linux-kernel - Re: [PATCH 0/2] KVM: kvm-coco-queue: Support protected TSC

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <c4df36dc-9924-e166-ec8b-ee48e4f6833e@amd.com>
Date: Mon, 14 Oct 2024 20:17:19 +0530
From: "Nikunj A. Dadhania" <nikunj@....com>
To: Isaku Yamahata <isaku.yamahata@...el.com>, kvm@...r.kernel.org,
 pbonzini@...hat.com
Cc: Sean Christopherson <seanjc@...gle.com>, chao.gao@...el.com,
 rick.p.edgecombe@...el.com, yan.y.zhao@...el.com,
 linux-kernel@...r.kernel.org, isaku.yamahata@...il.com,
 Marcelo Tosatti <mtosatti@...hat.com>, Tom Lendacky <thomas.lendacky@....com>
Subject: Re: [PATCH 0/2] KVM: kvm-coco-queue: Support protected TSC

Hi Isaku,

On 10/12/2024 1:25 PM, Isaku Yamahata wrote:
> This patch series is for the kvm-coco-queue branch.  The change for TDX KVM is
> included at the last.  The test is done by create TDX vCPU and run, get TSC
> offset via vCPU device attributes and compare it with the TDX TSC OFFSET
> metadata.  Because the test requires the TDX KVM and TDX KVM kselftests, don't
> include it in this patch series.
> 
> 
> Background
> ----------
> X86 confidential computing technology defines protected guest TSC so that the
> VMM can't change the TSC offset/multiplier once vCPU is initialized and the
> guest can trust TSC.  The SEV-SNP defines Secure TSC as optional.  TDX mandates
> it.  The TDX module determines the TSC offset/multiplier.  The VMM has to
> retrieve them.
> 
> On the other hand, the x86 KVM common logic tries to guess or adjust the TSC
> offset/multiplier for better guest TSC and TSC interrupt latency at KVM vCPU
> creation (kvm_arch_vcpu_postcreate()), vCPU migration over pCPU
> (kvm_arch_vcpu_load()), vCPU TSC device attributes (kvm_arch_tsc_set_attr()) and
> guest/host writing to TSC or TSC adjust MSR (kvm_set_msr_common()).
> 
> 
> Problem
> -------
> The current x86 KVM implementation conflicts with protected TSC because the
> VMM can't change the TSC offset/multiplier.  Disable or ignore the KVM
> logic to change/adjust the TSC offset/multiplier somehow.
> 
> Because KVM emulates the TSC timer or the TSC deadline timer with the TSC
> offset/multiplier, the TSC timer interrupts are injected to the guest at the
> wrong time if the KVM TSC offset is different from what the TDX module
> determined.
> 
> Originally the issue was found by cyclic test of rt-test [1] as the latency in
> TDX case is worse than VMX value + TDX SEAMCALL overhead.  It turned out that
> the KVM TSC offset is different from what the TDX module determines.

Can you provide what is the exact command line to reproduce this problem ? 
Any links to this reported issue ?

> 
> 
> Solution
> --------
> The solution is to keep the KVM TSC offset/multiplier the same as the value of
> the TDX module somehow.  Possible solutions are as follows.
> - Skip the logic
>   Ignore (or don't call related functions) the request to change the TSC
>   offset/multiplier.
>   Pros
>   - Logically clean.  This is similar to the guest_protected case.
>   Cons
>   - Needs to identify the call sites.
> 
> - Revert the change at the hooks after TSC adjustment
>   x86 KVM defines the vendor hooks when the TSC offset/multiplier are
>   changed.  The callback can revert the change.
>   Pros
>   - We don't need to care about the logic to change the TSC offset/multiplier.
>   Cons:
>   - Hacky to revert the KVM x86 common code logic.
> 
> Choose the first one.  With this patch series, SEV-SNP secure TSC can be
> supported.

I am not sure how will this help SNP Secure TSC, as the GUEST_TSC_OFFSET and 
GUEST_TSC_SCALE are only available to the guest.

Regards,
Nikunj