linux-kernel - Re: [PATCH 2/2] KVM: VMX: Extend VMX's #AC handding

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Date:   Thu, 30 Jan 2020 07:18:14 -0800
From:   Andy Lutomirski <luto@...capital.net>
To:     Xiaoyao Li <xiaoyao.li@...el.com>
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        "H. Peter Anvin" <hpa@...or.com>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Sean Christopherson <sean.j.christopherson@...el.com>,
        x86@...nel.org, linux-kernel@...r.kernel.org, kvm@...r.kernel.org
Subject: Re: [PATCH 2/2] KVM: VMX: Extend VMX's #AC handding



> On Jan 30, 2020, at 4:24 AM, Xiaoyao Li <xiaoyao.li@...el.com> wrote:
> 
> There are two types of #AC can be generated in Intel CPUs:
> 1. legacy alignment check #AC;
> 2. split lock #AC;
> 
> Legacy alignment check #AC can be injected to guest if guest has enabled
> alignemnet check.
> 
> When host enables split lock detection, i.e., split_lock_detect!=off,
> guest will receive an unexpected #AC when there is a split_lock happens in
> guest since KVM doesn't virtualize this feature to guest.
> 
> Since the old guests lack split_lock #AC handler and may have split lock
> buges. To make guest survive from split lock, applying the similar policy
> as host's split lock detect configuration:
> - host split lock detect is sld_warn:
>   warning the split lock happened in guest, and disabling split lock
>   detect around VM-enter;
> - host split lock detect is sld_fatal:
>   forwarding #AC to userspace. (Usually userspace dump the #AC
>   exception and kill the guest).

A correct userspace implementation should, with a modern guest kernel, forward the exception. Otherwise you’re introducing a DoS into the guest if the guest kernel is fine but guest userspace is buggy.

What’s the intended behavior here?