linux-kernel - Re: Dealing with the NMI mess

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Message-ID: <CALCETrUf-EWOAZkAOm_mBWDMhTU4n=qtgm18Zhqhuv1jeg1f0g@mail.gmail.com>
Date:	Mon, 7 Sep 2015 10:22:56 -0700
From:	Andy Lutomirski <luto@...capital.net>
To:	"Maciej W. Rozycki" <macro@...ux-mips.org>
Cc:	Paolo Bonzini <pbonzini@...hat.com>,
	Ingo Molnar <mingo@...nel.org>, Borislav Petkov <bp@...en8.de>,
	Peter Zijlstra <peterz@...radead.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Willy Tarreau <w@....eu>, Steven Rostedt <rostedt@...dmis.org>,
	X86 ML <x86@...nel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Brian Gerst <brgerst@...il.com>
Subject: Re: Dealing with the NMI mess

On Mon, Sep 7, 2015 at 10:01 AM, Maciej W. Rozycki <macro@...ux-mips.org> wrote:
>  These are all implementation-specific details, including the INT1
> instruction, which is why I am not at all surprised that they are omitted
> from architecture manuals.

That bit is BS, though.  The INT1 instruction, executed in user mode
(CPL3) with no hardware debugger attached, will enter the kernel
through a gate at vector 1, *even if that gate has DPL == 0*.

If there's an instruction that bypasses hardware protection
mechanisms, then Intel should document it rather than relying on OS
writers to know enough folklore to get it right.

Heck, SDM Volume 3 6.12.1.1 says "The processor checks the DPL of the
interrupt or trap gate only if an exception or interrupt is generated
with an INT n, INT 3, or INTO instruction."  It does not say "the
processor does not check the DPL of the interrupt or trap gate if the
exception or interrupt is generated with the undocumented ICEBP
instruction."

--Andy
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