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

[Zeng Heng]

On 2023/1/10 20:57, Borislav Petkov wrote:
> On Tue, Jan 10, 2023 at 08:32:07PM +0800, Zeng Heng wrote:
> > mce is registered on NMI handler by inject_init().
> That's a handler for the NMI raised by raise_mce(). That's for the injection
> case, which is simulated. If you're fixing the injection case, then surely not
> with a bogus boot NMI handler.

OK, mce-injection is the simulated one.

> > Yes, exactly. The following procedure is like:
> >
> > panic() -> relocate_kernel() -> identity_mapped() -> x86 purgatory image ->
> > EFI loader -> secondary kernel
> I'm doubtful now as you're injecting errors so you're not really in #MC context
> but in this contrived context which is actually an NMI one. So we need to think
> about how to fix this case.
>
> Certainly not with an empty NMI handler...
>
> Regardless, we should do
>
> diff --git a/arch/x86/kernel/cpu/mce/core.c b/arch/x86/kernel/cpu/mce/core.c
> index 7832a69d170e..57fe376ed049 100644
> --- a/arch/x86/kernel/cpu/mce/core.c
> +++ b/arch/x86/kernel/cpu/mce/core.c
> @@ -286,6 +286,8 @@ static noinstr void mce_panic(const char *msg, struct mce *final, char *exp)
>   	if (!fake_panic) {
>   		if (panic_timeout == 0)
>   			panic_timeout = mca_cfg.panic_timeout;
> +
> +		mce_wrmsrl(MSR_IA32_MCG_STATUS, 0);
>   		panic(msg);
>   	} else
>   		pr_emerg(HW_ERR "Fake kernel panic: %s\n", msg);
>
> so that we not run kexec in #MC context.
>
> Hmmm.

I don't have ready test case for real MCE to verify whether it has 
exited #MC context before panic() or not.

In mce-inject case that based on NMI, it doesn't work as mentioned indeed.

B.R.,

Zeng Heng