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

[KaiGai Kohei <kaigai@kaigai.gr.jp>]

> 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

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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-- 
KaiGai Kohei <kaigai@kaigai.gr.jp>