linux-kernel - Re: [PATCH 1/3] perf/core: Make sure the ring-buffer is mapped in all page-tables

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Date:   Fri, 20 Jul 2018 12:33:14 -0700
From:   Andy Lutomirski <luto@...nel.org>
To:     Thomas Gleixner <tglx@...utronix.de>
Cc:     Andy Lutomirski <luto@...nel.org>, Joerg Roedel <joro@...tes.org>,
        Ingo Molnar <mingo@...nel.org>,
        "H . Peter Anvin" <hpa@...or.com>, X86 ML <x86@...nel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Linux-MM <linux-mm@...ck.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Dave Hansen <dave.hansen@...el.com>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        Juergen Gross <jgross@...e.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Borislav Petkov <bp@...en8.de>, Jiri Kosina <jkosina@...e.cz>,
        Boris Ostrovsky <boris.ostrovsky@...cle.com>,
        Brian Gerst <brgerst@...il.com>,
        David Laight <David.Laight@...lab.com>,
        Denys Vlasenko <dvlasenk@...hat.com>,
        Eduardo Valentin <eduval@...zon.com>,
        Greg KH <gregkh@...uxfoundation.org>,
        Will Deacon <will.deacon@....com>,
        "Liguori, Anthony" <aliguori@...zon.com>,
        Daniel Gruss <daniel.gruss@...k.tugraz.at>,
        Hugh Dickins <hughd@...gle.com>,
        Kees Cook <keescook@...gle.com>,
        Andrea Arcangeli <aarcange@...hat.com>,
        Waiman Long <llong@...hat.com>, Pavel Machek <pavel@....cz>,
        "David H . Gutteridge" <dhgutteridge@...patico.ca>,
        Joerg Roedel <jroedel@...e.de>,
        Arnaldo Carvalho de Melo <acme@...nel.org>,
        Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
        Jiri Olsa <jolsa@...hat.com>,
        Namhyung Kim <namhyung@...nel.org>
Subject: Re: [PATCH 1/3] perf/core: Make sure the ring-buffer is mapped in all page-tables

On Fri, Jul 20, 2018 at 12:27 PM, Thomas Gleixner <tglx@...utronix.de> wrote:
> On Fri, 20 Jul 2018, Andy Lutomirski wrote:
>> > On Jul 20, 2018, at 6:22 AM, Joerg Roedel <joro@...tes.org> wrote:
>> >
>> > From: Joerg Roedel <jroedel@...e.de>
>> >
>> > The ring-buffer is accessed in the NMI handler, so we better
>> > avoid faulting on it. Sync the vmalloc range with all
>> > page-tables in system to make sure everyone has it mapped.
>> >
>> > This fixes a WARN_ON_ONCE() that can be triggered with PTI
>> > enabled on x86-32:
>> >
>> >    WARNING: CPU: 4 PID: 0 at arch/x86/mm/fault.c:320 vmalloc_fault+0x220/0x230
>> >
>> > This triggers because with PTI enabled on an PAE kernel the
>> > PMDs are no longer shared between the page-tables, so the
>> > vmalloc changes do not propagate automatically.
>>
>> It seems like it would be much more robust to fix the vmalloc_fault()
>> code instead.
>
> Right, but now the obvious fix for the issue at hand is this. We surely
> should revisit this.

If you commit this under this reasoning, then please at least make it say:

/* XXX: The vmalloc_fault() code is buggy on PTI+PAE systems, and this
is a workaround. */

Let's not have code in the kernel that pretends to make sense but is
actually voodoo magic that works around bugs elsewhere.  It's no fun
to maintain down the road.