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Date: Fri, 24 Apr 2015 09:27:03 -0700
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Subject: Re: Regression: Requiring CAP_SYS_ADMIN for /proc/<pid>/pagemap
 causes application-level breakage
From: Linus Torvalds <torvalds@linux-foundation.org>
To: Andy Lutomirski <luto@amacapital.net>
Cc: Mark Williamson <mwilliamson@undo-software.com>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>,
        Pavel Emelyanov <xemul@parallels.com>,
        Konstantin Khlebnikov <khlebnikov@openvz.org>,
        Andrew Morton <akpm@linux-foundation.org>,
        Mark Seaborn <mseaborn@chromium.org>,
        Linux API <linux-api@vger.kernel.org>,
        Finn Grimwood <fgrimwood@undo-software.com>,
        Daniel James <djames@undo-software.com>
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On Fri, Apr 24, 2015 at 9:10 AM, Andy Lutomirski <luto@amacapital.net> wrote:
>
> Even though I've been accused (correctly?) of suggesting that, I'm not
> sure I like it anymore.  Suppose I map some anonymous memory, learn
> its (scrambled) pfn, then unmap it and remap a setuid file.  Now I can
> tell whether I've mapped the setuid file at the same pfn that was
> mapped as my anonymous memory.  IIRC that's sufficient for one of the
> variants of Mark's attack.

Ack. So we really do have to zero out the pfn entirely for security
reasons, and not just because it's less effort.

                         Linus
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