linux-kernel - Re: [RFC] x86_64: A real proposal for iret-less return to kernel

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Message-ID: <20140521235143.GC21575@pd.tnic>
Date:	Thu, 22 May 2014 01:51:43 +0200
From:	Borislav Petkov <bp@...en8.de>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	"Luck, Tony" <tony.luck@...el.com>,
	Andy Lutomirski <luto@...capital.net>,
	Jiri Kosina <jkosina@...e.cz>,
	Thomas Gleixner <tglx@...utronix.de>,
	Steven Rostedt <rostedt@...dmis.org>,
	Andi Kleen <andi@...stfloor.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"H. Peter Anvin" <hpa@...or.com>, Ingo Molnar <mingo@...nel.org>
Subject: Re: [RFC] x86_64: A real proposal for iret-less return to kernel

On Thu, May 22, 2014 at 08:30:33AM +0900, Linus Torvalds wrote:
> If the OS then decides to take down the whole machine, the OS - not
> the hardware - can choose to do something that will punch through
> other CPU's NMI blocking (notably, init/reset), but the hardware doing
> this on its own is just broken if true.

Not that it is any consolation but MCE is not broadcast on AMD.

Regardless, exceptions like MCE cannot be held pending and do pierce the
NMI handler on both.

Now, if the NMI handler experiences a non-broadcast MCE on the same CPU,
while running, we're simply going to panic as we're in kernel space
anyway.

The only problem is if the NMI handler gets interrupted while running
on a bystander CPU. And I think we could deal with this because the
bystander would not see an MCE and will return safely. We just need
to make sure that it returns back to the said NMI handler and not to
userspace. Unless I'm missing something ...

Oh yeah, fun :-\

-- 
Regards/Gruss,
    Boris.

Sent from a fat crate under my desk. Formatting is fine.
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