Re: [PATCHv2 0/4] x86/mce: protect nr_cpus from rebooting by broadcast mce

[Pingfan Liu]

On Fri, Aug 30, 2019 at 04:11:56PM +0200, Borislav Petkov wrote:
> On Tue, Aug 27, 2019 at 11:02:19AM +0800, Pingfan Liu wrote:
> > v1 -> v2: fix compile warning and error on x86_32
> > 
> > 
> > This series include two related groups:
> > [1-3/4]: protect nr_cpus from rebooting by broadcast mce
> > [4/4]: improve "kexec -l" robustness against broadcast mce
> > 
> > When I tried to fix [1], Thomas raised concern about the nr_cpus' vulnerability
> > to unexpected rebooting by broadcast mce. After analysis, I think only the
> > following first case suffers from the rebooting by broadcast mce. [1-3/4] aims
> > to fix that issue.
> > 
> > *** Back ground ***
> > 
> > On x86 it's required to have all logical CPUs set CR4.MCE=1. Otherwise, a
> > broadcast MCE observing CR4.MCE=0b on any core will shutdown the machine.
> > 
> > The option 'nosmt' has already complied with the above rule by Thomas's patch.
> > For detail, refer to 506a66f3748 (Revert "x86/apic: Ignore secondary threads if
> > nosmt=force")
> > 
> > But for nr_cpus option, the exposure to broadcast MCE is a little complicated,
> > and can be categorized into three cases.
> 
> One thing is not clear to me: are you "fixing" a hypothetical case here
> or have you *actually* experienced an MCE happening while kdumping with
> nr_cpus < num_online_cpus()?
No, I do not hit this issue by myself.

But from the following two commits:
commit 5bc329503e8191c91c4c40836f062ef771d8ba83
Author: Xunlei Pang <xlpang@xxxxxxxxxx>
Date:   Mon Mar 13 10:50:19 2017 +0100

    x86/mce: Handle broadcasted MCE gracefully with kexec
And
commit 506a66f374891ff08e064a058c446b336c5ac760
Author: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Date:   Fri Jun 29 16:05:47 2018 +0200

    Revert "x86/apic: Ignore secondary threads if nosmt=force"

This issue is in practice.

BTW, clarify one thing kdumping (kexec -p) will not suffer from mce
issue as described in case 2. Only "kexec -l" will.

> 
> Btw, pls do not use lkml.org to refer to previous mails but
> 
> http://lkml.kernel.org/r/<Message-ID>
OK, I will.

Thanks,
	Pingfan