Date:	Fri, 18 Nov 2011 15:25:12 -0800
From:	Matt Helsley <matthltc@...ibm.com>
To:	Pavel Emelyanov <xemul@...allels.com>
Cc:	Matt Helsley <matthltc@...ibm.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Cyrill Gorcunov <gorcunov@...nvz.org>,
	Glauber Costa <glommer@...allels.com>,
	Andi Kleen <andi@...stfloor.org>, Tejun Heo <tj@...nel.org>,
	Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [PATCH 0/4] Checkpoint/Restore: Show in proc IDs of objects that
 can be shared between tasks

On Wed, Nov 16, 2011 at 12:25:16PM +0400, Pavel Emelyanov wrote:
> On 11/16/2011 09:44 AM, Matt Helsley wrote:
> > On Tue, Nov 15, 2011 at 03:35:58PM +0400, Pavel Emelyanov wrote:
> >> While doing the checkpoint-restore in the userspace one need to determine
> >> whether various kernel objects (like mm_struct-s of file_struct-s) are shared
> >> between tasks and restore this state.
> >>
> >> The 2nd step can for now be solved by using respective CLONE_XXX flags and
> >> the unshare syscall, while there's currently no ways for solving the 1st one.
> >>
> >> One of the ways for checking whether two tasks share e.g. an mm_struct is to
> >> provide some mm_struct ID of a task to its proc file. The best from the
> >> performance point of view ID is the object address in the kernel, but showing
> >> them to the userspace is not good for performance reasons.
> > 
> > (I think you meant "not good for security reasons."...)
> > 
> >> The previous attempt to solve this was to generate an ID for slab/slub and then
> >> mix it up with the object index on the slab page. This attempt wasn't met
> >> warmly by slab maintainers, so here's the 2nd approach.
> >>
> >> The object address is XOR-ed with a "random" value of the same size and then
> >> shown in proc. Providing this poison is not leaked into the userspace then
> >> ID seem to be safe.
> > 
> > Really? There's no way to quickly derive the random number from known
> > allocation patterns and thereby break the obfuscation scheme?
> > To start we can note that the low N bits are directly exposed in the ID
> > of anything that requires 2^N-byte alignment.
> > 
> > I think it's really a question of whether the high order bits can be derived.
> > 
> > And of course the random number only needs to be derived once per boot
> > before it reveals the address of everything with an ID.
> 
> Tejun already proposed to split ID space and use different poisons for them.
> 
> > Some wild speculation:
> > 
> > I bet you could use some cpu affinity, mem policy, slab info, mmap
> > tricks, etc. to derive more low bits of the random number. You can probably
> > get even more when you consider objects that don't fit evenly in slabs.
> > Speaking of slabs, is there some way to use the fact that nearby slab objects
> > will share their high ID bits?
> 
> OK, let's assume we found out that two mm_struct IDs have higher bits equal, what
> can we do next to split address bits from the poison ones?

Perhaps we can figure out where things are likely to be allocated by
looking at the booted kernel in /boot.

Do we really need to spend time discussing precisely how this ID scheme
can be attacked? I think we're better off just switching to the sha* hash
scheme or 64-bit counter instead.

It would also be good to specify that the IDs presented to userspace
are identifying strings (names) -- not identifying numbers. This way the ABI
will be forward and backward compatible if the kernel ever needs to change
the way it generates them.

Cheers,
	-Matt

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