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

[Nadav Amit]

> On Jan 17, 2019, at 12:47 PM, Andy Lutomirski <luto@xxxxxxxxxx> wrote:
> 
> On Thu, Jan 17, 2019 at 12:27 PM Andy Lutomirski <luto@xxxxxxxxxx> wrote:
>> On Wed, Jan 16, 2019 at 4:33 PM Rick Edgecombe
>> <rick.p.edgecombe@xxxxxxxxx> wrote:
>>> From: Nadav Amit <namit@xxxxxxxxxx>
>>> 
>>> 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.
> 
> Much better solution: do unuse_temporary_mm() and *then*
> flush_tlb_mm_range().  This is entirely non-sketchy and should be just
> about optimal, too.

This solution sounds nice and clean. The fact the global-bit was set didnât
matter before (since __flush_tlb_one_user would get rid of it no matter
what), but would matter now, so Iâll change it too.

Thanks!

Nadav