linux-kernel - Re: Linux 3.18.2 / xen 4.4.1 dom0

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Date:	Thu, 22 Jan 2015 09:53:04 -0500
From:	Boris Ostrovsky <boris.ostrovsky@...cle.com>
To:	Borislav Petkov <bp@...en8.de>,
	James Dingwall <james@...gwall.me.uk>
CC:	linux-kernel@...r.kernel.org
Subject: Re: Linux 3.18.2 / xen 4.4.1 dom0 - microcode oops

On 01/22/2015 09:48 AM, Boris Ostrovsky wrote:
> On 01/22/2015 03:20 AM, Borislav Petkov wrote:
>> Hmm,
>>
>> and I thought we fixed all that fun. It seems not :-\
>>
>> Boris, this paravirt_enabled() thing doesn't seem to work or why are we
>> even calling microcode_exit()?
>
> Looks like something is unloading microcode driver (init scripts 
> perhaps) and so we are trying to unregister device that we never 
> registered (because we had early return from microcode_init() when we 
> loaded it).
>
> I actually suspect the same bug would be triggered if dis_ucode_ldr is 
> true on baremetal.
>
> So we need something like:
>
> --- a/arch/x86/kernel/cpu/microcode/core.c
> +++ b/arch/x86/kernel/cpu/microcode/core.c
> @@ -625,6 +625,9 @@ static void __exit microcode_exit(void)
>  {
>         struct cpuinfo_x86 *c = &cpu_data(0);
>
> +       if (paravirt_enabled() || dis_ucode_ldr)
> +               return 0;

Plain 'return', of course.

Alternatively, we could return an error (-EINVAL?) from microcode_init() 
when either of these two conditions is true.

-boris

> +
>         microcode_dev_exit();
>
>         unregister_hotcpu_notifier(&mc_cpu_notifier);
>
>

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