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

[hpa@zytor.com]

On January 17, 2019 1:43:54 PM PST, Nadav Amit <nadav.amit@gmail.com> wrote:
>> On Jan 17, 2019, at 12:47 PM, Andy Lutomirski <luto@kernel.org>
>wrote:
>> 
>> On Thu, Jan 17, 2019 at 12:27 PM Andy Lutomirski <luto@kernel.org>
>wrote:
>>> On Wed, Jan 16, 2019 at 4:33 PM Rick Edgecombe
>>> <rick.p.edgecombe@intel.com> wrote:
>>>> From: Nadav Amit <namit@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.
>> 
>> 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
>didn’t
>matter before (since __flush_tlb_one_user would get rid of it no matter
>what), but would matter now, so I’ll change it too.
>
>Thanks!
>
>Nadav

You can just disable the global bit at the top level, obviously.

This approach also should make it far easier to do batching if desired.
-- 
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