#+TITLE: How BSD Authentication Works
#+DATE: 2020-06-26T18:31:36-04:00
#+DRAFT: true
#+DESCRIPTION:
#+TAGS[]: openbsd
#+KEYWORDS[]: openbsd
#+SLUG:
#+SUMMARY:
#+SHOWTOC: true
[[https://web.archive.org/web/20170327150148/http://www.penzin.net/bsdauth/]]
* History
OpenBSD is quite different from many other Unix-like operating systems
in many ways, but one way which I find interesting is the
authentication system. Most systems from AIX, Solaris, and Linux to
most BSDs including MacOS use some form of a system called Pluggable
Authentication Module (PAM). The two main implementations of PAM are
[[http://www.linux-pam.org/][Linux PAM]] and [[https://www.openpam.org/][OpenPAM]]. PAM modules are created a dynamically loaded
shared objects, which communicate using a set of standard
interfaces ([[https://linux.die.net/man/3/pam][Linux-PAM]] and [[https://www.freebsd.org/cgi/man.cgi?query=pam&apropos=0&sektion=3&manpath=FreeBSD+12.1-RELEASE+and+Ports&arch=default&format=html][OpenPAM]]). PAM is configured using the [[https://linux.die.net/man/5/pam.d][pam.d]]
directory and [[https://www.freebsd.org/cgi/man.cgi?query=pam.conf&sektion=5&apropos=0&manpath=FreeBSD+12.1-RELEASE+and+Ports][pam.conf]].
OpenBSD on the other hand uses a mechanism called BSD
Authentication. It was originally developed for a proprietary
operating system called [[https://en.wikipedia.org/wiki/BSD/OS][BSD/OS]] by [[https://en.wikipedia.org/wiki/Berkeley_Software_Design][Berkeley Software Design Inc.]], who
later donated the system. It was adopted by OpenBSD in release
2.9. BSD Auth is comparatively much simpler than PAM. Modules or,
authentication "styles", are instead stand alone applications or
scripts that communicate over IPC (=PF_LOCAL, SOCK_STREAM=,
specifically). The program or script has no ability to interfere
with the parent and can very easily revoke permissions using
[[https://man.openbsd.org/pledge][=pledge(2)=]] or [[https://man.openbsd.org/unveil][=unveil(2)=]]. The BSD Authentication system of
configured through [[https://man.openbsd.org/login.conf][=login.conf(5)=]].
* Why
This one is pretty difficult, since there seems to be very little
information about how BSD Auth works apart from the source code
itself. This is my best attempt to understand the flow of BSD Auth
from what I've read.
* BSD Auth Modules
These programs or scripts are located in =/usr/libexec/auth/= with the
naming convention =login_<style>=. They take arguments in the form of
#+BEGIN_SRC shell
login_<style> [-s service] [-v key=value] user [class]
#+END_SRC
- =<style>= is the authentication method. This could be =passwd=,
=radius=, =skey=, =yubikey=, etc. There's more information about
available styles in [[https://man.openbsd.org/login.conf][=login.conf(5)=]] under the [[https://man.openbsd.org/login.conf#AUTHENTICATION][=AUTHENTICATION=]]
header.
- =service= is the service type. Typically authentication methods will
accept one of three values here, =login=, =challenge=, or
=response=. =login= is the default if it's not specified, and is
usually the right choice. Read the style's man page for details.
- =-v key=value= is an optional argument. There is no limit to the
number of =-v= arguments. This is used to pass extra data to the
program under certain circumstances.
- =user= is the name of the user to be authenticated.
- =class= is optional and specifies the class of the user to be
authenticated.
=login= and =su= pass in extra data as =-v= flags.
#+CAPTION: Taken from [[https://man.openbsd.org/login.conf][=login.conf(5)=]]
#+BEGIN_SRC
The login(1) program provides the following through the -v option:
auth_type The type of authentication to use.
fqdn The hostname provided to login by the -h option.
hostname The name login(1) will place in the utmp file for the
remote hostname.
local_addr The local IP address given to login(1) by the -L option.
lastchance Set to "yes" when a user's password has expired but the
user is being given one last chance to login and update
the password.
login This is a new login session (as opposed to a simple
identity check).
remote_addr The remote IP address given to login(1) by the -R option.
style The style of authentication used for this user (see
approval scripts below).
