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

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Date:	Fri, 24 Apr 2015 17:46:25 +0100
From:	Mark Williamson <mwilliamson@...o-software.com>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	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>,
	Andy Lutomirski <luto@...capital.net>,
	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

Hi Linus,

Thanks for responding so quickly!

On Fri, Apr 24, 2015 at 5:08 PM, Linus Torvalds
<torvalds@...ux-foundation.org> wrote:
> So the one exception to the regression rule is "security fixes", but
> even for security fixes we do try to be as reasonable as humanly
> possible to make them not break things.

Understood - there are clear reasons something had to be done here.

> Now, as you mentioned, one option is to not outright disallow accesses
> to the /proc/PID/pagemap, but to at least hide the page frame numbers.
> However, I don't believe that we have a good enough scrambling model
> to make that reasonable. Remember: any attacker will be able to see
> our scrambling code, so it would need to be both cryptographically
> secure *and* use a truly random per-VM secret key. Quite frankly,
> that's a _lot_ of effort for dubious gain...

*nod*

> So the "just show physical addresses as zero for non-root users"
> (instead of the outright ban on opening the file) is likely the only
> really viable alternative.
>
> It sounds like that could work for you. So if you can modify the app
> to do that, and send me a tested kernel patch that moves the
> permission check into the read phase (remember to use the open-time
> credentials in "file->f_cred" rather than the read-time credentials in
> "current" - otherwise you can trick some suid program to read the fily
> that an unauthorized user opened), then we can have this fixed. Does
> that sound reasonable?

That sounds very reasonable, thank you!  We'll cook up a patch and get
back to you.

Thanks,
Mark
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