> +       __flush_tlb_one_user(poking_addr);
> +       if (cross_page_boundary) {
> +               pte_clear(poking_mm, poking_addr + PAGE_SIZE, ptep + 1);
> +               __flush_tlb_one_user(poking_addr + PAGE_SIZE);
> +       }

In principle, another CPU could still have the old translation.  Your
mutex probably makes this impossible, but it makes me nervous.
Ideally you'd use flush_tlb_mm_range(), but I guess you can't do that
with IRQs off.  Hmm.  I think you should add an inc_mm_tlb_gen() here.
Arguably, if you did that, you could omit the flushes, but maybe
that's silly.

If we start getting new users of use_temporary_mm(), we should give
some serious thought to the SMP semantics.

Also, you're using PAGE_KERNEL.  Please tell me that the global bit
isn't set in there.

--Andy