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

[Pingfan Liu]

On Wed, Aug 07, 2019 at 11:00:41AM +0800, Dave Young wrote:
> Add Tony and Xunlei in cc.
> On 08/05/19 at 04:58pm, Pingfan Liu wrote:
> > 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.
> 
> I did not understand and read the MCE details, but we previously had a
> MCE problem, Xunlei fixed in below commit:
> commit 5bc329503e8191c91c4c40836f062ef771d8ba83
> Author: Xunlei Pang <xlpang@xxxxxxxxxx>
> Date:   Mon Mar 13 10:50:19 2017 +0100
> 
>     x86/mce: Handle broadcasted MCE gracefully with kexec
> 
> I wonder if this is same issue or not. Also the old discussion is in
> below thread:
> https://lore.kernel.org/patchwork/patch/753530/
> 
> Tony raised similar questions, but I'm not sure if it is still a problem
> or it has been fixed.
> 
Xunlei's patch is the precondition of the stability for the case 2: boot up by "kexec -p nr_cpus="

For case1/3, extra effort is needed.

Thanks,
	Pingfan
> > 
> > *** 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.
> > 
> > -1. boot up by BIOS. Since no one set CR4.MCE=1, nr_cpus risks rebooting by
> > broadcast MCE.
> > 
> > -2. boot up by "kexec -p nr_cpus=".  Since the 1st kernel has all cpus'
> > CR4.MCE=1 set before kexec -p, nr_cpus is free of rebooting by broadcast MCE.
> > Furthermore, the crashed kernel's wreckage, including page table and text, is
> > not touched by capture kernel. Hence if MCE event happens on capped cpu,
> > do_machine_check->__mc_check_crashing_cpu() runs smoothly and returns
> > immediately, the capped cpu is still pinned on "halt".
> > 
> > -3. boot up by "kexec -l nr_cpus=". As "kexec -p", it is free of rebooting by
> > broadcast MCE. But the 1st kernel's wreckage is discarded and changed.  when
> > capped cpus execute do_machine_check(), they may crack the new kernel.  But
> > this is not related with broadcast MCE, and need an extra fix.
> > 
> > *** Solution ***
> > "nr_cpus" can not follow the same way as "nosmt".  Because nr_cpus limits the
> > allocation of percpu area and some other kthread memory, which is critical to
> > cpu hotplug framework.  Instead, developing a dedicated SIPI callback
> > make_capped_cpu_stable() for capped cpu, which does not lean on percpu area to
> > work.
> > 
> > [1]: https://lkml.org/lkml/2019/7/5/3
> > 
> > To: Gleixner <tglx@xxxxxxxxxxxxx>
> > To: Andy Lutomirski <luto@xxxxxxxxxx>
> > Cc: Ingo Molnar <mingo@xxxxxxxxxx>
> > Cc: Borislav Petkov <bp@xxxxxxxxx>
> > Cc: "H. Peter Anvin" <hpa@xxxxxxxxx>
> > Cc: Dave Hansen <dave.hansen@xxxxxxxxxxxxxxx>
> > Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
> > To: x86@xxxxxxxxxx
> > Cc: Masami Hiramatsu <mhiramat@xxxxxxxxxx>
> > Cc: Qian Cai <cai@xxxxxx>
> > Cc: Vlastimil Babka <vbabka@xxxxxxx>
> > Cc: Daniel Drake <drake@xxxxxxxxxxxx>
> > Cc: Jacob Pan <jacob.jun.pan@xxxxxxxxxxxxxxx>
> > Cc: Michal Hocko <mhocko@xxxxxxxx>
> > Cc: Eric Biederman <ebiederm@xxxxxxxxxxxx>
> > Cc: linux-kernel@xxxxxxxxxxxxxxx
> > Cc: Dave Young <dyoung@xxxxxxxxxx>
> > Cc: Baoquan He <bhe@xxxxxxxxxx>
> > Cc: kexec@xxxxxxxxxxxxxxxxxxx
> > 
> > ---
> > Pingfan Liu (4):
> >   x86/apic: correct the ENO in generic_processor_info()
> >   x86/apic: record capped cpu in generic_processor_info()
> >   x86/smp: send capped cpus to a stable state when smp_init()
> >   x86/smp: disallow MCE handler on rebooting AP
> > 
> >  arch/x86/include/asm/apic.h  |  1 +
> >  arch/x86/include/asm/smp.h   |  3 ++
> >  arch/x86/kernel/apic/apic.c  | 23 ++++++++----
> >  arch/x86/kernel/cpu/common.c |  7 ++++
> >  arch/x86/kernel/smp.c        |  8 +++++
> >  arch/x86/kernel/smpboot.c    | 83 ++++++++++++++++++++++++++++++++++++++++++++
> >  kernel/smp.c                 |  6 ++++
> >  7 files changed, 124 insertions(+), 7 deletions(-)
> > 
> > -- 
> > 2.7.5
> > 
> 
> Thanks
> Dave