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Subject: Re: [RFC][PATCH v2 00/21] x86/pti: Defer CR3 switch to C code
To:     Borislav Petkov <bp@alien8.de>
Cc:     tglx@linutronix.de, mingo@redhat.com, hpa@zytor.com,
        x86@kernel.org, dave.hansen@linux.intel.com, luto@kernel.org,
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        rppt@linux.vnet.ibm.com, graf@amazon.de, mgross@linux.intel.com,
        kuzuno@gmail.com
References: <20201116144757.1920077-1-alexandre.chartre@oracle.com>
 <20201116201711.GE1131@zn.tnic>
 <44a88648-738a-4a4b-9c25-6b70000e037c@oracle.com>
 <20201117165539.GG5719@zn.tnic>
 <890f6b7e-a268-2257-edcb-5eacc7db3d8e@oracle.com>
 <20201117182809.GK5719@zn.tnic>
From:   Alexandre Chartre <alexandre.chartre@oracle.com>
Message-ID: <33a1c6ee-0122-0b0c-ed2d-1578b29ef7c1@oracle.com>
Date:   Tue, 17 Nov 2020 20:02:51 +0100
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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.