linux-kernel - Re: [PATCH] x86/mm: Do not split_large_page() for set_kernel_text

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Date:   Mon, 26 Aug 2019 07:33:08 -0400
From:   Steven Rostedt <rostedt@...dmis.org>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Song Liu <songliubraving@...com>, linux-kernel@...r.kernel.org,
        linux-mm@...ck.org, kernel-team@...com, stable@...r.kernel.org,
        Thomas Gleixner <tglx@...utronix.de>,
        Dave Hansen <dave.hansen@...el.com>,
        Andy Lutomirski <luto@...capital.net>,
        Nadav Amit <namit@...are.com>,
        Daniel Bristot de Oliveira <bristot@...hat.com>
Subject: Re: [PATCH] x86/mm: Do not split_large_page() for
 set_kernel_text_rw()

On Fri, 23 Aug 2019 11:36:37 +0200
Peter Zijlstra <peterz@...radead.org> wrote:

> On Thu, Aug 22, 2019 at 10:23:35PM -0700, Song Liu wrote:
> > As 4k pages check was removed from cpa [1], set_kernel_text_rw() leads to
> > split_large_page() for all kernel text pages. This means a single kprobe
> > will put all kernel text in 4k pages:
> > 
> >   root@ ~# grep ffff81000000- /sys/kernel/debug/page_tables/kernel
> >   0xffffffff81000000-0xffffffff82400000     20M  ro    PSE      x  pmd
> > 
> >   root@ ~# echo ONE_KPROBE >> /sys/kernel/debug/tracing/kprobe_events
> >   root@ ~# echo 1 > /sys/kernel/debug/tracing/events/kprobes/enable
> > 
> >   root@ ~# grep ffff81000000- /sys/kernel/debug/page_tables/kernel
> >   0xffffffff81000000-0xffffffff82400000     20M  ro             x  pte
> > 
> > To fix this issue, introduce CPA_FLIP_TEXT_RW to bypass "Text RO" check
> > in static_protections().
> > 
> > Two helper functions set_text_rw() and set_text_ro() are added to flip
> > _PAGE_RW bit for kernel text.
> > 
> > [1] commit 585948f4f695 ("x86/mm/cpa: Avoid the 4k pages check completely")  
> 
> ARGH; so this is because ftrace flips the whole kernel range to RW and
> back for giggles? I'm thinking _that_ is a bug, it's a clear W^X
> violation.

Since ftrace did this way before text_poke existed and way before
anybody cared (back in 2007), it's not really a bug.

Anyway, I believe Nadav has some patches that converts ftrace to use
the shadow page modification trick somewhere.

Or we also need the text_poke batch processing (did that get upstream?).

Mapping in 40,000 pages one at a time is noticeable from a human stand
point.

-- Steve