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

From: Pingfan Liu <kernelfans@gmail.com>
To: Borislav Petkov <bp@alien8.de>
Cc: Gleixner <tglx@linutronix.de>, Andy Lutomirski <luto@kernel.org>,
	x86@kernel.org, Ingo Molnar <mingo@redhat.com>,
	"H. Peter Anvin" <hpa@zytor.com>,
	Dave Hansen <dave.hansen@linux.intel.com>,
	Peter Zijlstra <peterz@infradead.org>,
	Masami Hiramatsu <mhiramat@kernel.org>, Qian Cai <cai@lca.pw>,
	Vlastimil Babka <vbabka@suse.cz>,
	Daniel Drake <drake@endlessm.com>,
	Jacob Pan <jacob.jun.pan@linux.intel.com>,
	Michal Hocko <mhocko@suse.com>,
	Eric Biederman <ebiederm@xmission.com>,
	linux-kernel@vger.kernel.org, Dave Young <dyoung@redhat.com>,
	Baoquan He <bhe@redhat.com>,
	kexec@lists.infradead.org
Subject: Re: [PATCHv2 0/4] x86/mce: protect nr_cpus from rebooting by broadcast mce
Date: Mon, 2 Sep 2019 12:26:42 +0800	[thread overview]
Message-ID: <20190902042642.GA22792@mypc> (raw)
In-Reply-To: <20190830141156.GB30413@zn.tnic>

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@redhat.com>
Date:   Mon Mar 13 10:50:19 2017 +0100

    x86/mce: Handle broadcasted MCE gracefully with kexec
And
commit 506a66f374891ff08e064a058c446b336c5ac760
Author: Thomas Gleixner <tglx@linutronix.de>
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