linux-kernel - Re: [PATCH v3 2/3] Fix undefined operation fault that can hang a cpu on crash or panic

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Message-ID: <CALCETrVZx4VA9rg-Hn7KdER866ZOtZtmTkR0MSacnj5jGO-Pag@mail.gmail.com>
Date:   Sun, 5 Jul 2020 11:22:42 -0700
From:   Andy Lutomirski <luto@...nel.org>
To:     "David P. Reed" <dpreed@...pplum.com>
Cc:     Sean Christopherson <sean.j.christopherson@...el.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        X86 ML <x86@...nel.org>, "H. Peter Anvin" <hpa@...or.com>,
        Allison Randal <allison@...utok.net>,
        Enrico Weigelt <info@...ux.net>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Kate Stewart <kstewart@...uxfoundation.org>,
        "Peter Zijlstra (Intel)" <peterz@...radead.org>,
        Randy Dunlap <rdunlap@...radead.org>,
        Martin Molnar <martin.molnar.programming@...il.com>,
        Andy Lutomirski <luto@...nel.org>,
        Alexandre Chartre <alexandre.chartre@...cle.com>,
        Jann Horn <jannh@...gle.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v3 2/3] Fix undefined operation fault that can hang a cpu
 on crash or panic

On Sat, Jul 4, 2020 at 1:38 PM David P. Reed <dpreed@...pplum.com> wrote:
>
> Fix: Mask undefined operation fault during emergency VMXOFF that must be
> attempted to force cpu exit from VMX root operation.
> Explanation: When a cpu may be in VMX root operation (only possible when
> CR4.VMXE is set), crash or panic reboot tries to exit VMX root operation
> using VMXOFF. This is necessary, because any INIT will be masked while cpu
> is in VMX root operation, but that state cannot be reliably
> discerned by the state of the cpu.
> VMXOFF faults if the cpu is not actually in VMX root operation, signalling
> undefined operation.
> Discovered while debugging an out-of-tree x-visor with a race. Can happen
> due to certain kinds of bugs in KVM.

Can you re-wrap lines to 68 characters?  Also, the Fix: and
Explanation: is probably unnecessary.  You could say:

Ignore a potential #UD failut during emergency VMXOFF ...

When a cpu may be in VMX ...

>
> Fixes: 208067 <https://bugzilla.kernel.org/show_bug.cgi?id=208067>
> Reported-by: David P. Reed <dpreed@...pplum.com>

It's not really necessary to say that you, the author, reported the
problem, but I guess it's harmless.

> Suggested-by: Thomas Gleixner <tglx@...utronix.de>
> Suggested-by: Sean Christopherson <sean.j.christopherson@...el.com>
> Suggested-by: Andy Lutomirski <luto@...nel.org>
> Signed-off-by: David P. Reed <dpreed@...pplum.com>
> ---
>  arch/x86/include/asm/virtext.h | 20 ++++++++++++++------
>  1 file changed, 14 insertions(+), 6 deletions(-)
>
> diff --git a/arch/x86/include/asm/virtext.h b/arch/x86/include/asm/virtext.h
> index 0ede8d04535a..0e0900eacb9c 100644
> --- a/arch/x86/include/asm/virtext.h
> +++ b/arch/x86/include/asm/virtext.h
> @@ -30,11 +30,11 @@ static inline int cpu_has_vmx(void)
>  }
>
>
> -/* Disable VMX on the current CPU
> +/* Exit VMX root mode and isable VMX on the current CPU.

s/isable/disable/


>  /* Disable VMX if it is supported and enabled on the current CPU
> --
> 2.26.2
>

Other than that:

Reviewed-by: Andy Lutomirski <luto@...nel.org>

--Andy