linux-kernel - Re: [PATCH] mm,memory_failure: Always pin the page in madvise_inject

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Message-ID: <20201208023446.GA10757@hori.linux.bs1.fc.nec.co.jp>
Date:   Tue, 8 Dec 2020 02:34:46 +0000
From:   HORIGUCHI NAOYA(堀口　直也) 
        <naoya.horiguchi@....com>
To:     Andrew Morton <akpm@...ux-foundation.org>
CC:     Oscar Salvador <osalvador@...e.de>,
        "n-horiguchi@...jp.nec.com" <n-horiguchi@...jp.nec.com>,
        "vbabka@...e.cz" <vbabka@...e.cz>,
        "dan.j.williams@...el.com" <dan.j.williams@...el.com>,
        "linux-mm@...ck.org" <linux-mm@...ck.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] mm,memory_failure: Always pin the page in
 madvise_inject_error

On Mon, Dec 07, 2020 at 06:22:00PM -0800, Andrew Morton wrote:
> On Mon,  7 Dec 2020 10:48:18 +0100 Oscar Salvador <osalvador@...e.de> wrote:
> 
> > madvise_inject_error() uses get_user_pages_fast to translate the
> > address we specified to a page.
> > After [1], we drop the extra reference count for memory_failure() path.
> > That commit says that memory_failure wanted to keep the pin in order
> > to take the page out of circulation.
> > 
> > The truth is that we need to keep the page pinned, otherwise the
> > page might be re-used after the put_page() and we can end up messing
> > with someone else's memory.
> > 
> > E.g:
> > 
> > CPU0
> > process X					CPU1
> >  madvise_inject_error
> >   get_user_pages
> >    put_page
> > 					page gets reclaimed
> > 					process Y allocates the page
> >   memory_failure
> >    // We mess with process Y memory
> > 
> > madvise() is meant to operate on a self address space, so messing with
> > pages that do not belong to us seems the wrong thing to do.
> > To avoid that, let us keep the page pinned for memory_failure as well.
> > 
> > Pages for DAX mappings will release this extra refcount in
> > memory_failure_dev_pagemap.
> 
> Does the bug have any known user-visible effects?  Is a deliberate
> exploit conceivable?
> 
> IOW, cc:stable and if so, why?

This interface is a testing feature and only available only for privileged
(CAP_SYS_ADMIN) users, so I don't think that this bug is critical. But if
someone think it need to go to stable, I'm fine with that.

Thanks,
Naoya Horiguchi