The su(1) program provides the following through the -v option:
wheel Set to either "yes" or "no" to indicate if the user is in
group wheel when they are trying to become root. Some
authentication types require the user to be in group
wheel when using the su(1) program to become super user.
#+END_SRC
The auth module communicates with its caller through file descriptor 3.
* Documentation
All of the high level authentication functions are described in
[[https://man.openbsd.org/authenticate][=authenticate(3)=]], with the lower level functions being described in
[[https://man.openbsd.org/auth_subr][=auth_subr(3)=]].
* auth_userokay
The highest level function, and easiest to use is =auth_userokay=. It
takes four character arrays as arguments, =name=, =style=, =type=, and
=password=. It returns either a =0= for failure, of a non-zero value
for success.
This function lives inside =/lib/libc/gen/authenticate.c=
#+BEGIN_SRC c
int auth_userokay(char *name, char *style, char *type, char *password);
#+END_SRC
- =name= is the name of the user to be authenticated
- =style= is the login method to be used
- If =style= is =NULL=, the user's default login style will be
used. By default this is =passwd= on normal accounts.
- The style can be one of the installed authentication methods, like
=passwd=, =radius=, =skey=, =yubikey=, etc.
- There's more information about available styles in =login.conf(5)=
- Styles can also be installed through BSD Auth module packages
- =type= is the authentication type
- Types are defined in =login.conf= and define a group of allowed
auth styles
- If =type= is =NULL=, use the auth type for the user's login
class. The default type is =auth-default=, which allows
=psaswd= and =skey= auth methods.
- There's more information about how to add methods in =login.conf(5)=
- =password= is the password to test
- If =password= is =NULL=, then the user is interactively
prompted. This is required for auth styles using
challenge-response methods.
- If =password= is specified, then it's non-interactively tested
=auth_userokay= is just a wrapper around =auth_usercheck=, which
returns a finished auth session of type =auth_session_t=. It closes
the auth session using =auth_close= and returns the value returned
from closing.
* auth_session_t
#+BEGIN_SRC c
struct auth_session_t {
char *name; /* name of use being authenticated */
char *style; /* style of authentication used */
char *class; /* class of user */
char *service; /* type of service being performed */
char *challenge; /* last challenge issued */
int flags; /* see below */
struct passwd *pwd; /* password entry for user */
struct timeval now; /* time of authentication */
int state; /* authenticated state */
struct rmfiles *rmlist; /* list of files to remove on failure */
struct authopts *optlist; /* list of options to scripts */
struct authdata *data; /* additional data to send to scripts */
char spool[MAXSPOOLSIZE]; /* data returned from login script */
int index; /* how much returned thus far */
int fd; /* connection to authenticator */
va_list ap0; /* argument list to auth_call */
va_list ap; /* additional arguments to auth_call */
};
#+END_SRC
Where =authdata=, =authopts=, and =rmfiles= are defined as
#+BEGIN_SRC c
struct rmfiles {
struct rmfiles *next;
char *file;
};
struct authopts {
struct authopts *next;
char *opt;
};
struct authdata {
struct authdata *next;
void *ptr;
size_t len;
};
#+END_SRC
** auth_setdata
** auth_setitem
** auth_setoption
** auth_setstate
* auth_open
The =auth_open= function is used by several functions to create a
new auth session. It allocates an =auth_session_t= struct on the
heap, sets its default =service= to =login=, and it's =fd= to =-1=,
and returns the pointer.
* auth_usercheck
#+BEGIN_SRC c
auth_session_t *auth_usercheck(char *name, char *style, char *type, char *password)
#+END_SRC
=auth_usercheck= checks the user name against the passwd db. It also
checks the login class against the =login.conf= db, along with
confirming the login styles available.
