SYSTRACE(4) OpenBSD Programmer's Manual SYSTRACE(4)
NAME
systrace - enforce and generate policies for system calls
SYNOPSIS
pseudo-device systrace [count]
DESCRIPTION
systrace attaches to processes and enforces policies for system calls. A
pseudo-device, /dev/systrace, allows userland processes to control the
behavior of systrace through an ioctl(2) interface.
SYSTEM CALL POLICIES
systrace can assign the following policies to system calls:
SYSTR_POLICY_ASK Send a message of the type SYSTR_MSG_ASK, and put
the process to sleep until a STRIOCANSWER ioctl(2)
is made.
SYSTR_POLICY_PERMIT Immediately allow the system call.
SYSTR_POLICY_NEVER Immediately return an error code.
SYSTRACE MESSAGES
A read(2) operation on the systrace pseudo-device will block if there are
no pending messages, or return the following structure:
struct str_message {
int msg_type;
#define SYSTR_MSG_ASK 1
#define SYSTR_MSG_RES 2
#define SYSTR_MSG_EMUL 3
#define SYSTR_MSG_CHILD 4
#define SYSTR_MSG_UGID 5
#define SYSTR_MSG_POLICYFREE 6
#define SYSTR_MSG_EXECVE 7
pid_t msg_pid;
u_int16_t msg_seqnr; /* answer has to match seqnr */
short msg_policy;
union {
struct str_msg_emul msg_emul;
struct str_msg_ugid msg_ugid;
struct str_msg_ask msg_ask;
struct str_msg_child msg_child;
struct str_msg_execve msg_execve;
} msg_data;
};
struct str_msg_emul {
char emul[SYSTR_EMULEN];
};
struct str_msg_ugid {
uid_t uid;
gid_t gid;
};
struct str_msg_execve {
char path[MAXPATHLEN];
};
struct str_msg_ask {
int code;
int argsize;
register_t args[SYSTR_MAXARGS];
register_t rval[2];
int result;
};
struct str_msg_child {
pid_t new_pid; /* -1 if child exited */
};
These messages are all sent to the userland control process.
SYSTR_MSG_ASK This message is sent whenever the kernel does not
have a cached simple policy for system call number
code within the currently set emulation.
SYSTR_MSG_RES This message is sent whenever a system call is
flagged with SYSTR_FLAGS_RESULT.
SYSTR_MSG_EMUL This message is sent whenever the emulation of a
process changes.
SYSTR_MSG_CHILD This message is sent whenever a process gains or
loses a child. In the latter case, the event is
raised when the child exits, but not when it is
reaped.
SYSTR_MSG_UGID This message is sent whenever the effective UID or
GID has changed during the execution of a system
call.
SYSTR_MSG_POLICYFREE This is sent whenever the kernel frees the policy
identified by msg_policy.
SYSTR_MSG_EXECVE This message is sent whenever, before a call to
execve(2) a process is privileged (technically, the
process has the P_SUGID or P_SUGIDEXEC flag set),
but after the call these privileges have been
dropped. The new image name is specified in the
path argument.
IOCTL INTERFACE
systrace supports the following ioctl(2) commands:
STRIOCCLONE int *
Return a systrace file descriptor for further ioctl(2) opera-
tions.
STRIOCATTACH pid_t *
Attach to a process, unless:
1. It's the process that's doing the attaching.
2. It's a system process.
3. It's being traced already.
4. You do not own the process and you're not root.
5. It's init(8), and the kernel was not compiled with
option INSECURE.
STRIOCDETACH pid_t *
Wake up a process if it is waiting for an answer, and detach from
it.
STRIOCANSWER struct systrace_answer *
Tell systrace what to do with a system call that was assigned a
policy of SYSTR_POLICY_ASK.
struct systrace_answer {
pid_t stra_pid; /* PID of process being traced */
u_int16_t stra_seqnr;
short reserved;
uid_t stra_seteuid; /* Elevated privileges for syscall */
uid_t stra_setegid;
int stra_policy; /* Policy to assign */
int stra_error; /* Return value of denied syscall
(will return EPERM if zero) */
int stra_flags;
#define SYSTR_FLAGS_RESULT 0x001 /* Report syscall result */
#define SYSTR_FLAGS_SETEUID 0x002
#define SYSTR_FLAGS_SETEGID 0x004
};
STRIOCREPORT pid_t *
Report the current emulation a process is using inside the
msg_emul structure.
STRIOCREPLACE struct systrace_replace *
Arrange for system call arguments to be replaced by arguments
supplied by the monitoring process.
struct systrace_replace {
pid_t strr_pid;
u_int16_t strr_seqnr;
int16_t reserved;
int strr_nrepl; /* # of arguments to replace */
caddr_t strr_base; /* Base user memory */
size_t strr_len; /* Length of memory */
int strr_argind[SYSTR_MAXARGS]; /* Argument indexes */
size_t strr_off[SYSTR_MAXARGS]; /* Argument offsets */
size_t strr_offlen[SYSTR_MAXARGS]; /* Argument sizes */
int32_t strr_flags[SYSTR_MAXARGS];
};
STRIOCIO struct systrace_io *
Copy data in/out of the process being traced.
struct systrace_io {
pid_t strio_pid; /* PID of process being traced */
int strio_op;
#define SYSTR_READ 1
#define SYSTR_WRITE 2
void *strio_offs;
void *strio_addr;
size_t strio_len;
};
STRIOCPOLICY struct systrace_policy *
Manipulate the set of policies.
struct systrace_policy {
int strp_op;
#define SYSTR_POLICY_NEW 1 /* Allocate a new policy */
#define SYSTR_POLICY_ASSIGN 2 /* Assign policy to process */
#define SYSTR_POLICY_MODIFY 3 /* Modify an entry */
int strp_num;
union {
struct {
short code;
#define SYSTR_POLICY_ASK 0
#define SYSTR_POLICY_PERMIT 1
#define SYSTR_POLICY_NEVER 2
short policy;
} assign;
pid_t pid;
int maxents;
} strp_data;
#define strp_pid strp_data.pid
#define strp_maxents strp_data.maxents
#define strp_code strp_data.assign.code
#define strp_policy strp_data.assign.policy
};
The SYSTR_POLICY_NEW operation allocates a new policy of
strp_maxents entries with each initialized to SYSTR_POLICY_ASK,
and returns the new policy number into strp_num.
