When ACLs Attack: Cross-Platform File Permissions

Andrew LeonardISB IT ExchangeJuly 13, 2012

In the beginning: "Traditional" Unix Permissions

drwxr-x---

Read

WriteExecute

UserGroup

Other

Not permitted

Playing Nice With Other (Unix) Users

Some tactics:● drwxrws---: Use setgid bit to force files and

directories created within a directory to inherit its group id, rather than be assigned user's primary group id. (c. 1972?)

● umask 002: Don't limit group permissions, or read/execute permissions for others. (c. 1982?)

● drwxrwxrwt: Only the item's owner, directory's owner, or root can rm or mv contained files. (c. 1986)

● User private groups: Group containing single user, allows private files when setting umask 002. (Red Hat c. 2002)

Take dates above with a giant grain of salt, they could be way off.

POSIX.1e ACLs

● Allow setting permissions for multiple users and groups per file.

● Set explicit defaults (beyond the setgid bit).

user::rwx

user:aleonard:rwx

group::r-x

mask::rwx

other::r-x

default:user::rwx

default:user:aleonard:rwx

default:group::r-x

default:mask::rwx

default:other::r-x

Meanwhile, in Redmond...

NTFS ACLs:

Standard Permissions Advanced Permissions

● Modify● Read & Execute● Read● Write● List Folder Contents

● Full Control● Traverse Folder/Execute

File● List Folder/Read Data● Read Attributes● Read Extended Attributes● Create Files/Write Data● Create Folders/Append Data● Write Attributes● Write Extended Attributes● Delete Subfolders & Files● Delete● Read Permissions● Change Permissions● Take Ownership

Enter NFSv4 ACLsA Standard, as of 2000:http://ietfreport.isoc.org/idref/draft-falkner-nfsv4-acls/https://tools.ietf.org/html/rfc3010https://tools.ietf.org/html/rfc3530

# file: .# owner: root# group: bifx group:bifx:rwxpDdaARWcCos:fd----:allow owner@:rwxp--aARWcCos:------:allow group@:rwxp--a-R-c--s:------:allow everyone@:r-x---a-R-c--s:------:allow

● Each Access Control Entry (ACE) is made up of four parts: identifier, access rights, flags, type (allow, deny, audit, alarm).

● ACEs are traversed in order.● Access rights are NTFS compatible.

NFSv4 Privileges and AbbreviationsPrivilege (abbreviation):

Access Privileges (Linux, FreeBSD/Solaris) Flags (Linux, FreeBSD/Solaris)

● read_data, list_directory (r, r)● write_data, add_file (w, w)● execute (x, x)● append_data, add_subdirectory (a, p)● delete_child (D, D) ● delete (d, d) ● read_attributes (t, a) ● write_attributes (T, A) ● read_xattr (n, R)● write_xattr (N, W)● read_acl (c, c) ● write_acl (C, C) ● write_owner (o, o) ● synchronize (y, s)

● file_inherit (f, f) ● dir_inherit (d, d) ● inherit_only (i, i) ● no_propagate (n, n)

But: ACLs Aren't All Rainbows and Unicorns

Assuming the trade-off of flexibility for additional complexity is acceptable:● Does your file system support them? Do your clients?● Tools to manipulate ACLs are inconsistent and

sometimes inefficient.● Does your backup software preserve ACLs?● Do your everyday file system utilities handle them

correctly?● Does your vendor understand them? How buggy is

their implementation?● How do new-style ACLs interact with legacy permission

schemes?

Specifics: Seattle BioMed NAS Environment

● NetApp○ NFSv3/SMBv2○ Mix of "office" and "science" data.○ Home directories, group shares.

● FreeBSD/ZFS○ ZFS v28○ NFSv3, NFSv4/SMBv1○ Serves NFSv4 via newnfs○ Uses Samba for SMB○ Larger shares, mostly scientific data

Specifics: Our Client Environment

Pretty standard stuff, in order of prevalence:● Windows Desktops - SMBv1, SMBv2

○ 7, XP○ 2000, NT4, sigh.

● OS X - SMB, no NFS● Linux - NFSv3, NFSv4

○ CentOS○ Ubuntu

Details: ACLs and NetApp

● NetApp has three different security modes you can choose from at a volume or qtree level.○ "unix" mode: Unix-style permission bits.○ "ntfs" mode: "For CIFS requests, Windows NT permissions

determine user access. For NFS requests, the filer generates and stores a set of UNIX-style permission bits that are at least as restrictive as the Windows NT permissions. The filer grants NFS access only if the UNIX-style permission bits allow the user access."

○ They also have a third "mixed" mode, but nobody seems to use it: "A file's security style depends on whether the permission was last set from CIFS or NFS."

