Date: Tue, 27 Sep 2011 23:00:37 +0300 From: Pekka Enberg <penberg@...helsinki.fi> To: David Rientjes <rientjes@...gle.com> Cc: Vasiliy Kulikov <segoon@...nwall.com>, kernel-hardening@...ts.openwall.com, Christoph Lameter <cl@...ux-foundation.org>, Matt Mackall <mpm@...enic.com>, linux-mm@...ck.org, Kees Cook <kees@...ntu.com>, Dave Hansen <dave@...ux.vnet.ibm.com>, Valdis.Kletnieks@...edu, Linus Torvalds <torvalds@...ux-foundation.org>, Alan Cox <alan@...ux.intel.com>, linux-kernel@...r.kernel.org Subject: Re: [PATCH 1/2] mm: restrict access to slab files under procfs and sysfs On Tue, Sep 27, 2011 at 9:21 PM, David Rientjes <rientjes@...gle.com> wrote: > On Tue, 27 Sep 2011, Vasiliy Kulikov wrote: > >> Historically /proc/slabinfo and files under /sys/kernel/slab/* have >> world read permissions and are 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 and /sys/kernel/slab/* 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 >> >> >> To keep compatibility with old permission model where non-root >> monitoring daemon could watch for kernel memleaks though slabinfo one >> should do: >> >> groupadd slabinfo >> usermod -a -G slabinfo $MONITOR_USER >> >> And add the following commands to init scripts (to mountall.conf in >> Ubuntu's upstart case): >> >> chmod g+r /proc/slabinfo /sys/kernel/slab/*/* >> chgrp slabinfo /proc/slabinfo /sys/kernel/slab/*/* >> >> Signed-off-by: Vasiliy Kulikov <segoon@...nwall.com> >> Reviewed-by: Kees Cook <kees@...ntu.com> >> Reviewed-by: Dave Hansen <dave@...ux.vnet.ibm.com> >> CC: Christoph Lameter <cl@...two.org> >> CC: Pekka Enberg <penberg@...helsinki.fi> >> CC: Valdis.Kletnieks@...edu >> CC: Linus Torvalds <torvalds@...ux-foundation.org> >> CC: David Rientjes <rientjes@...gle.com> >> CC: Alan Cox <alan@...ux.intel.com> > > Acked-by: David Rientjes <rientjes@...gle.com> Applied, thanks!
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