Re: FSGSBASE causing panic on 5.9-rc1

[Andy Lutomirski]

On Thu, Aug 20, 2020 at 8:21 AM Tom Lendacky <thomas.lendacky@xxxxxxx> 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@xxxxxxxxxx> wrote:
> >>>
> >>> On Wed, Aug 19, 2020 at 11:19 AM Tom Lendacky <thomas.lendacky@xxxxxxx> 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@xxxxxxxxxx>
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.

Also, you're crashing when you run a guest, right?  Can you try
running the x86 sefltests on a bad kernel without running any guests?

--Andy