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

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Message-ID: <c4cc63b5-2861-5370-781f-5d664b6ffed5@amd.com>
Date:   Thu, 20 Aug 2020 14:04:41 -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 1:41 PM, Tom Lendacky wrote:
> 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:
>>>>
>>>>
>>>> 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.

I added a quick hack to save TSC_AUX to a new variable in the SVM struct 
and then restore it right after VMEXIT (just after where GS is restored in 
svm_vcpu_enter_exit()) and my guest is no longer crashing.

Thanks,
Tom

> 
> Thanks,
> Tom
> 
>>