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

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Message-ID: <f1dfbfb4-d2d5-bf30-600f-9e756a352860@intel.com>
Date:   Thu, 13 Jun 2019 09:20:53 -0700
From:   Dave Hansen <dave.hansen@...el.com>
To:     Andy Lutomirski <luto@...nel.org>,
        Alexander Graf <graf@...zon.com>, Nadav Amit <namit@...are.com>
Cc:     Marius Hillenbrand <mhillenb@...zon.de>,
        kvm list <kvm@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Kernel Hardening <kernel-hardening@...ts.openwall.com>,
        Linux-MM <linux-mm@...ck.org>, Alexander Graf <graf@...zon.de>,
        David Woodhouse <dwmw@...zon.co.uk>,
        the arch/x86 maintainers <x86@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>
Subject: Re: [RFC 00/10] Process-local memory allocations for hiding KVM
 secrets

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();