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d29216842a
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>
102 lines
3.1 KiB
C
102 lines
3.1 KiB
C
/*
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*
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* Definitions for mount interface. This describes the in the kernel build
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* linkedlist with mounted filesystems.
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*
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* Author: Marco van Wieringen <mvw@planets.elm.net>
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*
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*/
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#ifndef _LINUX_MOUNT_H
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#define _LINUX_MOUNT_H
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#include <linux/types.h>
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#include <linux/list.h>
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#include <linux/nodemask.h>
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#include <linux/spinlock.h>
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#include <linux/seqlock.h>
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#include <linux/atomic.h>
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struct super_block;
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struct vfsmount;
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struct dentry;
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struct mnt_namespace;
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#define MNT_NOSUID 0x01
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#define MNT_NODEV 0x02
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#define MNT_NOEXEC 0x04
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#define MNT_NOATIME 0x08
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#define MNT_NODIRATIME 0x10
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#define MNT_RELATIME 0x20
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#define MNT_READONLY 0x40 /* does the user want this to be r/o? */
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#define MNT_SHRINKABLE 0x100
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#define MNT_WRITE_HOLD 0x200
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#define MNT_SHARED 0x1000 /* if the vfsmount is a shared mount */
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#define MNT_UNBINDABLE 0x2000 /* if the vfsmount is a unbindable mount */
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/*
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* MNT_SHARED_MASK is the set of flags that should be cleared when a
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* mount becomes shared. Currently, this is only the flag that says a
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* mount cannot be bind mounted, since this is how we create a mount
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* that shares events with another mount. If you add a new MNT_*
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* flag, consider how it interacts with shared mounts.
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*/
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#define MNT_SHARED_MASK (MNT_UNBINDABLE)
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#define MNT_USER_SETTABLE_MASK (MNT_NOSUID | MNT_NODEV | MNT_NOEXEC \
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| MNT_NOATIME | MNT_NODIRATIME | MNT_RELATIME \
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| MNT_READONLY)
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#define MNT_ATIME_MASK (MNT_NOATIME | MNT_NODIRATIME | MNT_RELATIME )
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#define MNT_INTERNAL_FLAGS (MNT_SHARED | MNT_WRITE_HOLD | MNT_INTERNAL | \
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MNT_DOOMED | MNT_SYNC_UMOUNT | MNT_MARKED)
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#define MNT_INTERNAL 0x4000
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#define MNT_LOCK_ATIME 0x040000
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#define MNT_LOCK_NOEXEC 0x080000
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#define MNT_LOCK_NOSUID 0x100000
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#define MNT_LOCK_NODEV 0x200000
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#define MNT_LOCK_READONLY 0x400000
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#define MNT_LOCKED 0x800000
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#define MNT_DOOMED 0x1000000
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#define MNT_SYNC_UMOUNT 0x2000000
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#define MNT_MARKED 0x4000000
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#define MNT_UMOUNT 0x8000000
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struct vfsmount {
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struct dentry *mnt_root; /* root of the mounted tree */
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struct super_block *mnt_sb; /* pointer to superblock */
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int mnt_flags;
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};
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struct file; /* forward dec */
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struct path;
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extern int mnt_want_write(struct vfsmount *mnt);
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extern int mnt_want_write_file(struct file *file);
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extern int mnt_clone_write(struct vfsmount *mnt);
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extern void mnt_drop_write(struct vfsmount *mnt);
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extern void mnt_drop_write_file(struct file *file);
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extern void mntput(struct vfsmount *mnt);
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extern struct vfsmount *mntget(struct vfsmount *mnt);
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extern struct vfsmount *mnt_clone_internal(struct path *path);
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extern int __mnt_is_readonly(struct vfsmount *mnt);
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extern bool mnt_may_suid(struct vfsmount *mnt);
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struct path;
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extern struct vfsmount *clone_private_mount(struct path *path);
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struct file_system_type;
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extern struct vfsmount *vfs_kern_mount(struct file_system_type *type,
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int flags, const char *name,
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void *data);
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extern void mnt_set_expiry(struct vfsmount *mnt, struct list_head *expiry_list);
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extern void mark_mounts_for_expiry(struct list_head *mounts);
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extern dev_t name_to_dev_t(const char *name);
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extern unsigned int sysctl_mount_max;
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#endif /* _LINUX_MOUNT_H */
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