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authorEric W. Biederman <ebiederm@xmission.com>2016-09-28 00:27:17 -0500
committerEric W. Biederman <ebiederm@xmission.com>2016-09-30 12:46:48 -0500
commitd29216842a85c7970c536108e093963f02714498 (patch)
treeacf8843f0c807e80fc3b15653f3b22136984c69a /kernel/sysctl.c
parent2ed6afdee798658fe3c33b50c4a79d1bde45f1d8 (diff)
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mnt: Add a per mount namespace limit on the number of mounts
CAI Qian <caiqian@redhat.com> pointed out that the semantics of shared subtrees make it possible to create an exponentially increasing number of mounts in a mount namespace. mkdir /tmp/1 /tmp/2 mount --make-rshared / for i in $(seq 1 20) ; do mount --bind /tmp/1 /tmp/2 ; done Will create create 2^20 or 1048576 mounts, which is a practical problem as some people have managed to hit this by accident. As such CVE-2016-6213 was assigned. Ian Kent <raven@themaw.net> described the situation for autofs users as follows: > The number of mounts for direct mount maps is usually not very large because of > the way they are implemented, large direct mount maps can have performance > problems. There can be anywhere from a few (likely case a few hundred) to less > than 10000, plus mounts that have been triggered and not yet expired. > > Indirect mounts have one autofs mount at the root plus the number of mounts that > have been triggered and not yet expired. > > The number of autofs indirect map entries can range from a few to the common > case of several thousand and in rare cases up to between 30000 and 50000. I've > not heard of people with maps larger than 50000 entries. > > The larger the number of map entries the greater the possibility for a large > number of active mounts so it's not hard to expect cases of a 1000 or somewhat > more active mounts. So I am setting the default number of mounts allowed per mount namespace at 100,000. This is more than enough for any use case I know of, but small enough to quickly stop an exponential increase in mounts. Which should be perfect to catch misconfigurations and malfunctioning programs. For anyone who needs a higher limit this can be changed by writing to the new /proc/sys/fs/mount-max sysctl. Tested-by: CAI Qian <caiqian@redhat.com> Signed-off-by: "Eric W. Biederman" <ebiederm@xmission.com>
Diffstat (limited to 'kernel/sysctl.c')
-rw-r--r--kernel/sysctl.c9
1 files changed, 9 insertions, 0 deletions
diff --git a/kernel/sysctl.c b/kernel/sysctl.c
index b43d0b27c1fe..03f18cc15697 100644
--- a/kernel/sysctl.c
+++ b/kernel/sysctl.c
@@ -65,6 +65,7 @@
#include <linux/sched/sysctl.h>
#include <linux/kexec.h>
#include <linux/bpf.h>
+#include <linux/mount.h>
#include <asm/uaccess.h>
#include <asm/processor.h>
@@ -1838,6 +1839,14 @@ static struct ctl_table fs_table[] = {
.mode = 0644,
.proc_handler = proc_doulongvec_minmax,
},
+ {
+ .procname = "mount-max",
+ .data = &sysctl_mount_max,
+ .maxlen = sizeof(unsigned int),
+ .mode = 0644,
+ .proc_handler = proc_dointvec_minmax,
+ .extra1 = &one,
+ },
{ }
};