Re: [PATCH] capabilities: introduce per-process capability bounding set (v10)

[Andrew Morgan <morgan@kernel.org>]

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KaiGai Kohei wrote:
>> There is already a pam_cap module in the libcap2 package. Can we merge
>> this functionality?
> 
> I think it is a good idea.
> 
> However, this module already have a feature to modify inheritable
> capability set.
> How does it to be described in the "/etc/security/capability.conf"?
> 
> One idea is like a following convention:
> 
> # compatible configuration. We can omit "i:" at the head of line
> cap_setfcap                 tak
> # It drops any capabilities from b-set except for cap_net_raw and
> cap_fowner
> b:cap_net_raw,cap_fowner    ymj
> # It drops only cap_dac_override from b-set.
> b:-cap_dac_override         kaigai
> # It drops only cap_sys_admin from b-set of any user within users group.
> b:-cap_sys_admin            group:users

I like the idea of a separate line for bounds.

For ease of parsing, perhaps '!' or some other symbol prefix to the line
could be used to identify lines that refer to cap_bound?

In other modules, @groupname is used to capture a group association.

Lines like this should be supported:

!cap_net_raw         @regularusers    # suppress from cap_bset
cap_net_raw          @pingers morgan  # add to pI

where morgan is not in group @pingers but is in group @regularusers.

Cheers

Andrew

> 
> Thanks,
> 
>> Cheers
>>
>> Andrew
>>
>> serge@hallyn.com wrote:
>>> Quoting KaiGai Kohei (kaigai@kaigai.gr.jp):
>>>> 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]#
>>> Neat.  A bigger-stick version of not adding the account to
>>> group wheel.  I'll use that.
>>>
>>> Is there any reason not to have a separate /etc/login.capbounds
>>> config file, though, so the account can still have a full name?
>>> Did you only use that for convenience of proof of concept, or
>>> is there another reason?
>>>
>>>> # BTW, I replaced the James's address in the Cc: list,
>>>> # because MTA does not accept it.
>>> Thanks!  I don't know what happened to my alias for him...
>>>
>>> thanks,
>>> -serge
>>>
>>>> -- 
>>>> 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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> 
> 

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