linux-kernel - Re: FSGSBASE causing panic on 5.9-rc1

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Date:   Thu, 20 Aug 2020 13:41:24 -0500
From:   Tom Lendacky <thomas.lendacky@....com>
To:     Jim Mattson <jmattson@...gle.com>
Cc:     Andy Lutomirski <luto@...nel.org>,
        Sean Christopherson <sean.j.christopherson@...el.com>,
        Joerg Roedel <joro@...tes.org>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        X86 ML <x86@...nel.org>,
        "Chang S. Bae" <chang.seok.bae@...el.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Sasha Levin <sashal@...nel.org>,
        Borislav Petkov <bp@...en8.de>,
        Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...nel.org>
Subject: Re: FSGSBASE causing panic on 5.9-rc1

On 8/20/20 1:39 PM, Jim Mattson wrote:
> On Thu, Aug 20, 2020 at 11:38 AM Jim Mattson <jmattson@...gle.com> wrote:
>>
>> On Thu, Aug 20, 2020 at 11:34 AM Tom Lendacky <thomas.lendacky@....com> wrote:
>>>
>>> On 8/20/20 11:30 AM, Tom Lendacky wrote:
>>>> On 8/20/20 11:17 AM, Tom Lendacky wrote:
>>>>> On 8/20/20 10:55 AM, Andy Lutomirski wrote:
>>>>>> On Thu, Aug 20, 2020 at 8:21 AM Tom Lendacky <thomas.lendacky@....com>
>>>>>> wrote:
>>>>>>>
>>>>>>> On 8/20/20 10:10 AM, Sean Christopherson wrote:
>>>>>>>> On Wed, Aug 19, 2020 at 05:21:33PM -0700, Andy Lutomirski wrote:
>>>>>>>>> On Wed, Aug 19, 2020 at 2:25 PM Andy Lutomirski <luto@...nel.org>
>>>>>>>>> wrote:
>>>>>>>>>>
>>>>>>>>>> On Wed, Aug 19, 2020 at 11:19 AM Tom Lendacky
>>>>>>>>>> <thomas.lendacky@....com> wrote:
>>>>>>>>>>>
>>>>>>>>>>> On 8/19/20 1:07 PM, Tom Lendacky wrote:
>>>>>>>>>>>> It looks like the FSGSBASE support is crashing my second
>>>>>>>>>>>> generation EPYC
>>>>>>>>>>>> system. I was able to bisect it to:
>>>>>>>>>>>>
>>>>>>>>>>>> b745cfba44c1 ("x86/cpu: Enable FSGSBASE on 64bit by default and
>>>>>>>>>>>> add a chicken bit")
>>>>>>>>>>>>
>>>>>>>>>>>> The panic only happens when using KVM. Doing kernel builds or stress
>>>>>>>>>>>> on bare-metal appears fine. But if I fire up, in this case, a
>>>>>>>>>>>> 64-vCPU
>>>>>>>>>>>> guest and do a kernel build within the guest, I get the following:
>>>>>>>>>>>
>>>>>>>>>>> I should clarify that this panic is on the bare-metal system, not
>>>>>>>>>>> in the
>>>>>>>>>>> guest. And that specifying nofsgsbase on the bare-metal command
>>>>>>>>>>> line fixes
>>>>>>>>>>> the issue.
>>>>>>>>>>
>>>>>>>>>> I certainly see some oddities:
>>>>>>>>>>
>>>>>>>>>> We have this code:
>>>>>>>>>>
>>>>>>>>>> static void svm_vcpu_put(struct kvm_vcpu *vcpu)
>>>>>>>>>> {
>>>>>>>>>>            struct vcpu_svm *svm = to_svm(vcpu);
>>>>>>>>>>            int i;
>>>>>>>>>>
>>>>>>>>>>            avic_vcpu_put(vcpu);
>>>>>>>>>>
>>>>>>>>>>            ++vcpu->stat.host_state_reload;
>>>>>>>>>>            kvm_load_ldt(svm->host.ldt);
>>>>>>>>>> #ifdef CONFIG_X86_64
>>>>>>>>>>            loadsegment(fs, svm->host.fs);
>>>>>>>>>>            wrmsrl(MSR_KERNEL_GS_BASE, current->thread.gsbase);
>>>>>>>>
>>>>>>>> Pretty sure current->thread.gsbase can be stale, i.e. this needs:
>>>>>>>>
>>>>>>>>         current_save_fsgs();
>>>>>>>
>>>>>>> I did try adding current_save_fsgs() in svm_vcpu_load(), saving the
>>>>>>> current->thread.gsbase value to a new variable in the svm struct. I then
>>>>>>> used that variable in the wrmsrl below, but it still crashed.
>>>>>>
>>>>>> Can you try bisecting all the way back to:
>>>>>>
>>>>>> commit dd649bd0b3aa012740059b1ba31ecad28a408f7f
>>>>>> Author: Andy Lutomirski <luto@...nel.org>
>>>>>> Date:   Thu May 28 16:13:48 2020 -0400
>>>>>>
>>>>>>       x86/cpu: Add 'unsafe_fsgsbase' to enable CR4.FSGSBASE
>>>>>>
>>>>>> and adding the unsafe_fsgsbase command line option while you bisect.
>>>>>
>>>>> I'll give that a try.
>>>
>>> Bisecting with unsafe_fsgsbase identified:
>>>
>>> c82965f9e530 ("x86/entry/64: Handle FSGSBASE enabled paranoid entry/exit")
>>>
>>> But I'm thinking that could be because it starts using GET_PERCPU_BASE,
>>> which on Rome would use RDPID. So is SVM restoring TSC_AUX_MSR too late?
>>> That would explain why I don't see the issue on Naples, which doesn't
>>> support RDPID.
>>
>> It looks to me like SVM loads the guest TSC_AUX from vcpu_load to
>> vcpu_put, with this comment:
>>
>> /* This assumes that the kernel never uses MSR_TSC_AUX */
>> if (static_cpu_has(X86_FEATURE_RDTSCP))
>>          wrmsrl(MSR_TSC_AUX, svm->tsc_aux);
> 
> Correction: It never restores TSC_AUX, AFAICT.

It does, it's in the host_save_user_msrs array.

Thanks,
Tom

>