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

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Date:   Thu, 20 Aug 2020 18:43:10 +0000
From:   "Bae, Chang Seok" <chang.seok.bae@...el.com>
To:     Tom Lendacky <thomas.lendacky@....com>
CC:     "Christopherson, Sean J" <sean.j.christopherson@...el.com>,
        "Andy Lutomirski" <luto@...nel.org>,
        Joerg Roedel <joro@...tes.org>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        "Linux Kernel Mailing List" <linux-kernel@...r.kernel.org>,
        X86 ML <x86@...nel.org>, 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>,
        "Hansen, Dave" <dave.hansen@...el.com>,
        "Shankar, Ravi V" <ravi.v.shankar@...el.com>
Subject: Re: FSGSBASE causing panic on 5.9-rc1


> On Aug 20, 2020, at 08:21, Tom Lendacky <thomas.lendacky@....com> wrote:
> On 8/20/20 10:10 AM, Sean Christopherson wrote:
>> 
>> 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.

Then, current->thread.gsbase is from __rdgsbase_inactive() which is
user GSBASE.

If you do the wrmsrl below, it overwrites the current GSBASE with the 
user value.

>> 	wrmsrl(MSR_KERNEL_GS_BASE, current->thread.gsbase);

Thanks,
Chang