Subject: ntsec documentation [was Re: grep 2.4 and permission denied]
From: Corinna Vinschen <[email-masked]>
To: [email-masked]
Date: Sat, 06 May 2000 13:01:31 +0200

Jay Bromley wrote:
> [...]
> Also, I am curious as to what setting the CYGWIN=ntsec environment
> setting really does. I haven't been able to find too much about this.
> I would appreciate any pointers about how permissions work in cygwin.

Hi all,

this is a good reason to publish the latest version of the ntsec
documentation. It's part of the developers snapshots but hasn't
yet find it's way to the net documentation. I have attached the
html version of the document.

Please mail questions/suggestions/patches/flames regarding ntsec or
it's documentation to this mailing list.

Have fun,
Corinna

Cygwin User's Guide

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Chapter 2. Setting Up Cygwin

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NT security and the ntsec usage

The design goal of the ntsec patch was to get a more UNIX like
permission structure based upon the security features of Windows NT.
To describe the changes, I will give a short overview of NT security
in chapter one.

Chapter two discusses the changes in ntsec related to privileges on
processes.

Chapter three shows the basics of UNIX like setting of
file permissions.

Chapter four talks about the advanced settings introduced in
release 1.1

Chapter five illustrates the permission mapping leak of Windows NT.

Chapter six describes in short the new acl API since release 1.1

The setting of UNIX like object permissions is controlled by the new
CYGWIN variable setting (no)ntsec.
On NT ntsec is now turned on by default.

NT security

The NT security allows a process to allow or deny access of
different kind to `objects'. `Objects' are files, processes,
threads, semaphores, etc.

The main data structure of NT security is the `security descriptor'
(SD) structure. It explains the permissions, that are granted (or denied)
to an object and contains information, that is related to so called
`security identifiers' (SID).

A SID is a unique identifier for users, groups and domains.
SIDs are comparable to UNIX UIDs and GIDs, but are more complicated
because they are unique across networks. Example:

SID of a system `foo':

S-1-5-21-165875785-1005667432-441284377

SID of a user `johndoe' of the system `foo':

S-1-5-21-165875785-1005667432-441284377-1023

The above example shows the convention for printing SIDs. The leading
`S' should show that it is a SID. The next number is a version number which
is always 1. The next number is the so called `top-level authority' that
identifies the source that issued the SID.

While each system in a NT network has it's own SID, the situation
is modified in NT domains: The SID of the domain controller is the
base SID for each domain user. If an NT user has one account as domain
user and another account on his local machine, this accounts are under
any circumstances DIFFERENT, regardless of the usage of the same user
name and password!

SID of a domain `bar':

S-1-5-21-186985262-1144665072-740312968

SID of a user `johndoe' in the domain `bar':

S-1-5-21-186985262-1144665072-740312968-1207

The last part of the SID, the so called `relative identifier' (RID),
is by default used as UID and/or GID under cygwin. As the name and the
above example implies, this id is unique only relative to one system or
domain.

Note, that it's possible, that an user has the same RID on two
different systems. The resulting SIDs are nevertheless different, so
the SIDs are representing different users in an NT network.

There is a big difference between UNIX IDs and NT SIDs, the existence of
the so called `well known groups'. For example UNIX has no GID for the
group of `all users'. NT has an SID for them, called `Everyone' in the
English versions. The SIDs of well-known groups are not unique across
an NT network but their meanings are unmistakable.
Examples of well-known groups:

everyone S-1-1-0
creator/owner S-1-3-0
batch process (via `at') S-1-5-3
authenticated users S-1-5-11
system S-1-5-18

The last important group of SIDs are the `predefined groups'. This
groups are used mainly on systems outside of domains to simplify the
administration of user permissions. The corresponding SIDs are not unique
across the network so they are interpreted only locally:

administrators S-1-5-32-544
users S-1-5-32-545
guests S-1-5-32-546
...

Now, how are permissions given to objects? A process may assign an SD
to the object. The SD of an object consists of three parts:

the SID of the owner

the SID of the group

a list of SIDs with their permissions, called
`access control list' (ACL)

UNIX is able to create three different permissions, the permissions
for the owner, for the group and for the world. In contrast the ACL
has a potentially infinite number of members. Every member is a so called
`access control element' (ACE). An ACE contains three parts:

the type of the ACE

permissions, described with a DWORD

the SID, for which the above mentioned permissions are
set

The two important types of ACEs are the `access allowed ACE' and the
`access denied ACE'. The ntsec patch only uses `access allowed ACEs'.

The possible permissions on objects are more complicated than in
UNIX. For example, the permission to delete an object is different
from the write permission.

With the aforementioned method NT is able to grant or revoke permissions
to objects in a far more specific way. But what about cygwin? In a POSIX
environment it would be fine to have the security behavior of a POSIX
system. The NT security model is MOSTLY able to reproduce the POSIX model.
The ntsec patch tries to do this in cygwin.

You ask "Mostly? Why mostly???" Because there's a leak in the NT model.
I will describe that in detail in chapter 4.

