(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@xxxxxxxxxxxx> > --- > 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 -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Fight unfair telecom internet charges in Canada: sign http://stopthemeter.ca/ Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>