linux-kernel - Re: [patch 2/2] x86/kvm/vmx: Prevent split lock detection induced #AC wreckage

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Message-ID: <87y2rdn7vq.fsf@nanos.tec.linutronix.de>
Date:   Thu, 02 Apr 2020 18:56:57 +0200
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Sean Christopherson <sean.j.christopherson@...el.com>
Cc:     LKML <linux-kernel@...r.kernel.org>, x86@...nel.org,
        "Kenneth R. Crudup" <kenny@...ix.com>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Fenghua Yu <fenghua.yu@...el.com>,
        Xiaoyao Li <xiaoyao.li@...el.com>,
        Nadav Amit <namit@...are.com>,
        Thomas Hellstrom <thellstrom@...are.com>,
        Tony Luck <tony.luck@...el.com>,
        "Peter Zijlstra \(Intel\)" <peterz@...radead.org>,
        Jessica Yu <jeyu@...nel.org>,
        Steven Rostedt <rostedt@...dmis.org>
Subject: Re: [patch 2/2] x86/kvm/vmx: Prevent split lock detection induced #AC wreckage

Sean Christopherson <sean.j.christopherson@...el.com> writes:
> On Thu, Apr 02, 2020 at 02:33:00PM +0200, Thomas Gleixner wrote:
>> Mark the module with MOD_INFO(sld_safe, "Y") so the module loader does not
>> force SLD off.
>
> Some comments below.  But, any objection to taking Xiaoyao's patches that
> do effectively the same things, minus the MOD_INFO()?  I'll repost them in
> reply to this thread.

If they are sane, I don't have a problem. But TBH, I really couldn't be
bothered to actually scan my mails whether there surfaced something sane
by now. Writing that up was just faster :)

I'll have look.

>> +static bool guest_handles_ac(struct kvm_vcpu *vcpu)
>> +{
>> +	/*
>> +	 * If guest has alignment checking enabled in CR0 and activated in
>> +	 * eflags, then the #AC originated from CPL3 and the guest is able
>> +	 * to handle it. It does not matter whether this is a regular or
>> +	 * a split lock operation induced #AC.
>> +	 */
>> +	if (vcpu->arch.cr0 & X86_CR0_AM &&
>
> Technically not required since KVM doesn't let the gets toggle CR0.AM at
> will, but going through kvm_read_cr0{_bits}() is preferred.

You're the expert here.

>> +	    vmx_get_rflags(vcpu) & X86_EFLAGS_AC)
>
> I don't think this is correct.  A guest could trigger a split-lock #AC at
> CPL0 with EFLAGS.AC=1 and CR0.AM=1, and then panic because it didn't expect
> #AC at CPL0.

Indeed.

Thanks,

        tglx