linux-kernel - Re: Dealing with the NMI mess

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Date:	Fri, 24 Jul 2015 19:21:00 +0200
From:	Willy Tarreau <w@....eu>
To:	Andy Lutomirski <luto@...capital.net>
Cc:	Peter Zijlstra <peterz@...radead.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Steven Rostedt <rostedt@...dmis.org>, X86 ML <x86@...nel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Borislav Petkov <bp@...en8.de>,
	Thomas Gleixner <tglx@...utronix.de>,
	Brian Gerst <brgerst@...il.com>
Subject: Re: Dealing with the NMI mess

On Fri, Jul 24, 2015 at 07:10:18PM +0200, Willy Tarreau wrote:
> The OS has to set the RSP by itself before doing SYSRET, which opens a
> race between "mov rsp" and "sysret", but if we only take that path once
> we figure we come from NMI (using just IF+RSP), we know that IRQs and
> NMIs are still disabled and cannot strike at this instant. Maybe MCEs
> can, but they would execute within the NMI's stack just as if they were
> triggered inside the NMI as well so I don't see a problem here.

OK too bad I just found the response here in the code :-(

     * SYSRET can't restore RF.  SYSRET can restore TF, but unlike IRET,
     * restoring TF results in a trap from userspace immediately after
     * SYSRET.  This would cause an infinite loop whenever #DB happens
     * with register state that satisfies the opportunistic SYSRET
     * conditions.

Willy

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