linux-kernel - Re: [PATCH RFC] mm: add MAP_EXCLUSIVE to create exclusive user mappings

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Message-ID: <20191028130018.GA7192@rapoport-lnx>
Date:   Mon, 28 Oct 2019 14:00:19 +0100
From:   Mike Rapoport <rppt@...nel.org>
To:     "Kirill A. Shutemov" <kirill@...temov.name>
Cc:     linux-kernel@...r.kernel.org,
        Alexey Dobriyan <adobriyan@...il.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Andy Lutomirski <luto@...nel.org>,
        Arnd Bergmann <arnd@...db.de>, Borislav Petkov <bp@...en8.de>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        James Bottomley <jejb@...ux.ibm.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>,
        "H. Peter Anvin" <hpa@...or.com>, linux-api@...r.kernel.org,
        linux-mm@...ck.org, x86@...nel.org,
        Mike Rapoport <rppt@...ux.ibm.com>
Subject: Re: [PATCH RFC] mm: add MAP_EXCLUSIVE to create exclusive user
 mappings

On Mon, Oct 28, 2019 at 03:31:24PM +0300, Kirill A. Shutemov wrote:
> On Sun, Oct 27, 2019 at 12:17:32PM +0200, Mike Rapoport wrote:
> > From: Mike Rapoport <rppt@...ux.ibm.com>
> > 
> > The mappings created with MAP_EXCLUSIVE are visible only in the context of
> > the owning process and can be used by applications to store secret
> > information that will not be visible not only to other processes but to the
> > kernel as well.
> > 
> > The pages in these mappings are removed from the kernel direct map and
> > marked with PG_user_exclusive flag. When the exclusive area is unmapped,
> > the pages are mapped back into the direct map.
> 
> I probably blind, but I don't see where you manipulate direct map...

__get_user_pages() calls __set_page_user_exclusive() which in turn calls
set_direct_map_invalid_noflush() that makes the page not present.
 
> -- 
>  Kirill A. Shutemov

-- 
Sincerely yours,
Mike.