[PATCH 4.4 10/17] mnt: Add a per mount namespace limit on the number of mounts

Greg Kroah-Hartman
2017-04-28T08:31:01Z

From: Eric W. Biederman <ebiederm@xmission.com>

commit d29216842a85c7970c536108e093963f02714498 upstream.

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>
[bwh: Backported to 4.4: adjust context]
Signed-off-by: Ben Hutchings <ben.hutchings@codethink.co.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
---
Documentation/sysctl/fs.txt | 7 ++++++
fs/mount.h | 2 +
fs/namespace.c | 50 +++++++++++++++++++++++++++++++++++++++++++-
fs/pnode.c | 2 -
fs/pnode.h | 1
include/linux/mount.h | 2 +
kernel/sysctl.c | 9 +++++++
7 files changed, 71 insertions(+), 2 deletions(-)

--- a/Documentation/sysctl/fs.txt
+++ b/Documentation/sysctl/fs.txt
@@ -265,6 +265,13 @@ aio-nr can grow to.

==============================================================

+mount-max:
+
+This denotes the maximum number of mounts that may exist
+in a mount namespace.
+
+==============================================================
+

2. /proc/sys/fs/binfmt_misc
----------------------------------------------------------
--- a/fs/mount.h
+++ b/fs/mount.h
@@ -13,6 +13,8 @@ struct mnt_namespace {
u64 seq; /* Sequence number to prevent loops */
wait_queue_head_t poll;
u64 event;
+ unsigned int mounts; /* # of mounts in the namespace */
+ unsigned int pending_mounts;
};

struct mnt_pcp {
--- a/fs/namespace.c
+++ b/fs/namespace.c
@@ -27,6 +27,9 @@
#include "pnode.h"
#include "internal.h"

+/* Maximum number of mounts in a mount namespace */
+unsigned int sysctl_mount_max __read_mostly = 100000;
+
static unsigned int m_hash_mask __read_mostly;
static unsigned int m_hash_shift __read_mostly;
static unsigned int mp_hash_mask __read_mostly;
@@ -925,6 +928,9 @@ static void commit_tree(struct mount *mn

list_splice(&head, n->list.prev);

+ n->mounts += n->pending_mounts;
+ n->pending_mounts = 0;
+
__attach_mnt(mnt, parent);
touch_mnt_namespace(n);
}
@@ -1445,11 +1451,16 @@ static void umount_tree(struct mount *mn
propagate_umount(&tmp_list);

while (!list_empty(&tmp_list)) {
+ struct mnt_namespace *ns;
bool disconnect;
p = list_first_entry(&tmp_list, struct mount, mnt_list);
list_del_init(&p->mnt_expire);
list_del_init(&p->mnt_list);
- __touch_mnt_namespace(p->mnt_ns);
+ ns = p->mnt_ns;
+ if (ns) {
+ ns->mounts--;
+ __touch_mnt_namespace(ns);
+ }
p->mnt_ns = NULL;
if (how & UMOUNT_SYNC)
p->mnt.mnt_flags |= MNT_SYNC_UMOUNT;
@@ -1850,6 +1861,28 @@ static int invent_group_ids(struct mount
return 0;
}

+int count_mounts(struct mnt_namespace *ns, struct mount *mnt)
+{
+ unsigned int max = READ_ONCE(sysctl_mount_max);
+ unsigned int mounts = 0, old, pending, sum;
+ struct mount *p;
+
+ for (p = mnt; p; p = next_mnt(p, mnt))
+ mounts++;
+
+ old = ns->mounts;
+ pending = ns->pending_mounts;
+ sum = old + pending;
+ if ((old > sum) ||
+ (pending > sum) ||
+ (max < sum) ||
+ (mounts > (max - sum)))
+ return -ENOSPC;
+
+ ns->pending_mounts = pending + mounts;
+ return 0;
+}
+
/*
* @source_mnt : mount tree to be attached
* @nd : place the mount tree @source_mnt is attached
@@ -1919,6 +1952,7 @@ static int attach_recursive_mnt(struct m
struct path *parent_path)
{
HLIST_HEAD(tree_list);
+ struct mnt_namespace *ns = dest_mnt->mnt_ns;
struct mountpoint *smp;
struct mount *child, *p;
struct hlist_node *n;
@@ -1931,6 +1965,13 @@ static int attach_recursive_mnt(struct m
if (IS_ERR(smp))
return PTR_ERR(smp);

+ /* Is there space to add these mounts to the mount namespace? */
+ if (!parent_path) {
+ err = count_mounts(ns, source_mnt);
+ if (err)
+ goto out;
+ }
+
if (IS_MNT_SHARED(dest_mnt)) {
err = invent_group_ids(source_mnt, true);
if (err)
@@ -1970,11 +2011,14 @@ static int attach_recursive_mnt(struct m
out_cleanup_ids:
while (!hlist_empty(&tree_list)) {
child = hlist_entry(tree_list.first, struct mount, mnt_hash);
+ child->mnt_parent->mnt_ns->pending_mounts = 0;
umount_tree(child, UMOUNT_SYNC);
}
unlock_mount_hash();
cleanup_group_ids(source_mnt, NULL);
out:
+ ns->pending_mounts = 0;
+
read_seqlock_excl(&mount_lock);
put_mountpoint(smp);
read_sequnlock_excl(&mount_lock);
@@ -2804,6 +2848,8 @@ static struct mnt_namespace *alloc_mnt_n
init_waitqueue_head(&new_ns->poll);
new_ns->event = 0;
new_ns->user_ns = get_user_ns(user_ns);
+ new_ns->mounts = 0;
+ new_ns->pending_mounts = 0;
return new_ns;
}

@@ -2853,6 +2899,7 @@ struct mnt_namespace *copy_mnt_ns(unsign
q = new;
while (p) {
q->mnt_ns = new_ns;
+ new_ns->mounts++;
if (new_fs) {
if (&p->mnt == new_fs->root.mnt) {
new_fs->root.mnt = mntget(&q->mnt);
@@ -2891,6 +2938,7 @@ static struct mnt_namespace *create_mnt_
struct mount *mnt = real_mount(m);
mnt->mnt_ns = new_ns;
new_ns->root = mnt;
+ new_ns->mounts++;
list_add(&mnt->mnt_list, &new_ns->list);
} else {
mntput(m);
--- a/fs/pnode.c
+++ b/fs/pnode.c
@@ -259,7 +259,7 @@ static int propagate_one(struct mount *m
read_sequnlock_excl(&mount_lock);
}
hlist_add_head(&child->mnt_hash, list);
- return 0;
+ return count_mounts(m->mnt_ns, child);
}

/*
--- a/fs/pnode.h
+++ b/fs/pnode.h
@@ -54,4 +54,5 @@ void mnt_change_mountpoint(struct mount
struct mount *copy_tree(struct mount *, struct dentry *, int);
bool is_path_reachable(struct mount *, struct dentry *,
const struct path *root);
+int count_mounts(struct mnt_namespace *ns, struct mount *mnt);
#endif /* _LINUX_PNODE_H */
--- a/include/linux/mount.h
+++ b/include/linux/mount.h
@@ -95,4 +95,6 @@ extern void mark_mounts_for_expiry(struc

extern dev_t name_to_dev_t(const char *name);

+extern unsigned int sysctl_mount_max;
+
#endif /* _LINUX_MOUNT_H */
--- 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>
@@ -1749,6 +1750,14 @@ 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,
+ },
{ }
};