The creation of explicit object security is a bit complicated, so
typically only two simple variations are used:

default permissions, computed by the operating system

each permission to everyone

For parameters to functions that create or open securable objects another
data structure is used, the `security attributes' (SA). This structure
contains an SD and a flag, that specifies whether the returned handle
to the created or opened object is inherited to child processes or not.
This property is not important for the ntsec patch description, so in
this document SDs and SAs are more or less identical.

Process privileges

Any process started under control of cygwin has a semaphore attached
to it, that is used for signaling purposes. The creation of this semaphore
can be found in sigproc.cc, function `getsem'. The first parameter to the
function call `CreateSemaphore' is an SA. Without ntsec patch this SA
assigns default security to the semaphore. There is a simple disadvantage:
Only the owner of the process may send signals to it. Or, in other words,
if the owner of the process is not a member of the administrators' group,
no administrator may kill the process! This is especially annoying, if
processes are started via service manager.

The ntsec patch now assigns an SA to the process control semaphore, that
has each permission set for the user of the process, for the
administrators' group and for `system', which is a synonym for the
operating system itself. The creation of this SA is done by the function
`sec_user', that can be found in `shared.cc'. Each member of the
administrators' group is now allowed to send signals to any process
created in cygwin, regardless of the process owner.

Moreover, each process now has the appropriate security settings, when
it is started via `CreateProcess'. You will find this in function
`spawn_guts' in module `spawn.cc'. The security settings for starting a
process in another user context have to add the sid of the new user, too.
In the case of the `CreateProcessAsUser' call, sec_user creates an SA with
an additional entry for the sid of the new user.

File permissions

If ntsec is turned on, file permissions are set as in UNIX. An SD is
assigned to the file containing the owner and group and ACEs for the
owner, the group and `Everyone'.

The complete settings of UNIX like permissions can be found in the file
`security.cc'. The two functions `get_nt_attribute' and `set_nt_attribute'
are the main code. The reading and writing of the SDs is done by the
functions `read_sd' and `write_sd'. `write_sd' uses the function `BackupRead'
instead of the simpler function `SetFileSecurity' because the latter is
unable to set owners different from the caller.

If you are creating a file `foo' outside of cygwin, you will see something
like the following on ls -ln:

If your login is member of the administrators' group:

rwxrwxrwx 1 544 513 ... foo

if not:

rwxrwxrwx 1 1000 513 ... foo

Note the user and group IDs. 544 is the UID of the administrators' group.
This is a `feature' :-P of WinNT. If one is a member of
the administrators' group, every file, that he has created is owned by the
administrators' group, instead by him.

The second example shows the UID of the first user, that has been
created with NT's the user administration tool. The users and groups are
sequentially numbered, starting with 1000. Users and groups are using the
same numbering scheme, so a user and a group don't share the same ID.

In both examples the GID 513 is of special interest. This GID is a
well known group with different naming in local systems and domains.
Outside of domains the group is named 'None' (`Kein' in German, `Aucun'
in French, etc.), in domains it is named 'Domain Users'. Unfortunately,
the group `None' is never shown in the user admin tool outside of domains!
This is very confusing but it seems that this has no negativ influences.

To work correctly the ntsec patch depends on reasoned files
/etc/passwd/ and /etc/group.
In cygwin release 1.0 the names and the IDs must correspond to the
appropriate NT IDs! The IDs used in cygwin are the RID of the NT SID, as
mentioned earlier.
An SID of e.g. the user `corinna' on my NT workstation:

S-1-5-21-165875785-1005667432-441284377-1000

Note the last number: It's the RID 1000, the cygwin's UID.

Unfortunately, workstations and servers outside of domains are not
able to set primary groups! In these cases, where there is no correlation
of users to primary groups, NT returns 513 (None) as primary group,
regardless of the membership to existing local groups.

when using mkpasswd -l -g on such systems, you
have to change the primary group by hand if `None' as primary group is
not what you want (and I'm sure, it's not what you want!)

To get help in creating correct passwd and group files, look at
the following examples, that are part of my files. With the exception
of my personal user entry, all entries are well known entries. For a
better understanding, the names are translated to the equivalents of the
English NT version.

Example 2-1. /etc/passwd

everyone:*:0:0:::
system:*:18:18:::
administrator::500:544::/home/root:/bin/bash
guest:*:501:546:::
administrators:*:544:544::/home/root:
corinna::1000:547:Corinna Vinschen:/home/corinna:/bin/tcsh

Example 2-2. /etc/group

everyone::0:
system::18:
none::513:
administrators::544:
users::545:
guests::546:
powerusers::547:

Groups may be mentioned in the passwd file, too. This has two
advantages:

Because NT assigns them to files as owners, a
ls -l is often better readable.

Moreover it's possible to assigned them to files as
owners with cygwin's chown.

The group `system' is the aforementioned synonym for the operating system
itself and is normally the owner of processes, that are started through
service manager. The same is true for files, that are created by
processes, which are started through service manager.

New since Cygwin release 1.1

In Cygwin release 1.1 a new technique of using the
/etc/passwd and /etc/group
is introduced.

