linux-kernel - Re: [RFC][PATCH v2 00/21] x86/pti: Defer CR3 switch to C code

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Date:   Tue, 17 Nov 2020 20:02:51 +0100
From:   Alexandre Chartre <alexandre.chartre@...cle.com>
To:     Borislav Petkov <bp@...en8.de>
Cc:     tglx@...utronix.de, mingo@...hat.com, hpa@...or.com,
        x86@...nel.org, dave.hansen@...ux.intel.com, luto@...nel.org,
        peterz@...radead.org, linux-kernel@...r.kernel.org,
        thomas.lendacky@....com, jroedel@...e.de, konrad.wilk@...cle.com,
        jan.setjeeilers@...cle.com, junaids@...gle.com, oweisse@...gle.com,
        rppt@...ux.vnet.ibm.com, graf@...zon.de, mgross@...ux.intel.com,
        kuzuno@...il.com
Subject: Re: [RFC][PATCH v2 00/21] x86/pti: Defer CR3 switch to C code



On 11/17/20 7:28 PM, Borislav Petkov wrote:
> On Tue, Nov 17, 2020 at 07:12:07PM +0100, Alexandre Chartre wrote:
>> Yes. L1TF/MDS allow some inter cpu-thread attacks which are not mitigated at
>> the moment. In particular, this allows a guest VM to attack another guest VM
>> or the host kernel running on a sibling cpu-thread. Core Scheduling will
>> mitigate the guest-to-guest attack but not the guest-to-host attack.
> 
> I see in vmx_vcpu_enter_exit():
> 
>          /* L1D Flush includes CPU buffer clear to mitigate MDS */
>          if (static_branch_unlikely(&vmx_l1d_should_flush))
>                  vmx_l1d_flush(vcpu);
>          else if (static_branch_unlikely(&mds_user_clear))
>                  mds_clear_cpu_buffers();
> 
> Is that not enough?

No. This prevents the guest VM from gathering data from the host kernel on the
same cpu-thread. But there's no mitigation for a guest VM running on a cpu-thread
attacking another cpu-thread (which can be running another guest VM or the
host kernel) from the same cpu-core. You cannot use flush/clear barriers because
the two cpu-threads are running in parallel.

alex.