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Commit c50fd34e authored by Eric W. Biederman's avatar Eric W. Biederman Committed by Greg Kroah-Hartman
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mnt: Add a per mount namespace limit on the number of mounts 



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: default avatarCAI Qian <caiqian@redhat.com>
Signed-off-by: default avatar"Eric W. Biederman" <ebiederm@xmission.com>
[bwh: Backported to 4.4: adjust context]
Signed-off-by: default avatarBen Hutchings <ben.hutchings@codethink.co.uk>
Signed-off-by: default avatarGreg Kroah-Hartman <gregkh@linuxfoundation.org>
parent 59e0cd11

Documentation/sysctl/fs.txt

+7 −0
Original line number Diff line number Diff line
@@ -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

----------------------------------------------------------

fs/mount.h

+2 −0
Original line number Diff line number Diff line
@@ -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 {

fs/namespace.c

+49 −1
Original line number Diff line number Diff line
@@ -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 *mnt) 


	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 *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;
@@ -1850,6 +1861,28 @@ 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
@@ -1919,6 +1952,7 @@ 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 mountpoint *smp;

	struct mount *child, *p;

	struct hlist_node *n;
@@ -1931,6 +1965,13 @@ static int attach_recursive_mnt(struct mount *source_mnt, 
	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 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;



	read_seqlock_excl(&mount_lock);

	put_mountpoint(smp);

	read_sequnlock_excl(&mount_lock);
@@ -2804,6 +2848,8 @@ static struct mnt_namespace *alloc_mnt_ns(struct user_namespace *user_ns) 
	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(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);
@@ -2891,6 +2938,7 @@ 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);

fs/pnode.c

+1 −1
Original line number Diff line number Diff line
@@ -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);

}



/*

fs/pnode.h

+1 −0
Original line number Diff line number Diff line
@@ -54,4 +54,5 @@ void mnt_change_mountpoint(struct mount *parent, struct mountpoint *mp, 
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 */