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Date: Mon, 19 Sep 2011 20:18:37 +0400
From: Vasiliy Kulikov <segoon@...nwall.com>
To: Pekka Enberg <penberg@...helsinki.fi>
Cc: Andrew Morton <akpm@...ux-foundation.org>,
kernel-hardening@...ts.openwall.com, Kees Cook <kees@...ntu.com>,
Cyrill Gorcunov <gorcunov@...il.com>,
Al Viro <viro@...iv.linux.org.uk>,
Christoph Lameter <cl@...ux-foundation.org>,
Matt Mackall <mpm@...enic.com>, linux-kernel@...r.kernel.org,
linux-mm@...ck.org, Dan Rosenberg <drosenberg@...curity.com>,
Theodore Tso <tytso@....edu>, Alan Cox <alan@...ux.intel.com>,
Jesper Juhl <jj@...osbits.net>,
Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: Re: [RFC PATCH 2/2] mm: restrict access to
/proc/slabinfo
On Mon, Sep 19, 2011 at 19:11 +0300, Pekka Enberg wrote:
> >> What's different about the patch now?
> >
> > The exploitation you're talking about is an exploitation of kernel heap
> > bugs. Dan's previous "make slabinfo 0400" patch tried to complicate
> > attacker's life by hiding information about how many free object are
> > left in the slab. With this information an attacker may compute how he
> > should spray the slab to position slab object to increase his chances of
> > overwriting specific memory areas - pointers, etc.
> >
> > I don't speak about how much/whether closing slabinfo complicates this
> > task, though. My idea is orthogonal to the Dan's idea. I claim that
> > with 0444 slabinfo any user may get information about in-system activity
> > that he shouldn't learn. In short, one may learn precisely when other
> > user reads directory contents, opens files, how much files there are in
> > the specific _private_ directory, how much files _private_ ecryptfs or
> > fuse mount point contains, etc. This breaks user's assumption that
> > the number of files in a private directory is a private information.
> > There are a bit more thoughts in the patch description.
>
> Yes, I read your patch description and I think it's convincing enough
> to warrant a config option but not changing the default.
>
> However, if the encryptfs and infoleaks really are serious enough to
> hide /proc/slabinfo, I think you should consider switching over to
> kmalloc() instead of kmem_cache_alloc() to make sure nobody can
> gain access to the information.
kmalloc() is still visible in slabinfo as kmalloc-128 or so.
> >> > One note: only to _kernel_ developers. It means it is a strictly
> >> > debugging feature, which shouldn't be enabled in the production systems.
> >>
> >> It's pretty much _the_ interface for debugging kernel memory leaks in
> >> production systems and we ask users for it along with /proc/meminfo
> >> when debugging many memory management related issues. When we
> >> temporarily dropped /proc/slabinfo with the introduction of SLUB, people
> >> complained pretty loudly.
> >
> > Could you point to the discussion, please? I cannot find the patch for
> > 0400 slabinfo even in the linux-history repository.
>
> We dropped the whole file for SLUB:
>
> http://lwn.net/Articles/263337/
Ah, I've misunderstood you.
> [ I didn't find the original discussion that motivated the above
> patch but it should be somewhere in LKML archives around
> that time. ]
>
> Making it root-only will have pretty much the same kind of
> out-of-the-box behavior.
>
> >> I'd be willing to consider this patch if it's a config option that's not enabled
> >> by default; otherwise you need to find someone else to merge the patch.
> >> You can add some nasty warnings to the Kconfig text to scare the users
> >> into enabling it. ;-)
> >
> > How do you see this CONFIG_ option? CONFIG_PROCFS_COMPAT_MODES (or _PERMS),
> > defaults to Y? If we find more procfs files with dangerous permissions,
> > we may move it under "ifndef CONFIG_PROCFS_COMPAT_PERMS".
>
> I guess CONFIG_RESTRICT_PROCFS type of thing makes most sense
> since the problem is not only about SLAB. If you want to make it slab-only
> config option, I'm fine with that too.
OK, then I'll prepare a patch with a configure option, if no other
objections.
> Please note that you need to restrict sysfs files for SLUB as well.
Sure.
Thank you for the comments!
--
Vasiliy Kulikov
http://www.openwall.com - bringing security into open computing environments
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