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>
This commit is contained in:
Eric W. Biederman 2016-09-28 00:27:17 -05:00
parent 2ed6afdee7
commit d29216842a
7 changed files with 70 additions and 2 deletions

View File

@ -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
----------------------------------------------------------

View File

@ -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 {

View File

@ -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);

View File

@ -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);
}
/*

View File

@ -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 */

View File

@ -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 */

View File

@ -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,
},
{ }
};