Both files may now contain SIDs of users and groups. They
are saved in the last field of pw_gecos in /etc/passwd
and in the gr_passwd field in /etc/group.

This has the following advantages:

ntsec works better in domain environments.

Accounts (users and groups) may get another name in
cygwin that their NT account name. The name in /etc/passwd
or /etc/group is transparently used by cygwin
applications (eg. chown, chmod,
ls):

root::500:513::/home/root:/bin/sh

instead of

adminstrator::500:513::/home/root:/bin/sh

Caution: If you like to use the account as login account via
telnet etc. you have to remain the name unchanged or
you have to use a special version of login which will
be part of the release 1.1 soon.

Cygwin UIDs and GIDs are now not necessarily the RID
part of the NT SID:

root::0:513:S-1-5-21-54355234-56236534-345635656-500:/home/root:/bin/sh

instead of

root::500:513::/home/root:/bin/sh

As in U*X systems UIDs and GIDs numbering scheme now
don't influence each other. So it's possible to have same Id's for a
user and a group:

Example 2-3. /etc/passwd:

root::0:0:S-1-5-21-54355234-56236534-345635656-500:/home/root:/bin/sh

Example 2-4. /etc/group:

root:S-1-5-32-544:0:

The tools mkpasswd and mkgroup
create the needed entries by default. If you don't want that you can use
the options -s or --no-sids. In this
case ntsec behaves like the previous version.

Please note that the pw_gecos field in /etc/passwd
is defined as a comma seperated list. The SID has to be the last field!

As aforementioned you are able to use cygwin account names different
from the NT account names. If you want to login thru `telnet' or something
else you have to use the special login. You may then
add another field to pw_gecos which contains the NT user name including
it's domain. So you are able to login as each domain user. The syntax
is easy: Just add an entry of the form U-ntdomain\ntusername to the pw_gecos
field. Note that the SID must still remain the last field in pw_gecos!

the_king::1:1:Elvis Presley,U-STILLHERE\elvis,S-1-5-21-1234-5678-9012-1000:/bin/sh

For a local user just drop the domain:

the_king::1:1:Elvis Presley,U-elvis,S-1-5-21-1234-5678-9012-1000:/bin/sh

In each case the password of the user is taken from the NT user
database, NOT from the passwd file!

The mapping leak

Now its time to point out the leak in the NT permissions.
The official documentation explains in short the following:

access allow ACEs are accumulated regarding to the
group membership of the caller.

The order of ACEs is important. The system reads them
in sequence until either any needed right is denied or all needed rights
are granted. Later ACEs are then not taken into account.

ALl access denied ACEs _should_ precede any
access allowed ACE.

Note that the last rule is a preference, not a law. NT will correctly
deal with the ACL regardless of the sequence order. The second rule is
not modified to get the ACEs in the prefered order.

Unfortunately the security tab of the NT4 explorer is completely
unable to deal with access denied ACEs while the explorer of W2K rearranges
the order of the ACEs before you can read them. Thank God, the sort order
remains unchanged if one presses the Cancel button.

You still ask "Where is the leak?" NT ACLs are unable to reflect each
possible combination of POSIX permissions. Example:

rw-r-xrw-

1st try:

UserAllow: 110
GroupAllow: 101
OthersAllow: 110

Hmm, because of the accumulation of allow rights the user may
execute because the group may execute.

2st try:

UserDeny: 001
GroupAllow: 101
OthersAllow: 110

Now the user may read and write but not execute. Better? No!
Unfortunately the group may write now because others may write.

3rd try:

UserDeny: 001
GroupDeny: 010
GroupAllow: 001
OthersAllow: 110

Now the group may not write as intended but unfortunately the user may
not write anymore, too. How should this problem be solved? According to
the official rules a UserAllow has to follow the GroupDeny but it's
easy to see that this can never be solved that way.

The only chance:

UserDeny: 001
UserAllow: 010
GroupDeny: 010
GroupAllow: 001
OthersAllow: 110

Again: This works for both, NT4 and W2K. Only the GUIs aren't
able to deal with that order.

New acl API

For dealing with ACLs Cygwin now has the acl API as it's
implemented in newer versions of Solaris. The new data structure
for a single ACL entry (ACE in NT terminology) is defined in
sys/acl.h as:

typedef struct acl {
int a_type; /* entry type */
uid_t a_id; /* UID | GID */
mode_t a_perm; /* permissions */
} aclent_t;

The a_perm member of the aclent_t type contains only the bits
for read, write and execute as in the file mode. If eg. read permission
is granted, all read bits (S_IRUSR, S_IRGRP, S_IROTH) are set.
CLASS_OBJ or MASK ACL entries are not fully implemented yet.

The new API calls are

acl(2), facl(2)
aclcheck(3),
aclsort(3),
acltomode(3), aclfrommode(3),
acltopbits(3), aclfrompbits(3),
acltotext(3), aclfromtext(3)

Like in Solaris, Cygwin has two new commands for working with
ACLs on the command line: getfacl and
setfacl.

Online man pages for the aforementioned commands and API calls
can be found on eg. http://docs.sun.com

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