linux-kernel - Re: [PATCH 3.14 79/84] x86/nmi/64: Switch stacks on userspace NMI entry

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Message-ID: <CALCETrUqbZh=736tRJCidKtRO37oqCyLU_v71xCEn4sb_tNmbg@mail.gmail.com>
Date:	Tue, 29 Sep 2015 10:25:52 -0700
From:	Andy Lutomirski <luto@...capital.net>
To:	Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Cc:	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	stable <stable@...r.kernel.org>,
	Andy Lutomirski <luto@...nel.org>,
	Steven Rostedt <rostedt@...dmis.org>,
	Borislav Petkov <bp@...e.de>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...nel.org>
Subject: Re: [PATCH 3.14 79/84] x86/nmi/64: Switch stacks on userspace NMI entry

On Tue, Sep 29, 2015 at 8:19 AM, Greg Kroah-Hartman
<gregkh@...uxfoundation.org> wrote:
> 3.14-stable review patch.  If anyone has any objections, please let me know.
>
> ------------------
>
> From: Andy Lutomirski <luto@...nel.org>
>
> commit 9b6e6a8334d56354853f9c255d1395c2ba570e0a upstream.
>
> Returning to userspace is tricky: IRET can fail, and ESPFIX can
> rearrange the stack prior to IRET.

Just a heads up: under very strange circumstances (most likely
requires booting with paravirt-noreplace, which is hopefully only used
by Sasha), this causes a regression that's fixed by
83c133cf11fb0e68a51681447e372489f052d40e.  I'm not 100% sure that the
regression is observable at all, because the same strange
configuration may also hit the bug fixed by
fc57a7c68020dcf954428869eafd934c0ab1536f before the regression can
trigger.

Anyway, you've been warned.  I wouldn't worry too much about it, though.

--Andy
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