If the password is non-=NULL=, then it creates a new session using
=auth_open=. With the new session, =auth_usercheck= calls (with =as=
as the session struct)
#+BEGIN_SRC c
auth_setitem(as, AUTHV_SERVICE, "response");
auth_setdata(as, "", 1);
auth_setdata(as, password, strlen(password) + 1);
#+END_SRC
setting the service protocol to =response=, adding an empty line to
the session data, then adding the password as data. If the password is
=NULL=, it sets the =auth_session_t= pointer to =NULL=. It then passes
the user name, style, login class, and =NULL= char pointer to
=auth_verify=. The last two variables are received as variable
arguments. It then returns the auth session pointer the call
returns.
* auth_verify
#+BEGIN_SRC c
auth_session_t *auth_verify(auth_session_t *as, char *style, char *name, ...)
#+END_SRC
=auth_verify= creates an auth session using =auth_open= if =as= is
=NULL=. It then sets the user name and style of the session, if the
respective arguments are non-=NULL=. It then copies its variable
arguments to the auth session's =va_list ap=, which is used inside
of =auth_call=.
After that it constructs the path of the authentication module by
combining =_PATH_AUTHPROG=, which is defined in =login_cap.h= as
=/usr/libexec/auth/login_=, and the authentication style. For the
case of auth style =passwd=, it would result in the path
=/usr/libexec/auth/login_passwd=.
Then =auth_call= is called with the struct, the path to the auth
module, the auth style, the "-s" flag followed by the service (login,
challenge, response), a double dash, the user name, and a =NULL=
character pointer. The return value of =auth_call= is ignored and a
pointer to the auth session is returned immediately afterwards.
#+BEGIN_SRC c
auth_call(as, path, auth_getitem(as, AUTHV_STYLE), "-s",
auth_getitem(as, AUTHV_SERVICE), "--", name, (char *)NULL);
#+END_SRC
* auth_call
#+BEGIN_SRC c
int auth_call(auth_session_t *as, char *path, ...)
#+END_SRC
<<here>>
---
note: In the man page auth_subr it says
#+begin_quote
path The full path name of the login script to run. The call will
fail if path does not pass the requirements of the secure_path(3)
function.
#+end_quote
However I don't see this enforced anywhere, I even wrote a small test
script to prove that's the case on =vfwall ~/authtest=.
The manpage also says the path is limited to =/bin/= and =/usr/bin=,
which is also not the case.
Ask jcs about the file descriptor situation, I don't understand it
after reading both the man page and source.
---
Inside of =auth_call=, a socket pair of type =PF_LOCAL, SOCK_STREAM=
is created. This is called the "back channel", and is used to
communicate with the authentication module. The process then forks,
calling ~execve(path, argv, auth_environ)~, where the =argv= is
everything after =path= in the =auth_call= arguments. Any =authopts=
set in the auth session are also passed as arguments in the format =-v
opt1 -v opt2 -v opt3=, etc. =auth_environ= is defined at the top of
the file as
#+BEGIN_SRC c
static char *auth_environ[] = {
"PATH=" _PATH_DEFPATH,
"SHELL=" _PATH_BSHELL,
NULL,
};
#+END_SRC
Where both constants are defined in =paths.h= as
#+BEGIN_SRC c
#define _PATH_DEFPATH "/usr/bin:/bin:/usr/sbin:/sbin:/usr/X11R6/bin:/usr/local/bin:/usr/local/sbin"
#define _PATH_BSHELL "/bin/sh"
#+END_SRC
The =exec='d process then listens on FD 3, which is one half of the
=sockpair= that was created earlier.
In the non-exec'd process, first the contents of the auth session's
=*data= are read in one at a time.
The data received through the back channel is then put into the
=spool= of the auth session using =_auth_spool(as, pfd[0])=. After
that the spooled data is scanned for key words defined in
=login_cap.h=.
