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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.


    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.o -lpam
    # cp /lib/security

    modify /etc/passwd as follows:

    [kaigai@masu ~]$ ping
    PING ( 56(84) bytes of data.
    64 bytes from icmp_seq=1 ttl=64 time=1.23 ms
    64 bytes from icmp_seq=2 ttl=64 time=1.02 ms

    --- 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
    [tak@masu ~]$ export LANG=C
    [tak@masu ~]$ ping
    ping: icmp open socket: Operation not permitted

    [tak@masu ~]$ su
    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 <>


    * pam_cap_drop.c module -- drop capabilities bounding set
    * Copyright: 2007 KaiGai Kohei <>

    #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

    static char *captable[] = {

    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));
    syslog(LOG_NOTICE, "user %s drops %s\n", username, captable[i]);
    goto next;
    pos = strtok(NULL, ",");
    } 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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      Last update: 2007-12-01 02:51    [W:0.027 / U:31.596 seconds]
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