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SubjectRe: [PATCH] capabilities: introduce per-process capability bounding set (v10)
Serge E. Hallyn wrote:
> The capability bounding set is a set beyond which capabilities
> cannot grow. Currently cap_bset is per-system. It can be
> manipulated through sysctl, but only init can add capabilities.
> Root can remove capabilities. By default it includes all caps
> except CAP_SETPCAP.

Serge,

This feature makes me being interested in.
I think you intend to apply this feature for the primary process
of security container.
However, it is also worthwhile to apply when a session is starting up.

The following PAM module enables to drop capability bounding bit
specified by the fifth field in /etc/passwd entry.
This code is just an example now, but considerable feature.

build and install:
# gcc -Wall -c pam_cap_drop.c
# gcc -Wall -shared -Xlinker -x -o pam_cap_drop.so pam_cap_drop.o -lpam
# cp pam_cap_drop.so /lib/security

modify /etc/passwd as follows:

tak:x:1004:100:cap_drop=cap_net_raw,cap_chown:/home/tak:/bin/bash
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
example:
[kaigai@masu ~]$ ping 192.168.1.1
PING 192.168.1.1 (192.168.1.1) 56(84) bytes of data.
64 bytes from 192.168.1.1: icmp_seq=1 ttl=64 time=1.23 ms
64 bytes from 192.168.1.1: icmp_seq=2 ttl=64 time=1.02 ms
--- 192.168.1.1 ping statistics ---
2 packets transmitted, 2 received, 0% packet loss, time 999ms
rtt min/avg/max/mdev = 1.023/1.130/1.237/0.107 ms
[kaigai@masu ~]$ ssh tak@localhost
tak@localhost's password:
Last login: Sat Dec 1 10:09:29 2007 from masu.myhome.cx
[tak@masu ~]$ export LANG=C
[tak@masu ~]$ ping 192.168.1.1
ping: icmp open socket: Operation not permitted

[tak@masu ~]$ su
Password:
pam_cap_bset[6921]: user root does not have 'cap_drop=' property
[root@masu tak]# cat /proc/self/status | grep ^Cap
CapInh: 0000000000000000
CapPrm: 00000000ffffdffe
CapEff: 00000000ffffdffe
[root@masu tak]#
# BTW, I replaced the James's address in the Cc: list,
# because MTA does not accept it.
--
KaiGai Kohei <kaigai@kaigai.gr.jp>

************************************************************
pam_cap_drop.c
************************************************************
/*
* pam_cap_drop.c module -- drop capabilities bounding set
*
* Copyright: 2007 KaiGai Kohei <kaigai@kaigai.gr.jp>
*/
#include <errno.h>
#include <pwd.h>
#include <stdlib.h>
#include <stdio.h>
#include <string.h>
#include <syslog.h>
#include <sys/prctl.h>
#include <sys/types.h>

#include <security/pam_modules.h>

#ifndef PR_CAPBSET_DROP
#define PR_CAPBSET_DROP 24
#endif

static char *captable[] = {
"cap_chown",
"cap_dac_override",
"cap_dac_read_search",
"cap_fowner",
"cap_fsetid",
"cap_kill",
"cap_setgid",
"cap_setuid",
"cap_setpcap",
"cap_linux_immutable",
"cap_net_bind_service",
"cap_net_broadcast",
"cap_net_admin",
"cap_net_raw",
"cap_ipc_lock",
"cap_ipc_owner",
"cap_sys_module",
"cap_sys_rawio",
"cap_sys_chroot",
"cap_sys_ptrace",
"cap_sys_pacct",
"cap_sys_admin",
"cap_sys_boot",
"cap_sys_nice",
"cap_sys_resource",
"cap_sys_time",
"cap_sys_tty_config",
"cap_mknod",
"cap_lease",
"cap_audit_write",
"cap_audit_control",
"cap_setfcap",
NULL,
};

PAM_EXTERN int
pam_sm_open_session(pam_handle_t *pamh, int flags,
int argc, const char **argv)
{
struct passwd *pwd;
char *pos, *buf;
char *username = NULL;
/* open system logger */
openlog("pam_cap_bset", LOG_PERROR | LOG_PID, LOG_AUTHPRIV);
/* get the unix username */
if (pam_get_item(pamh, PAM_USER, (void *) &username) != PAM_SUCCESS || !username)
return PAM_USER_UNKNOWN;
/* get the passwd entry */
pwd = getpwnam(username);
if (!pwd)
return PAM_USER_UNKNOWN;
/* Is there "cap_drop=" ? */
pos = strstr(pwd->pw_gecos, "cap_drop=");
if (pos) {
buf = strdup(pos + sizeof("cap_drop=") - 1);
if (!buf)
return PAM_SESSION_ERR;
pos = strtok(buf, ",");
while (pos) {
int rc, i;
for (i=0; captable[i]; i++) {
if (!strcmp(pos, captable[i])) {
rc = prctl(PR_CAPBSET_DROP, i);
if (rc < 0) {
syslog(LOG_NOTICE, "user %s could not drop %s (%s)",
username, captable[i], strerror(errno));
break;
}
syslog(LOG_NOTICE, "user %s drops %s\n", username, captable[i]);
goto next;
}
}
break;
next:
pos = strtok(NULL, ",");
}
free(buf);
} else {
syslog(LOG_NOTICE, "user %s does not have 'cap_drop=' property", username);
}
return PAM_SUCCESS;
}
PAM_EXTERN int
pam_sm_close_session(pam_handle_t *pamh, int flags,
int argc, const char **argv)
{
/* do nothing */
return PAM_SUCCESS;
}
************************************************************
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