#+BEGIN_SRC c
#define BI_AUTH "authorize" /* Accepted authentication */
#define BI_REJECT "reject" /* Rejected authentication */
#define BI_CHALLENGE "reject challenge" /* Reject with a challenge */
#define BI_SILENT "reject silent" /* Reject silently */
#define BI_REMOVE "remove" /* remove file on error */
#define BI_ROOTOKAY "authorize root" /* root authenticated */
#define BI_SECURE "authorize secure" /* okay on non-secure line */
#define BI_SETENV "setenv" /* set environment variable */
#define BI_UNSETENV "unsetenv" /* unset environment variable */
#define BI_VALUE "value" /* set local variable */
#define BI_EXPIRED "reject expired" /* account expired */
#define BI_PWEXPIRED "reject pwexpired" /* password expired */
#define BI_FDPASS "fd" /* child is passing an fd */
#+END_SRC
The [[https://man.openbsd.org/login.conf][=login.conf(5)=]] man page once again goes into greater detail on
these values.
#+BEGIN_SRC
authorize The user has been authorized.
authorize secure
The user has been authorized and root should be allowed to
login even if this is not a secure terminal. This should only
be sent by authentication styles that are secure over insecure
lines.
reject Authorization is rejected. This overrides any indication that
the user was authorized (though one would question the wisdom
in sending both a reject and an authorize command).
reject challenge
Authorization was rejected and a challenge has been made
available via the value challenge.
reject silent
Authorization is rejected, but no error messages should be
generated.
remove file
If the login session fails for any reason, remove file before
termination.
setenv name value
If the login session succeeds, the environment variable name
should be set to the specified value.
unsetenv name
If the login session succeeds, the environment variable name
should be removed.
value name value
Set the internal variable name to the specified value. The
value should only contain printable characters. Several \
sequences may be used to introduce non printing characters.
These are:
\n A newline.
\r A carriage return.
\t A tab.
\xxx The character represented by the octal value xxx. The
value may be one, two, or three octal digits.
\c The string is replaced by the value of c. This allows
quoting an initial space or the \ character itself.
The following values are currently defined:
challenge
See section on challenges below.
errormsg
If set, the value is the reason authentication failed.
The calling program may choose to display this when
rejecting the user, but display is not required.
#+END_SRC
It is looking for lines that start with either =BI_AUTH=
(=authorize=), or =BI_REJECT= (=reject=). If the line is still longer,
it continues to scan for any other qualifiers such as =pwexpired= or
=silent=. The struct's =state= is set to one using the =AUTH_= values
from =login_cap.h= accordingly.
#+BEGIN_SRC c
/*
* bits which can be returned by authenticate()/auth_scan()
*/
#define AUTH_OKAY 0x01 /* user authenticated */
#define AUTH_ROOTOKAY 0x02 /* authenticated as root */
#define AUTH_SECURE 0x04 /* secure login */
#define AUTH_SILENT 0x08 /* silent rejection */
#define AUTH_CHALLENGE 0x10 /* a challenge was given */
#define AUTH_EXPIRED 0x20 /* account expired */
#define AUTH_PWEXPIRED 0x40 /* password expired */
#+END_SRC
** _auth_spool
** _recv_fd
* auth_close
=auth_close= is the function responsible for cleaning up the session
and taking care of the values returned though the back channel.
It first sets the environment variables returned through the back
channel by passing the auth session to =auth_setenv=. It then goes
through the =rmlist= of the session, deleting the files if the
session reported a failure. It then zeroes out all sensitive
information, and frees the various structs associated with the current
=auth_session_t=, and then the session itself. Finally it returns
the session's state =&='ed with =AUTH_ALLOW=.
* grapgh?
# Setting env on auth_close(as)
# partual rewrite below
The call graph for =auth_userokay= looks something like this:
#+BEGIN_SRC c
int auth_userokay(char *name, char *style, char *type, char *password)
#+END_SRC
calls ~auth_usercheck~ and then calls ~auth_close~ on the returned
~auth_session_t~. The value returned from ~auth_close~ is then
returned.
#+BEGIN_SRC c
auth_session_t *auth_usercheck(char *name, char *style, char *type, char *password)
#+END_SRC
Validates the checks that the user exists, gets the user's login
class, verifies the auth type, and that the auth style can be used.
It creates an auth session struct.
If the password is provided it sets the service type to =response=,
and adds the adds the password to the auth data. Otherwise it
leaves it empty.