linux-kernel - Re: [PATCH 17/30] x86, kaiser: map debug IDT tables

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Date:   Mon, 20 Nov 2017 21:54:13 +0100 (CET)
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Andy Lutomirski <luto@...nel.org>
cc:     Dave Hansen <dave.hansen@...ux.intel.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-mm@...ck.org" <linux-mm@...ck.org>,
        moritz.lipp@...k.tugraz.at,
        Daniel Gruss <daniel.gruss@...k.tugraz.at>,
        michael.schwarz@...k.tugraz.at, richard.fellner@...dent.tugraz.at,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Kees Cook <keescook@...gle.com>,
        Hugh Dickins <hughd@...gle.com>, X86 ML <x86@...nel.org>
Subject: Re: [PATCH 17/30] x86, kaiser: map debug IDT tables

On Mon, 20 Nov 2017, Andy Lutomirski wrote:

> On Fri, Nov 10, 2017 at 11:31 AM, Dave Hansen
> <dave.hansen@...ux.intel.com> wrote:
> >
> > From: Dave Hansen <dave.hansen@...ux.intel.com>
> >
> > The IDT is another structure which the CPU references via a
> > virtual address.  It also obviously needs these to handle an
> > interrupt in userspace, so these need to be mapped into the user
> > copy of the page tables.
> 
> Why would the debug IDT ever be used in user mode?  IIRC it's a total
> turd related to avoiding crap nesting inside NMI.  Or am I wrong?

No. It's called from the TRACE_IRQS macros in the ASM entry code and from
do_nmi().

> If it *is* used in user mode, then we have a bug and it should be in
> the IDT to avoid address leaks just like the normal IDT.

It's not so this can go away. Good catch.

Thanks,

	tglx