● (We're not using NFSv4 on NetApp today.)○ http://www.netapp.com/us/communities/tech-ontap/nfsv4-0408.html

Details: ACLs and ZFS

ZFS has native NFSv4 ACLs. Important issue:● What happens to an NFSv4 ACL when you chmod(2) is

important. If a file has an NFSv4 ACL, do you:○ Edit only the file's mode ("passthrough")?○ Remove any NFSv4 ACL ("discard")?○ Do something in-between ("groupmask")?○ Let the admin decide on a per-file system basis?

On ZFS, this is controlled by the "aclmode" property. Sun removed this shortly before the Oracle acquisition, enforcing "discard" on all ZFS file systems; however, FreeBSD and Illumos have added "aclmode" back.

http://arc.opensolaris.org/caselog/PSARC/2010/029/20100126_mark.shellenbaum

Usage notes: User mapping matters

You can't share files across platforms if you can't map identities across platforms.

We use Active Directory as our source of truth for users and groups; NetApp and FreeBSD systems access this information using LDAP.

Usage notes: General notes

● Simple permissions solve most of our use cases○ User home directories are on NetApp, using unix-

mode, 0700 permissions.○ Ntfs-mode qtrees work well for most groups.○ Many of our complex permission structures are

SMB-only and therefore use ntfs-mode.○ For those that need them, unix-mode qtrees are

often enough.● For everything else, there's NFSv4 ACLs on

ZFS.

Usage notes: NetApp + ntfs-mode

● What you see with an ntfs qtree over NFS is often not what you get:

$ ls -ld somedirdrwxrwxrwx 40 root root 8192 Mar 16 10:45 somedir$ cd somedir-bash: cd: somedir: Permission denied

● Some apps try to be good citizens and check permissions before carrying out an action, and then fail. Others complain when they can't set permissions within an ntfs-mode qtree.○ Setting cifs.ntfs_ignore_unix_security_ops (silently

discard NFS permission operations) and nfs.ntacl_display_permissive_perms (displayed permissions are based on the maximum access granted to any user) to 'on'can help here.

Usage notes: NetApp + unix-mode

For simple configurations, these cifs shares flags may get you what you need:

Make created files belong to a group:-forcegroup <groupname>

Set initial permissions of newly created files and directories:-umask <mask>-dir_umask <mask>-file_umask <mask>

Usage notes: ZFS on which operating system?

We're using FreeBSD 8-STABLE, as we often need fixes before they wind up in a -RELEASE.

The freebsd-fs@freebsd.org and freebsd-stable@freebsd.org mailing lists have been indispensable.

On the Solaris side of ZFS, there's always Oracle Solaris 11.As far as Ilumos, Nexenta is always an option,

and I hear there's neat stuff being built on OmniOS.There's also ZFS-on-Linux.

Usage Notes: ZFS file system properties

We generally set permissions at the top of a share, and have them inherited down into the share, so we: zfs set aclinherit=passthrough-xInherits all inheritable ACL entries without modification, but inherit execute permission only if the file creation mode specifies it.

zfs set aclmode=passthroughWhen chmod(2) is called, "no changes are made to the ACL other than creating or updating the necessary ACL entries to represent the new mode of the file or directory."

Usage notes: Samba configuration - simple permissions

We frequently use this idiom when configuring Samba shares, roughly equivalent to umask 007 and a setgid directory under NFS:

# Bitwise AND file/directory permissions with these masks:create mask = 0660directory mask = 2770# File/directory permission bits that will always be set:force create mode = 0660force directory mode = 2770# Assign group: force group = "somelab"# Limit permission bits that can be modified from Windows client - # these are forced on:force security mode = 0660force directory security mode = 2770

Usage notes: Samba configuration - complex ACLs

Set ACLs using native tools on ZFS as needed.

In smb.conf, remove force group, adjust mask and mode settings as appropriate... and let the NFSv4 ACLs at the file system level do the rest.

(Remember: Samba is just another application accessing files - v4 ACLs, including inheritance, are still applied.)

Usage notes: Samba on FreeBSD/ZFS config to allow ACL manipulation from Windows

We haven't heavily used this, but it seems to work.Build Samba WITH_ACL_SUPPORT=true from FreeBSD ports; add the following to smb.conf:

Global config:unix extensions = no

Within a share definition:nt acl support = yesinherit acls = nomap acl inherit = yesvfs objects = zfsaclnfs4:mode = specialnfs4:acedup = mergenfs4:chown = yes

Usage notes: Users increasingly want to manipulate their own ACLs

In general, our users haven't wanted to understand or manage their own ACLs, so IT has done it for them. However, we now have one group of users - an internal service provider - that wants to actively manage their own ACLs on a wide scale.● They use either nfs4_setfacl on Linux, or

an IT-supplied script to adjust permissions.● This is a fairly new development, so it's

unclear what pitfalls await.

Closing: NFSv4 in 2012

Despite being 12 years old, NFSv4 isn't widely- or well-supported. Commercial vendors don't dedicate more resources to it because users aren't using it heavily; users don't use it heavily because vendors aren't dedicating resources to it.

As a consequence, the best implementations and support today seem to be Open Source.