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Date: Thu, 9 Nov 2017 12:05:36 -0600
From: "Serge E. Hallyn" <>
To: chris hyser <>
Cc: "Serge E. Hallyn" <>,
	Daniel Micay <>,
	Mahesh Bandewar (महेश बंडेवार) <>,
	Mahesh Bandewar <>,
	LKML <>,
	Netdev <>,
	Kernel-hardening <>,
	Linux API <>,
	Kees Cook <>,
	"Eric W . Biederman" <>,
	Eric Dumazet <>,
	David Miller <>
Subject: Re: Re: [PATCH resend 2/2] userns: control
 capabilities of some user namespaces

Quoting chris hyser (
> On 11/06/2017 10:23 PM, Serge E. Hallyn wrote:
> >I think I definately prefer what I mentioned in the email to Boris.
> >Basically a "permanent capability bounding set".  The normal bounding
> >set gets reset to a full set on every new user_ns creation.  In this
> >proposal, it would instead be set to the calling task's permanent
> >capability set, which starts (at boot) full, and which privileged
> >tasks can pull capabilities out of.
> Actually, this may solve a similar problem I've been looking at. The
> idea was basically at strategic points in the kernel (possibly LSM
> hook sites, still evaluating, and probably syscall entry) validate
> that a task has not "magically" acquired capabilities that it or
> parent specifically said it cannot have and then take some action
> like say killing it immediately. Using your terms, basically make
> the "permanent capability set" a write-once privilege escalation
> defense. To handle the 0-day threat, perhaps make it writable but
> only with more "restrictive" values.

Would the existing capability bounding set not suffice for that?

The 'permanent' bounding set turns out to not be a good fit for
the problem being discussed in this thread, but please feel free
to start a new thread if you want to discuss your use case.

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