SYSTRACE(1) OpenBSD Reference Manual SYSTRACE(1)NAME
systrace - generate and enforce system call policies
SYNOPSIS
systrace [-AaCeitUuV] [-c user:group] [-d policydir] [-E logfile]
[-f file] [-g gui] [-p pid] command ...
DESCRIPTION
The systrace utility monitors and controls an application's access to the
system by enforcing access policies for system calls. The systrace
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.
The access policy can be generated interactively or obtained from a
policy file. Interactive policy generation will be performed by the
``notification user agent'', normally xsystrace(1), unless text mode is
specified via -t.
When running in ``automatic enforcement'' mode, operations not covered by
the policy raise an alarm and allow a user to refine the currently
configured policy.
The options are as follows:
-A Automatically generate a policy that allows every operation the
application executes. The created policy functions as a base
that can be refined.
-a Enables automatic enforcement of configured policies. An
operation not covered by policy is denied and logged via
syslog(3), or to stderr if the -e flag is specified.
-C Run systrace in cradle mode; currently, when multiple processes
are started with systrace protection, each systrace starts its
own UI (user interface) process. Cradle mode allows a user to
attach all systrace processes to one UI. This may be useful,
for example, in scenarios where systrace is being heavily used.
If a cradle server is not running, one is launched.
-c user:group
Specifies the user and 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.
-d policydir
Specifies an alternative location for the user's directory from
which policies are loaded and to which changed policies are
stored.
-E logfile
Logs all policy violations or specifically logged system calls
to logfile.
-e Specifies to log to stderr instead of syslog(3).
-f file The policies specified in file are added to the policies that
systrace 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.
-g gui Specifies an alternative location for the notification user
interface.
-i Inherits the policy after a call to execve(2). If this option
is not specified, a new program will get its own policy.
-p pid Specifies the pid of a process that systrace should attach to.
The full path name of the corresponding binary has to be
specified as command.
-t Uses text mode to ask for interactive policy generation.
-U Ignore user-configured policies and use only global system
policies.
-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 lstat() and access() are translated to
fsread().
-V Prints the version number of systrace.
POLICY
The policy is specified via the following grammar:
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"
The cmdstring is an arbitrary string enclosed with quotation marks. The
errorcode is used to return an errno(2) value to the system call when
using a deny action. The values ``inherit'' and ``detach'' have special
meanings when used with a permit rule for the execve system call. When
using ``inherit,'' the current policy is inherited for the new binary.
With ``detach,'' systrace detaches from a process after successfully
completing the execve system call.
The ask action specifies that the user should be prompted for a decision
every time that the rule matches.
The filter operations have the following meaning:
match Evaluates to true if file name globbing according to
fnmatch(3) succeeds.
eq Evaluates to true if the system call argument matches
cmdstring exactly.
neq This is the logical negation of eq.
sub Performs a substring match on the system call argument.
nsub This is the logical negation of sub.
inpath Evaluates to true if the system call argument is a subpath of
cmdstring.
re Evaluates to true if the system call arguments matches the
specified regular expression.
By appending the 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 execve system call.
Policy entries may contain an appended predicate. Predicates have the
following format:
", if" {"user", "group"} {"=", "!=", "<", ">" } {number, string}
A rule is added to the configured policy only if its predicate evaluates
to true.
The environment variables $HOME, $USER and $CWD are substituted in rules.
Comments, begun by an unquoted `#' character and continuing to the end of
the line, are ignored.
PRIVILEGE ELEVATION
With systrace 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,
native-bind: sockaddr eq "inet-[0.0.0.0]:22" then permit as root
allows an unprivileged application to bind to a reserved port. Privilege
elevation requires that the systrace process is executed as root.
The following statements can be appended after the permit in a policy to
elevate the privileges for the matching system call:
as user
as user:group
as :group
The effective uid and gid are elevated only for the duration of the
system call, and are restored to the old values afterwards (except for
the seteuid or setegid system calls).
FILES
/dev/systrace systrace device
/etc/systrace global systrace policies
$HOME/.systrace user specified policies, one per binary, with slashes in
the full pathname replaced by the underscore character.
EXAMPLES
An excerpt from a sample ls(1) policy might look as follows:
Policy: /bin/ls, Emulation: native
[...]
native-fsread: filename eq "$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
SEE ALSOsystrace(4)AUTHORS
The systrace utility was developed by Niels Provos.
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 systrace and escape policy
enforcement.
OpenBSD 4.9 May 31, 2007 OpenBSD 4.9