linux-kernel - Re: [PATCH] [RFC] Make it easier to harden /proc/

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Date:	Wed, 16 Mar 2011 14:27:57 -0700
From:	ebiederm@...ssion.com (Eric W. Biederman)
To:	Richard Weinberger <richard@....at>
Cc:	Arnd Bergmann <arnd@...db.de>, Kees Cook <kees.cook@...onical.com>,
	linux-kernel@...r.kernel.org, akpm@...ux-foundation.org,
	serge@...lyn.com, eparis@...hat.com, jmorris@...ei.org,
	eugeneteo@...nel.org, drosenberg@...curity.com
Subject: Re: [PATCH] [RFC] Make it easier to harden /proc/

Richard Weinberger <richard@....at> writes:

> Am Mittwoch 16 März 2011, 22:17:39 schrieb Eric W. Biederman:
>> Richard Weinberger <richard@....at> writes:
>> 
>> 2> Am Mittwoch 16 März 2011, 21:45:45 schrieb Arnd Bergmann:
>> >> On Wednesday 16 March 2011 21:08:16 Richard Weinberger wrote:
>> >> > Am Mittwoch 16 März 2011, 20:55:49 schrieb Kees Cook:
>> >> > > On Wed, Mar 16, 2011 at 08:31:47PM +0100, Richard Weinberger wrote:
>> >> > > > When containers like LXC are used a unprivileged and jailed
>> >> > > > root user can still write to critical files in /proc/.
>> >> > > > E.g: /proc/sys/kernel/{sysrq, panic, panic_on_oops, ... }
>> >> > > > 
>> >> > > > This new restricted attribute makes it possible to protect such
>> >> > > > files. When restricted is set to true root needs CAP_SYS_ADMIN
>> >> > > > to into the file.
>> >> > > 
>> >> > > I was thinking about this too. I'd prefer more fine-grained control
>> >> > > in this area, since some sysctl entries aren't strictly controlled
>> >> > > by CAP_SYS_ADMIN (e.g. mmap_min_addr is already checking
>> >> > > CAP_SYS_RAWIO).
>> >> > > 
>> >> > > How about this instead?
>> >> > 
>> >> > Good Idea.
>> >> > May we should also consider a per-directory restriction.
>> >> > Every file in /proc/sys/{kernel/, vm/, fs/, dev/} needs a protection.
>> >> > It would be much easier to set the protection on the parent directory
>> >> > instead of protecting file by file...
>> >> 
>> >> How does this interact with the per-namespace sysctls that Eric
>> >> Biederman added a few years ago?
>> > 
>> > Do you mean CONFIG_{UTS, UPC, USER, NET,}_NS?
>> > 
>> >> I had expected that any dangerous sysctl would not be visible in
>> >> an unpriviledge container anyway.
>> > 
>> > No way.
>> > That's why it's currently a very good idea to mount /proc/ read-only
>> > into a container.
>> 
>> However it is in the architecture.  The problem is that the user
>> namespace is not finished.  Once finished even root with all caps in a
>> container will have no more permissions than the unprivileged user that
>> created the user namespace.
>> 
>> Essentially the change is to make permissions checks become a comparison
>> of the tuple (user_ns, uid) instead of just comparisons by uid.  If we
>> want to fix permission problems with proc and containers please let's
>> focus on the completing the user namespace.
>
> Ok. What's the current status, where can I help?

Serge has been getting some of the pieces together and merging them to
Andrew.  I think he has the basic infrastructure in place.  Certainly he
has the infrastructure in place for per user namespace capabilities.

What should be left is the mechanics of making certain every permission
check in the kernel takes user namespaces properly into account.

Eric
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