On 11/6/2024 10:21 AM, Chris PeBenito wrote:
I've recently become aware of systemd's credentials feature[1]. In a
nutshell, the intent is to reduce privilege in units by systemd itself
copying the credentials (crypto materials, passwords, though in practice
it could be anything) and placing it in /run/credentials, with access
managed by namespacing. This is intended to eliminate the need for the
daemon in the unit directly accessing the data. My concern is the
possibility of inadvertently leaking credentials or abuse. i.e. putting in
LoadCredential=foobar:/etc/shadow
This illustrative, as systemd can't read shadow if it's confined, but
you could replace shadow with a private key or any other highly
confidential data systemd needs to access. The common response to my
concern is systemd units should be protected at high integrity; only
root can modify them, etc. However, I think we can do better to reduce
the possibility of errors.
I've discussed this with one of the systemd maintainers, and I'm
proposing this change:
1. pid1 forks (to pidN) to run the unit, as usual.
2. pidN does security_compute_create() using the current domain and the
label of the unit to get a new domain.
3. pidN does setcon() to the new domain.
4. pidN runs the unit as per usual (including the credentials stuff)
Then we'd need to add something like this to the policy:
allow init_t httpd_initrc_t:process dyntransition;
type_transition init_t httpd_unit_t:process httpd_initrc_t;
I have not yet prototyped this, but based on my discussion with the
systemd maintainers, this should be doable. I believe the added benefit
is we can decompose initrc_t's privilege and maybe even reduce init_t's
privilege.
Hearing no objections, I've done an initial implementation:
https://github.com/systemd/systemd/compare/main...pebenito:systemd:pidN-selinux-setcon
If there is no policy in place, it does not incur new denials. One nice
thing I found is that the unit name is available, so I used that in the
security_compute_create_name_raw() call. I tested by adding the
following systemd-networkd.service drop-in:
[Service]
LoadCredential=shadow:/etc/shadow
I added the following to the policy:
type systemd_networkd_initrc_t;
domain_type(systemd_networkd_initrc_t)
role system_r types systemd_networkd_initrc_t;
allow init_t self:process setcurrent;
domain_dyntrans_type(init_t)
allow init_t systemd_networkd_initrc_t:process dyntransition;
type_transition init_t systemd_networkd_unit_t:process
systemd_networkd_initrc_t;
domtrans_pattern(systemd_networkd_initrc_t, systemd_networkd_exec_t,
systemd_networkd_t)
These changes resulted in this denial:
Nov 13 15:10:54 azurelinux-vm audit[605]: AVC avc: denied { read } for
pid=605 comm="(sd-mkdcreds)" name="shadow" dev="sda2" ino=18058
scontext=system_u:system_r:systemd_networkd_initrc_t:s0
tcontext=system_u:object_r:shadow_t:s0 tclass=file permissive=1
The remaining policy for systemd_networkd_initrc_t would look like
(denials summarized by audit2allow):
allow systemd_networkd_initrc_t autofs_t:dir getattr;
allow systemd_networkd_initrc_t autofs_t:filesystem unmount;
allow systemd_networkd_initrc_t bin_t:dir { getattr search };
allow systemd_networkd_initrc_t bin_t:file { execute execute_no_trans
getattr map open read };
allow systemd_networkd_initrc_t boot_t:dir search;
allow systemd_networkd_initrc_t cgroup_t:dir { getattr search };
allow systemd_networkd_initrc_t cgroup_t:file { getattr mounton };
allow systemd_networkd_initrc_t cgroup_t:filesystem { getattr remount };
allow systemd_networkd_initrc_t device_t:dir mounton;
allow systemd_networkd_initrc_t devlog_t:sock_file write;
allow systemd_networkd_initrc_t dosfs_t:filesystem remount;
allow systemd_networkd_initrc_t fs_t:filesystem { remount unmount };
allow systemd_networkd_initrc_t home_root_t:dir { getattr mounton };
allow systemd_networkd_initrc_t init_runtime_t:dir { add_name create
getattr mounton remove_name rmdir search write };
allow systemd_networkd_initrc_t init_t:dir search;
allow systemd_networkd_initrc_t init_t:fd use;
allow systemd_networkd_initrc_t init_t:file { getattr ioctl open read };
allow systemd_networkd_initrc_t init_t:unix_stream_socket getattr;
allow systemd_networkd_initrc_t kernel_t:unix_dgram_socket sendto;
allow systemd_networkd_initrc_t kmsg_device_t:chr_file { getattr mounton };
allow systemd_networkd_initrc_t modules_object_t:dir { getattr mounton };
allow systemd_networkd_initrc_t proc_kmsg_t:file { getattr mounton };
allow systemd_networkd_initrc_t proc_t:file { getattr open read };
allow systemd_networkd_initrc_t proc_t:filesystem { mount remount unmount };
allow systemd_networkd_initrc_t root_t:dir mounton;
allow systemd_networkd_initrc_t self:capability { dac_read_search fowner
net_admin setgid setpcap setuid sys_resource };
allow systemd_networkd_initrc_t self:key { search setattr write };
allow systemd_networkd_initrc_t self:netlink_route_socket { bind create
getattr getopt nlmsg_read read setopt write };
allow systemd_networkd_initrc_t self:process { getcap setcap setfscreate
setrlimit };
allow systemd_networkd_initrc_t self:unix_dgram_socket { connect create
getopt setopt };
allow systemd_networkd_initrc_t shell_exec_t:file getattr;
allow systemd_networkd_initrc_t sysctl_fs_t:dir { getattr mounton search };
allow systemd_networkd_initrc_t sysctl_kernel_t:dir search;
allow systemd_networkd_initrc_t sysctl_kernel_t:file { getattr ioctl
open read };
allow systemd_networkd_initrc_t syslogd_runtime_t:dir search;
allow systemd_networkd_initrc_t systemd_networkd_runtime_t:dir { getattr
mounton open read search watch };
allow systemd_networkd_initrc_t systemd_networkd_runtime_t:file {
getattr open read };
allow systemd_networkd_initrc_t systemd_networkd_t:process2 nnp_transition;
### other than mounton this tmpfs dir/file access is for creating the
/run/credentials content
allow systemd_networkd_initrc_t tmpfs_t:dir { add_name create getattr
mounton open read remove_name search setattr write };
contents:
allow systemd_networkd_initrc_t tmpfs_t:file { create getattr open read
rename setattr write };
allow systemd_networkd_initrc_t tmpfs_t:filesystem { mount remount
unmount };
allow systemd_networkd_initrc_t unlabeled_t:dir mounton;
allow systemd_networkd_initrc_t user_home_dir_t:dir { getattr mounton };
allow systemd_networkd_initrc_t user_runtime_root_t:dir { getattr mounton };
This seems like a very promising way to break up initrc_t, limit
privileges, and prevent administrator errors. What do you think?
--
Chris
