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Date: Wed, 14 Sep 2011 17:16:30 +0400
From: Vasiliy Kulikov <segoon@...nwall.com>
To: kernel-hardening@...ts.openwall.com,
Andrew Morton <akpm@...ux-foundation.org>
Cc: Cyrill Gorcunov <gorcunov@...il.com>, Al Viro <viro@...iv.linux.org.uk>,
Christoph Lameter <cl@...ux-foundation.org>,
Pekka Enberg <penberg@...nel.org>, Matt Mackall <mpm@...enic.com>,
linux-kernel@...r.kernel.org, linux-mm@...ck.org,
Dave Hansen <dave@...ux.vnet.ibm.com>,
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: [RFC PATCH 2/2] mm: restrict access to /proc/slabinfo
(cc'ed all members of the previous discussion as currently lists might
not work as they should)
On Sat, Sep 10, 2011 at 20:41 +0400, Vasiliy Kulikov wrote:
> Historically /proc/slabinfo has 0444 permissions and is accessible to
> the world. slabinfo contains rather private information related both to
> the kernel and userspace tasks. Depending on the situation, it might
> reveal either private information per se or information useful to make
> another targeted attack. Some examples of what can be learned by
> reading/watching for /proc/slabinfo entries:
>
> 1) dentry (and different *inode*) number might reveal other processes fs
> activity. The number of dentry "active objects" doesn't strictly show
> file count opened/touched by a process, however, there is a good
> correlation between them. The patch "proc: force dcache drop on
> unauthorized access" relies on the privacy of dentry count.
>
> 2) different inode entries might reveal the same information as (1), but
> these are more fine granted counters. If a filesystem is mounted in a
> private mount point (or even a private namespace) and fs type differs from
> other mounted fs types, fs activity in this mount point/namespace is
> revealed. If there is a single ecryptfs mount point, the whole fs
> activity of a single user is revealed. Number of files in ecryptfs
> mount point is a private information per se.
>
> 3) fuse_* reveals number of files / fs activity of a user in a user
> private mount point. It is approx. the same severity as ecryptfs
> infoleak in (2).
>
> 4) sysfs_dir_cache similar to (2) reveals devices' addition/removal,
> which can be otherwise hidden by "chmod 0700 /sys/". With 0444 slabinfo
> the precise number of sysfs files is known to the world.
>
> 5) buffer_head might reveal some kernel activity. With other
> information leaks an attacker might identify what specific kernel
> routines generate buffer_head activity.
>
> 6) *kmalloc* infoleaks are very situational. Attacker should watch for
> the specific kmalloc size entry and filter the noise related to the unrelated
> kernel activity. If an attacker has relatively silent victim system, he
> might get rather precise counters.
>
> Additional information sources might significantly increase the slabinfo
> infoleak benefits. E.g. if an attacker knows that the processes
> activity on the system is very low (only core daemons like syslog and
> cron), he may run setxid binaries / trigger local daemon activity /
> trigger network services activity / await sporadic cron jobs activity
> / etc. and get rather precise counters for fs and network activity of
> these privileged tasks, which is unknown otherwise.
>
>
> Also hiding slabinfo is a one step to complicate exploitation of kernel
> heap overflows (and possibly, other bugs). The related discussion:
>
> http://thread.gmane.org/gmane.linux.kernel/1108378
>
>
> World readable slabinfo simplifies kernel developers' job of debugging
> kernel bugs (e.g. memleaks), but I believe it does more harm than
> benefits. For most users 0444 slabinfo is an unreasonable attack vector.
Please tell if anybody has complains about the restriction - whether it
forces someone besides kernel developers to do "chmod/chgrp". But if
someone want to debug the kernel, it shouldn't significantly influence
on common users, especially it shouldn't create security issues.
Thanks!
> Signed-off-by: Vasiliy Kulikov <segoon@...nwall.com>
> ---
> mm/slab.c | 3 ++-
> mm/slub.c | 2 +-
> 2 files changed, 3 insertions(+), 2 deletions(-)
>
> --
> diff --git a/mm/slab.c b/mm/slab.c
> index 6d90a09..560ffd0 100644
> --- a/mm/slab.c
> +++ b/mm/slab.c
> @@ -4584,7 +4584,8 @@ static const struct file_operations proc_slabstats_operations = {
>
> static int __init slab_proc_init(void)
> {
> - proc_create("slabinfo",S_IWUSR|S_IRUGO,NULL,&proc_slabinfo_operations);
> + proc_create("slabinfo", S_IWUSR | S_IRUSR, NULL,
> + &proc_slabinfo_operations);
> #ifdef CONFIG_DEBUG_SLAB_LEAK
> proc_create("slab_allocators", 0, NULL, &proc_slabstats_operations);
> #endif
> diff --git a/mm/slub.c b/mm/slub.c
> index 9f662d7..f440fc7 100644
> --- a/mm/slub.c
> +++ b/mm/slub.c
> @@ -5257,7 +5257,7 @@ static const struct file_operations proc_slabinfo_operations = {
>
> static int __init slab_proc_init(void)
> {
> - proc_create("slabinfo", S_IRUGO, NULL, &proc_slabinfo_operations);
> + proc_create("slabinfo", S_IRUSR, NULL, &proc_slabinfo_operations);
> return 0;
> }
> module_init(slab_proc_init);
--
Vasiliy Kulikov
http://www.openwall.com - bringing security into open computing environments
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