systrace(1)
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SYSTRACE(1) NetBSD General Commands Manual SYSTRACE(1)
NAME
systrace -- generate and enforce system call policies
SYNOPSIS
systrace [-AaCitUu] [-c uid:gid] [-d policydir] [-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 util-
ity might be used to trace an untrusted application's access to the sys-
tem. 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 pol-
icy 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 con-
figured 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 opera-
tion not covered by policy is denied and logged via syslog(3).
-C Run systrace in cradle mode; if a cradle server is not running,
one is launched. This decouples UIs from systrace, allowing for
re-attaching UIs.
-c uid:gid
Specifies the uid and gid that the monitored application should
be executed with, which must be specified as nonnegative inte-
gers (not as names). This is useful in conjunction with privi-
lege 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.
-f file The policies specified in file are added to the policies that
systrace knows about.
-g gui Specifies an alternative location for the notification user
interface.
-i Inherits the policy - child processes inherit policy of the par-
ent binary.
-p pid Specifies the pid of a process that systrace should attach to.
The full path name of the corresponding binary has to be speci-
fied as command.
-t Uses text mode to ask for interactive policy generation.
-U Ignore user configured policies and use only global system poli-
cies.
-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().
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 com-
pleting 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 is logged to syslog(3). 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 exe-
cuted 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 sys-
tem 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 ALSO
systrace(4)
HISTORY
The systrace utility first appeared in OpenBSD 3.2, and then in
NetBSD 2.0.
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 argu-
ments after they have been evaluated by systrace and escape policy
enforcement.
NetBSD 3.1 November 28, 2003 NetBSD 3.1
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