Re: [kernel-hardening] Re: [RFC PATCH 2/2] mm: restrict access to /proc/slabinfo

[Vasiliy Kulikov <segoon@openwall.com>]

On Mon, Sep 19, 2011 at 18:13 +0300, Pekka Enberg wrote:
> On Mon, Sep 19, 2011 at 5:46 PM, Vasiliy Kulikov <segoon@openwall.com> wrote:
> >> and
> >> concluded that it's not worth it doesn't really protect from anything
> >
> > Closing only slabinfo doesn't add any significant protection against
> > kernel heap exploits per se, no objections here.
> >
> > But as said in the desciption, the reason for this patch is not protecting
> > against exploitation heap bugs.  It is a source of infoleaks of kernel
> > and userspace activity, which should be forbidden to non-root users.
> 
> Last time we discussed this, the 'extra protection' didn't seem to be
> significant enough to justify disabling an useful kernel debugging
> interface by default.
> 
> 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.


> >> and causes harm to developers.
> >
> > 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.


> 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".

Thanks,

-- 
Vasiliy Kulikov
http://www.openwall.com - bringing security into open computing environments

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