Re: [PATCH -v2] x86/boot/compressed: Register dummy NMI handler in EFI boot loader, to avoid kdump crashes

[Zeng Heng]

On 2023/1/10 20:11, Borislav Petkov wrote:
> On Tue, Jan 10, 2023 at 01:01:06PM +0100, Ingo Molnar wrote:
> > From: Zeng Heng <zengheng4@xxxxxxxxxx>
> > Date: Tue, 10 Jan 2023 18:27:45 +0800
> > Subject: [PATCH] x86/boot/compressed: Register dummy NMI handler in EFI boot loader, to avoid kdump crashes
> >
> > If kdump is enabled, when using mce_inject to inject errors, EFI
> Why does "EFI" matter here? Any boot loader would do...
>
> > boot loader would decompress & load second kernel for saving the
> s/&/and/
>
> > vmcore file.
> >
> > For normal errors that is fine.
> Useless sentence.
>
> > However, in the MCE case, the panic
> > CPU that firstly enters into mce_panic() is running within NMI
> > interrupt context,
> "#MC context" it is non-maskable but that's not "NMI interrupt context"

mce is registered on NMI handler by inject_init().

And here is the context of mce-inject:

#0  relocate_kernel () at arch/x86/kernel/relocate_kernel_64.S:55
#1  0xffffffff81a57fc2 in machine_kexec (image=0xffff888101ef8400)
    at arch/x86/kernel/machine_kexec_64.c:391
#2  0xffffffff811a9573 in __crash_kexec (regs=regs@entry=0x0 
<fixed_percpu_data>)
    at kernel/kexec_core.c:1057
#3  0xffffffff81a5b4e4 in panic (fmt=fmt@entry=0xffffffff823211c8 "Fatal 
machine check")
    at kernel/panic.c:393
#4  0xffffffff81aa65f5 in mce_panic (
    msg=msg@entry=0xffffffff823211c8 "Fatal machine check",
    final=final@entry=0xffff88813ac9eec0, exp=<optimized out>)
    at arch/x86/kernel/cpu/mce/core.c:380
#5  0xffffffff8103863b in mce_reign () at 
arch/x86/kernel/cpu/mce/core.c:1042
#6  0xffffffff81aa682f in mce_end (order=order@entry=1)
    at arch/x86/kernel/cpu/mce/core.c:1175
#7  0xffffffff81aa6d57 in do_machine_check (regs=0xfffffe00000beef8)
    at arch/x86/kernel/cpu/mce/core.c:1567
#8  0xffffffffc0495167 in raise_exception (m=m@entry=0xffff88813ad9ef40,
    pregs=<optimized out>) at arch/x86/kernel/cpu/mce/inject.c:152
#9  0xffffffffc0495e7f in mce_raise_notify (cmd=<optimized out>, 
regs=<optimized out>)
    at arch/x86/kernel/cpu/mce/inject.c:168
#10 0xffffffff810204b8 in nmi_handle ()
#11 0xffffffff81aa5e62 in default_do_nmi 
(regs=regs@entry=0xfffffe00000beef8)
    at arch/x86/kernel/nmi.c:335
#12 0xffffffff81aa608d in exc_nmi (regs=0xfffffe00000beef8) at 
arch/x86/kernel/nmi.c:517
#13 0xffffffff81c014e8 in asm_exc_nmi () at arch/x86/entry/entry_64.S:1440


> > and the processor blocks delivery of subsequent
> > NMIs until the next execution of the IRET instruction.
> >
> > When the panic CPU takes long time in the panic processing route,
> I'm still unclear on the order of events here. It sounds like
>
> 1. MCE injected
> 2. panic
> 3. kdump gets loaded
>
> If that is the case, then I presume the flow is:
>
> mce_panic -> panic -> __crash_kexec()
>
> Yes?

Yes, exactly. The following procedure is like:

panic() -> relocate_kernel() -> identity_mapped() -> x86 purgatory image 
-> EFI loader -> secondary kernel

> If so, then we should make sure we have *exited* #MC context before calling
> panic() and not have to add hacks like this one of adding an empty NMI handler.
>
> But I'm only speculating as it is hard to make sense of all this text.
>
> Thx.