linux-kernel - Re: Regression: Requiring CAP_SYS_ADMIN for /proc/<pid>/pagemap causes application-level breakage

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Message-ID: <CA+55aFwx=mUtm3HWN=-=s23q3Q0CFDXiXPoeFKRzCN29dQ6oQw@mail.gmail.com>
Date:	Fri, 24 Apr 2015 09:27:03 -0700
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Andy Lutomirski <luto@...capital.net>
Cc:	Mark Williamson <mwilliamson@...o-software.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	"Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
	Pavel Emelyanov <xemul@...allels.com>,
	Konstantin Khlebnikov <khlebnikov@...nvz.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Mark Seaborn <mseaborn@...omium.org>,
	Linux API <linux-api@...r.kernel.org>,
	Finn Grimwood <fgrimwood@...o-software.com>,
	Daniel James <djames@...o-software.com>
Subject: Re: Regression: Requiring CAP_SYS_ADMIN for /proc/<pid>/pagemap
 causes application-level breakage

On Fri, Apr 24, 2015 at 9:10 AM, Andy Lutomirski <luto@...capital.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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