.\"	$OpenBSD: systrace.1,v 1.46 2015/09/12 15:52:37 schwarze Exp $
.\"
.\" Copyright 2002 Niels Provos <provos@citi.umich.edu>
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.\"    must display the following acknowledgement:
.\"      This product includes software developed by Niels Provos.
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.\" Manual page, using -mandoc macros
.\"
.Dd $Mdocdate: September 12 2015 $
.Dt SYSTRACE 1
.Os
.Sh NAME
.Nm systrace
.Nd generate and enforce system call policies
.Sh SYNOPSIS
.Nm systrace
.Op Fl AaCeitUuV
.Op Fl c Ar user : Ns Ar group
.Op Fl d Ar policydir
.Op Fl E Ar logfile
.Op Fl f Ar file
.Op Fl g Ar gui
.Op Fl p Ar pid
.Ar command ...
.Sh DESCRIPTION
The
.Nm
utility monitors and controls an application's access to the system by
enforcing access policies for system calls.
The
.Nm
utility might be used to trace an untrusted application's access to
the system.
Alternatively, it might be used to protect the system
from software bugs (such as buffer overflows) by constraining a
daemon's access to the system.
Its privilege elevation feature can be used to obviate the
need to run large, untrusted programs as root when only one or two
system calls require root privilege.
.Pp
The access policy can be generated interactively or obtained from a
policy file.
Interactive policy generation will be performed by the
.Dq notification user agent ,
normally
.Xr xsystrace 1 ,
unless text mode is specified via
.Fl t .
.Pp
When running in
.Dq automatic enforcement
mode, operations not covered by the policy raise an alarm and
allow a user to refine the currently configured policy.
.Pp
The options are as follows:
.Bl -tag -width Dfxfile
.It Fl A
Automatically generate a policy that allows every operation the
application executes.
The created policy functions as a base that can be refined.
.It Fl a
Enables automatic enforcement of configured policies.
An operation not covered by policy is denied and logged via
.Xr syslog 3 ,
or to
.Em stderr
if the
.Fl e
flag is specified.
.It Fl C
Run
.Nm
in cradle mode;
currently, when multiple processes are started with systrace protection, each
.Nm
starts its own UI
.Pq user interface
process.
Cradle mode allows a user to attach all
.Nm
processes to one UI.
This may be useful, for example, in scenarios where
.Nm
is being heavily used.
If a cradle server is not running, one is launched.
.It Fl c Ar user : Ns Ar group
Specifies the
.Ar user
and
.Ar group
that the monitored application should be executed with,
which may be either non-negative integers or names.
This is useful in conjunction with privilege elevation and requires
root privilege.
.It Fl d Ar policydir
Specifies an alternative location for the user's directory from
which policies are loaded and to which changed policies are stored.
.It Fl E Ar logfile
Logs all policy violations or specifically logged system calls to
.Ar logfile .
.It Fl e
Specifies to log to
.Em stderr
instead of
.Xr syslog 3 .
.It Fl f Ar file
The policies specified in
.Ar file
are added to the policies that
.Nm
knows about.
The dirname in the policy may contain an "*" to match any possible pathname.
The wildcard is removed from the policy database the first time that
a filename matches.
.It Fl g Ar gui
Specifies an alternative location for the notification user interface.
.It Fl i
Inherits the policy after a call to
.Xr execve 2 .
If this option is not specified, a new program will get its own policy.
.It Fl p Ar pid
Specifies the pid of a process that
.Nm
should attach to.
The full path name of the corresponding binary has to be specified
as
.Ar command .
.It Fl t
Uses text mode to ask for interactive policy generation.
.It Fl U
Ignore user-configured policies and use only global system policies.
.It Fl u
Do not perform aliasing on system call names.
Aliasing is enabled by default to group similar system calls into a
single compound name.
For example, system calls that read from the file system like
.Fn lstat
and
.Fn access
are translated to
.Fn fsread .
.It Fl V
Prints the version number of
.Nm .