The SYSTR_POLICY_ASSIGN operation attaches the policy identified
by strp_num to strp_pid, with a maximum of strp_maxents entries.
The SYSTR_POLICY_MODIFY operation changes the entry indexed by
strp_code to strp_policy.
STRIOCGETCWD pid_t *
Set the working directory of the current process to that of the
named process.
STRIOCRESCWD
Restore the working directory of the current process.
STRIOCINJECT
Inject a buffer into the stackgap of the traced process. This
accommodates for the manipulation of non-scalar arguments. The
actual replacement is not done until system call time, and its
presence in the stackgap is only guaranteed for the duration of
that system call.
struct systrace_inject {
/* On return, this contains the stackgap address. */
caddr_t stri_addr;
size_t stri_len;
pid_t stri_pid;
};
STRIOCSCRIPTNAME struct systrace_scriptname *
Set the path of executed scripts to sn_scriptname.
struct systrace_scriptname {
pid_t sn_pid;
char sn_scriptname[MAXPATHLEN];
};
FILES
/dev/systrace system call tracing facility
EXAMPLES
The following is an example program that traces another process, printing
out the path to any open(2) system calls it performs.
#include <sys/param.h>
#include <sys/ioctl.h>
#include <dev/systrace.h>
#include <err.h>
#include <fcntl.h>
#include <stdio.h>
#include <unistd.h>
/*
* Number of system calls that will be covered in our policy.
*/
#define NSYSCALLS 512
int
main(int argc, char *argv[])
{
struct systrace_policy strpol;
struct systrace_answer strans;
struct systrace_io strio;
struct str_message strmsg;
int fd, cfd, pid, i;
ssize_t n;
void *p;
char c;
if ((fd = open("/dev/systrace", O_RDONLY)) == -1)
err(1, "/dev/systrace");
/*
* Get a systrace descriptor.
*/
if (ioctl(fd, STRIOCCLONE, &cfd) == -1)
err(1, "STRIOCCLONE");
close(fd);
/* Gather the PID of a process to systrace from somewhere. */
/* ... */
if (ioctl(cfd, STRIOCATTACH, &pid) == -1)
err(1, "STRIOCATTACH");
/* Install one policy. */
strpol.strp_op = SYSTR_POLICY_NEW;
strpol.strp_maxents = NSYSCALLS;
if (ioctl(cfd, STRIOCPOLICY, &strpol) == -1)
err(1, "STRIOCPOLICY NEW");
strpol.strp_op = SYSTR_POLICY_ASSIGN;
strpol.strp_pid = pid;
if (ioctl(cfd, STRIOCPOLICY, &strpol) == -1)
err(1, "STRIOCPOLICY ASSIGN");
/* Permit all system calls. */
for (i = 0; i < NSYSCALLS; i++) {
strpol.strp_op = SYSTR_POLICY_MODIFY;
strpol.strp_code = i;
strpol.strp_policy = SYSTR_POLICY_PERMIT;
if (ioctl(cfd, STRIOCPOLICY, &strpol) == -1)
err(1, "STRIOCPOLICY MODIFY");
}
/* Ask us about open(2) system calls. */
strpol.strp_op = SYSTR_POLICY_MODIFY;
strpol.strp_code = 5; /* open(2) */
strpol.strp_policy = SYSTR_POLICY_ASK;
if (ioctl(cfd, STRIOCPOLICY, &strpol) == -1)
err(1, "STRIOCPOLICY MODIFY");
/*
* Now this process just answers requests for the operations the
* traced process performs that we have requested systrace to ask
* us about.
*/
while ((n = read(cfd, &strmsg, sizeof(strmsg))) ==
sizeof(strmsg)) {
switch (strmsg.msg_type) {
case SYSTR_MSG_ASK:
/* Print out the path argument to open(2). */
memcpy(&p, &strmsg.msg_data.msg_ask.args,
sizeof(p));
printf("open(");
do {
memset(&strio, 0, sizeof(strio));
strio.strio_pid = strmsg.msg_pid;
strio.strio_op = SYSTR_READ;
strio.strio_offs = p;
strio.strio_addr = &c;
strio.strio_len = 1;
if (ioctl(cfd, STRIOCIO, &strio) == -1)
err(1, "STRIOCIO");
putchar(c);
(unsigned char *)p += sizeof(char);
} while (c != '\0');
printf(")\n");
memset(&strans, 0, sizeof(strans));
strans.stra_pid = strmsg.msg_pid;
strans.stra_seqnr = strmsg.msg_seqnr;
strans.stra_policy = SYSTR_POLICY_PERMIT;
if (ioctl(cfd, STRIOCANSWER, &strans) == -1)
err(1, "STRIOCANSWER");
break;
}
}
if (n == -1)
err(1, "read");
close(cfd);
exit(0);
}
SEE ALSO
systrace(1), ioctl(2), read(2), options(4), securelevel(7)
HISTORY
The systrace facility first appeared in OpenBSD 3.2.
CAVEATS
When creating new policies, if strp_maxents is not large enough to accom-
modate any system calls needed for fundamental process operations, the
traced process will block forever.
OpenBSD 4.0 May 26, 2002 6