linux-kernel - Re: x86/microcode/intel: Division by zero panic in 4.9.79 and 4.4.114

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Date:   Thu, 8 Feb 2018 13:55:48 +0000
From:   Rolf Neugebauer <rolf.neugebauer@...ker.com>
To:     Borislav Petkov <bp@...en8.de>
Cc:     Greg KH <gregkh@...uxfoundation.org>,
        Jia Zhang <zhang.jia@...ux.alibaba.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Tony Luck <tony.luck@...el.com>,
        Ingo Molnar <mingo@...hat.com>,
        "H. Peter Anvin" <hpa@...or.com>, x86@...nel.org,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: x86/microcode/intel: Division by zero panic in 4.9.79 and 4.4.114

On Wed, Feb 7, 2018 at 6:12 PM, Borislav Petkov <bp@...en8.de> wrote:
> On Wed, Feb 07, 2018 at 04:31:59PM +0000, Rolf Neugebauer wrote:
>>  arch/x86/kernel/cpu/microcode/core.c | 28 +++++++++++++++++++---------
>>  1 file changed, 19 insertions(+), 9 deletions(-)
>>
>> diff --git a/arch/x86/kernel/cpu/microcode/core.c
>> b/arch/x86/kernel/cpu/microcode/core.c
>> index b3e94ef461fd..1b3e0aa4c511 100644
>> --- a/arch/x86/kernel/cpu/microcode/core.c
>> +++ b/arch/x86/kernel/cpu/microcode/core.c
>> @@ -44,7 +44,7 @@
>>
>>  static struct microcode_ops    *microcode_ops;
>>
>> -static bool dis_ucode_ldr;
>> +static bool dis_ucode_ldr = true;
>>
>>  static int __init disable_loader(char *str)
>>  {
>> @@ -81,6 +81,7 @@ struct cpu_info_ctx {
>>
>>  static bool __init check_loader_disabled_bsp(void)
>>  {
>> +       u32 a, b, c, d;
>>  #ifdef CONFIG_X86_32
>>         const char *cmdline = (const char *)__pa_nodebug(boot_command_line);
>>         const char *opt     = "dis_ucode_ldr";
>> @@ -93,8 +94,23 @@ static bool __init check_loader_disabled_bsp(void)
>>         bool *res = &dis_ucode_ldr;
>>  #endif
>>
>> -       if (cmdline_find_option_bool(cmdline, option))
>> -               *res = true;
>> +       if (!have_cpuid_p())
>> +               return *res;
>
> That might cause an issue, see 1f161f67a272c. Might wanna backport that
> commit too, just in case, for those old CPUID-less geodes.

On the 4.4 kernel, 1f161f67a272c ("x86/microcode: Do the family check
first") does not apply cleanly. Looks like it relies on 309aac77768c0
("x86/microcode: Decrease CPUID use") and 7a93a40be23e5
("x86/microcode: Remove local vendor variable") as well. These don't
apply cleanly either.

Should I just manually backport the functionality of 1f161f67a272c?

Rolf

>
> The rest looks ok.
>
> --
> Regards/Gruss,
>     Boris.
>
> Good mailing practices for 400: avoid top-posting and trim the reply.