CVE-2016-6213 (published 2016-12-28T07:59Z)

fs/namespace.c in the Linux kernel before 4.9 does not restrict how many mounts may exist in a mount namespace, which allows local users to cause a denial of service (memory consumption and deadlock) via MS_BIND mount system calls, as demonstrated by a loop that triggers exponential growth in the number of mounts.

Severity: MEDIUM
CVSS v2 base score: 4.7 (AV:L/AC:M/Au:N/C:N/I:N/A:C)
CVSS v3 base score: 4.7, MEDIUM (CVSS:3.0/AV:L/AC:H/PR:L/UI:N/S:U/C:N/I:N/A:H)

Weakness information:
cwe_id: CWE-400
cwe_name: Uncontrolled Resource Consumption
description: The product does not properly control the allocation and maintenance of a limited resource, thereby enabling an actor to influence the amount of resources consumed, eventually leading to the exhaustion of available resources.
extended_description: 
            Limited resources include memory, file system storage, database connection pool entries, and CPU. If an attacker can trigger the allocation of these limited resources, but the number or size of the resources is not controlled, then the attacker could cause a denial of service that consumes all available resources. This would prevent valid users from accessing the product, and it could potentially have an impact on the surrounding environment. For example, a memory exhaustion attack against an application could slow down the application as well as its host operating system.
            There are at least three distinct scenarios which can commonly lead to resource exhaustion:
               
                  Lack of throttling for the number of allocated resources
                  Losing all references to a resource before reaching the shutdown stage
                  Not closing/returning a resource after processing
               
            Resource exhaustion problems are often result due to an incorrect implementation of the following situations:
               
                  Error conditions and other exceptional circumstances.
                  Confusion over which part of the program is responsible for releasing the resource.
               
         
         

Fix information:
repo_url: https://github.com/torvalds/linux
commit_details: repo_url: https://github.com/torvalds/linux
author: Eric W. Biederman
author_date: 2016-09-28 00:27:17-05:00
msg: 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>
repository: repo_url: https://github.com/torvalds/linux
repo_name: torvalds/linux
description: Linux kernel source tree
repo_language: C
owner: torvalds
file_changes: filename: fs.txt
change_type: ModificationType.MODIFY
diff: @@ -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
 ----------------------------------------------------------

programming_language: Markdown
filename: mount.h
change_type: ModificationType.MODIFY
diff: @@ -14,6 +14,8 @@ 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 {

programming_language: C
filename: namespace.c
change_type: ModificationType.MODIFY
diff: @@ -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;
@@ -899,6 +902,9 @@ static void commit_tree(struct mount *mnt, struct mount *shadows)
 
 	list_splice(&head, n->list.prev);
 
+	n->mounts += n->pending_mounts;
+	n->pending_mounts = 0;
+
 	attach_shadowed(mnt, parent, shadows);
 	touch_mnt_namespace(n);
 }
@@ -1419,11 +1425,16 @@ static void umount_tree(struct mount *mnt, enum umount_tree_flags how)
 		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;
@@ -1840,6 +1851,28 @@ static int invent_group_ids(struct mount *mnt, bool recurse)
 	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
@@ -1909,10 +1942,18 @@ static int attach_recursive_mnt(struct mount *source_mnt,
 			struct path *parent_path)
 {
 	HLIST_HEAD(tree_list);
+	struct mnt_namespace *ns = dest_mnt->mnt_ns;
 	struct mount *child, *p;
 	struct hlist_node *n;
 	int err;
 
+	/* 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)
@@ -1949,11 +1990,13 @@ static int attach_recursive_mnt(struct mount *source_mnt,
  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;
 	return err;
 }
 
@@ -2776,6 +2819,8 @@ static struct mnt_namespace *alloc_mnt_ns(struct user_namespace *user_ns)
 	new_ns->event = 0;
 	new_ns->user_ns = get_user_ns(user_ns);
 	new_ns->ucounts = ucounts;
+	new_ns->mounts = 0;
+	new_ns->pending_mounts = 0;
 	return new_ns;
 }
 
@@ -2825,6 +2870,7 @@ struct mnt_namespace *copy_mnt_ns(unsigned long flags, struct mnt_namespace *ns,
 	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);
@@ -2863,6 +2909,7 @@ static struct mnt_namespace *create_mnt_ns(struct vfsmount *m)
 		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);

programming_language: C
filename: pnode.c
change_type: ModificationType.MODIFY
diff: @@ -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);
 }
 
 /*

programming_language: C
filename: pnode.h
change_type: ModificationType.MODIFY
diff: @@ -52,4 +52,5 @@ void mnt_set_mountpoint(struct mount *, struct mountpoint *,
 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 */

programming_language: C
filename: mount.h
change_type: ModificationType.MODIFY
diff: @@ -96,4 +96,6 @@ extern void mark_mounts_for_expiry(struct list_head *mounts);
 
 extern dev_t name_to_dev_t(const char *name);
 
+extern unsigned int sysctl_mount_max;
+
 #endif /* _LINUX_MOUNT_H */

programming_language: C
filename: sysctl.c
change_type: ModificationType.MODIFY
diff: @@ -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,
+	},
 	{ }
 };
 

programming_language: C