Date: Tue, 27 Sep 2011 11:21:22 -0700 (PDT) From: David Rientjes <rientjes@...gle.com> To: Vasiliy Kulikov <segoon@...nwall.com> cc: kernel-hardening@...ts.openwall.com, Christoph Lameter <cl@...ux-foundation.org>, Pekka Enberg <penberg@...nel.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, 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>
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