Re: [RFC 00/10] Process-local memory allocations for hiding KVM secrets

[Nadav Amit]

> On Jun 13, 2019, at 9:20 AM, Dave Hansen <dave.hansen@xxxxxxxxx> wrote:
> 
> On 6/13/19 9:13 AM, Andy Lutomirski wrote:
>>> It might make sense to use it for kmap_atomic() for debug purposes, as
>>> it ensures that other users can no longer access the same mapping
>>> through the linear map. However, it does come at quite a big cost, as we
>>> need to shoot down the TLB of all other threads in the system. So I'm
>>> not sure it's of general value?
>> What I meant was that kmap_atomic() could use mm-local memory so that
>> it doesn't need to do a global shootdown.  But I guess it's not
>> actually used for real on 64-bit, so this is mostly moot.  Are you
>> planning to support mm-local on 32-bit?
> 
> Do we *do* global shootdowns on kmap_atomic()s on 32-bit?  I thought we
> used entirely per-cpu addresses, so a stale entry from another CPU can
> get loaded in the TLB speculatively but it won't ever actually get used.
> I think it goes:
> 
> kunmap_atomic() ->
> __kunmap_atomic() ->
> kpte_clear_flush() ->
> __flush_tlb_one_kernel() ->
> __flush_tlb_one_user() ->
> __native_flush_tlb_one_user() ->
> invlpg
> 
> The per-cpu address calculation is visible in kmap_atomic_prot():
> 
>        idx = type + KM_TYPE_NR*smp_processor_id();

>From a security point-of-view, having such an entry is still not too good,
since the mapping protection might override the default protection. This
might lead to potential W+X cases, for example, that might stay for a long
time if they are speculatively cached in the TLB and not invalidated upon
kunmap_atomic().

Having said that, I am not too excited to deal with this issue. Do people
still care about x86/32-bit? In addition, if kunmap_atomic() is used when
IRQs are disabled, sending a TLB shootdown during kunmap_atomic() can cause
a deadlock.