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

Alexandre Chartre
2020-11-17T19:01:29Z

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.