.El
.Ss POLICY
The policy is specified via the following grammar:
.Bd -literal -offset 3n
filter = expression "then" action errorcode logcode
expression = symbol | "not" expression | "(" expression ")" |
    expression "and" expression | expression "or" expression
symbol = string typeoff "match" cmdstring |
    string typeoff "eq" cmdstring | string typeoff "neq" cmdstring |
    string typeoff "sub" cmdstring | string typeoff "nsub" cmdstring |
    string typeoff "inpath" cmdstring | string typeoff "re" cmdstring |
    "true"
typeoff = /* empty */ | "[" number "]"
action = "permit" | "deny" | "ask"
errorcode = /* empty */ | "[" string "]"
logcode = /* empty */ | "log"
.Ed
.Pp
The
.Va cmdstring
is an arbitrary string enclosed with quotation marks.
The
.Va errorcode
is used to return an
.Xr errno 2
value to the system call when using a
.Va deny
action.
The values
.Do
inherit
.Dc
and
.Do
detach
.Dc
have special meanings when used with a
.Va permit
rule for the
.Va execve
system call.
When using
.Do
inherit,
.Dc
the current policy is inherited for the new binary.
With
.Do
detach,
.Dc
.Nm
detaches from a process after successfully
completing
the
.Va execve
system call.
.Pp
The
.Va ask
action specifies that the user should be prompted for a decision
every time that the rule matches.
.Pp
The filter operations have the following meaning:
.Bl -hang -width Dinpath -offset AAA
.It match
Evaluates to true if file name globbing according to
.Xr fnmatch 3
succeeds.
.It eq
Evaluates to true if the system call argument matches
.Va cmdstring
exactly.
.It neq
This is the logical negation of
.Va eq .
.It sub
Performs a substring match on the system call argument.
.It nsub
This is the logical negation of
.Va sub .
.It inpath
Evaluates to true if the system call argument is a subpath of
.Va cmdstring .
.It re
Evaluates to true if the system call arguments matches
the specified regular expression.
.El
.Pp
By appending the
.Va log
statement to a rule, a matching system call and its arguments
are logged.
This is useful, for example, to log all invocations of the
.Va execve
system call.
.Pp
Policy entries may contain an appended predicate.
Predicates have the following format:
.Bd -literal -offset 3n
", if" {"user", "group"} {"=", "!=", "\*[Lt]", "\*[Gt]" } {number, string}
.Ed
.Pp
A rule is added to the configured policy only if its predicate
evaluates to true.
.Pp
The environment variables
.Ev $HOME ,
.Ev $USER
and
.Ev $CWD
are substituted in rules.
Comments, begun by an unquoted
.Sq \&#
character and continuing to the end of the line, are ignored.
.Sh PRIVILEGE ELEVATION
With
.Nm
it is possible to remove setuid or setgid binaries, and use the
privilege elevation feature instead.
Single system calls can be executed with higher privileges if
specified by the policy.
For example,
.Bd -literal -offset 3n
native-bind: sockaddr eq "inet-[0.0.0.0]:22" then permit as root
.Ed
.Pp
allows an unprivileged application to bind to a reserved port.
Privilege elevation requires that the
.Nm
process is executed as root.
.Pp
The following statements can be appended after the
.Va permit
in a policy to elevate the privileges for the matching system call:
.Bd -literal -offset 3n
as user
as user:group
as :group
.Ed
.Pp
The effective
.Va uid
and
.Va gid
are elevated only for the duration of the system call, and are restored
to the old values afterwards (except for the
.Va seteuid
or
.Va setegid
system calls).
.Sh FILES
.Bl -tag -width xHOME/xsystrace -compact
.It Pa /dev/systrace
systrace device
.It Pa /etc/systrace
global systrace policies
.It Pa $HOME/.systrace
user specified policies, one per binary, with slashes in the full pathname
replaced by the underscore character.
.El
.Sh EXAMPLES
An excerpt from a sample
.Xr ls 1
policy might look as follows:
.Bd -literal -offset 2n
Policy: /bin/ls, Emulation: native
[...]
   native-fsread: filename eq "$HOME" then permit
   native-fchdir: permit
[...]
   native-fsread: filename eq "/tmp" then permit
   native-stat: permit
   native-fsread: filename match "$HOME/*" then permit
   native-fsread: filename eq "/etc/pwd.db" then permit
[...]
   native-fsread: filename eq "/etc" then deny[eperm], if group != wheel
.Ed
.Sh SEE ALSO
.Xr systrace 4
.Sh AUTHORS
The
.Nm
utility was developed by
.An Niels Provos .
.Sh BUGS
Applications that use clone()-like system calls to share the complete
address space between processes may be able to replace system call
arguments after they have been evaluated by
.Nm
and escape policy enforcement.