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    SubjectRe: [RFC][PATCH v2 00/21] x86/pti: Defer CR3 switch to C code
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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.

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