4 ms·

Which is easy for you to state, were it not for the fact that the "simple service" is broken in various ways. Let's start with the implementation-specific brok

Which is easy for you to state, were it not for the fact that the "simple service" is broken in various ways. Let's start with the implementation-specific brokenness. I configured a log host with rsyslog. Rsyslog's configuration format, as well as documentation, is completely abysmal. And once in a while, rsyslog would use 100% CPU for no reason. A quick strace reveals that it keeps reading or writing to a file descriptor, but getting EAGAIN as error. Looks like a bug to me. And oh yeah, there's a race condition in rsyslog with regard to privilege lowering, so that sometimes it would great directories owned as root, and then it would lower its privilege, and then it would be unable to write to its own created directory. Just great. The above are all fixable. But what bothers me more is that syslog is entirely unauthenticated. Any process can write to syslog, claiming that it is any process. A malicious user can write a syslog entry saying that "init[1]: the system is unstable. please reboot now!". As an administrator, you will not be able to tell whether the message actually came from init (or PID 1) or not. A quick look at systemd's journal mechanism reveals systemd at least has thought about these problems: http://www.freedesktop.org/software/systemd/man/systemd.journal-fields.html http://www.freedesktop.org/software/systemd/man/systemd.jour.... It would log the process's PID, UID, GID and more, and the sender is unable to fake that.

spc476 13y ago
systemd can log the PID, UID and GID of a message because its using a Linux-specific feature of datagram based local sockets; such a feature cannot be supported on other Unix systems. But there are other reasons to use syslogd/rsyslogd/syslog-ng besides logging to local files, and that's to forward the logs to a centralized logging service. Of course, there are issues with that, as the default syslog protocol also allows any process, anywhere, to claim it is any other process (and if root, even fake its own source address) but it's being addressed (I know rsyslogd is, I'm not sure about other syslogd replacements). My issue with systemd is that forwarding logs to a central server is an afterthought, if even that. And the problem with that is that it won't be as secure or work as well if it's part of the design. And another issue with the "afterthought support for remote logging" is that I still need to run syslogd/rsyslogd/syslog-ng because I have a ton of other network equipment (routers) that can forward their logs via the syslog protocol (which I have used on networks I've managed---by doing that, I've been able to receive notifications of OSPF routing changes, for instance). And if I have syslogd/rsyslogd/syslog-ng already running, what are the benefits of systemd logging?
- FooBarWidget 13y ago
  Regarding PID/UID/GID logging: if it has to use a Linux-specific feature to be less broken in that regard, then I'm all for it. In my opinion, authentication is such an important security feature that other Unices should just implement similar mechanisms (or, preferably, the same mechanism). Right now I've only seen the BSD people complaining that systemd uses Linux-specific features, but doing nothing to solve the problem. Regarding central logging: indeed, that is a problem. Has Lennart ever spoken about this issue?
  - spc476 13y ago
    The impression I've gotten is that he isn't interested in centralized logging, as that is outside of what he is trying to do. I could be wrong though.
asveikau 13y ago
Reading this I thought: how does one authenticate a pid or uid in a daemon? I am assuming Unix domain sockets, which don't provide you with the caller's pid or uid. Doing some googling I found there are indeed nonportable interfaces for this. First I found getpeerucred which is a Sun thing. Then I found Linux has a SO_PEERCRED socket option. [Edit: and there seems to exist getpeereid and LOCAL_CRED on some BSDs] Still seems like a weird or otherwise shaky thing to me. It feels like it breaks the abstraction of a socket.
- spc476 13y ago
  Unix domain sockets are the way to pass open file descriptors between processes, though (why? I don't know, it just is). I think that because of that, the mechanism was extended to also pass along the PID, UID and GID. I did use the open file descriptor with SO_PEERCRED options to implement a Linux-only daemon to open privileged sockets on behalf of a process: https://github.com/spc476/ipacld https://github.com/spc476/ipacld (something I'm surprised has never been written before).
  - asveikau 13y ago
    I think the easier way is to create the socket with high privileges, then drop down to lower privilege.
    - spc476 13y ago
      But why have the keys to the entire kingdom when all you need is a single key to a single room? The daemon I wrote is very small, so there isn't much code to audit. The client that calls the daemon doesn't need to be root at all. The daemon can also make a lot of different checks (right UID, right GID, right executable image, etc.) before passing the privileged port back to the client. Edit: fixing the last sentence.
      - FooBarWidget 13y ago
        A lot of stuff is built on the "worse is better" philosophy. Introducing a daemon like yours will complicate the OS a little bit more. So instead, OS developers outsource the complexity to the apps, and only implement a quick hack to make this possible. In practice, the running-as-root-then-drop-privilege pattern works well enough so nobody bothered making a more complex, but potentially more secure, solution.

spc476 13y ago

systemd can log the PID, UID and GID of a message because its using a Linux-specific feature of datagram based local sockets; such a feature cannot be supported on other Unix systems. But there are other reasons to use syslogd/rsyslogd/syslog-ng besides logging to local files, and that's to forward the logs to a centralized logging service. Of course, there are issues with that, as the default syslog protocol also allows any process, anywhere, to claim it is any other process (and if root, even fake its own source address) but it's being addressed (I know rsyslogd is, I'm not sure about other syslogd replacements). My issue with systemd is that forwarding logs to a central server is an afterthought, if even that. And the problem with that is that it won't be as secure or work as well if it's part of the design. And another issue with the "afterthought support for remote logging" is that I still need to run syslogd/rsyslogd/syslog-ng because I have a ton of other network equipment (routers) that can forward their logs via the syslog protocol (which I have used on networks I've managed---by doing that, I've been able to receive notifications of OSPF routing changes, for instance). And if I have syslogd/rsyslogd/syslog-ng already running, what are the benefits of systemd logging?

FooBarWidget 13y ago
Regarding PID/UID/GID logging: if it has to use a Linux-specific feature to be less broken in that regard, then I'm all for it. In my opinion, authentication is such an important security feature that other Unices should just implement similar mechanisms (or, preferably, the same mechanism). Right now I've only seen the BSD people complaining that systemd uses Linux-specific features, but doing nothing to solve the problem. Regarding central logging: indeed, that is a problem. Has Lennart ever spoken about this issue?
- spc476 13y ago
  The impression I've gotten is that he isn't interested in centralized logging, as that is outside of what he is trying to do. I could be wrong though.

asveikau 13y ago

Reading this I thought: how does one authenticate a pid or uid in a daemon? I am assuming Unix domain sockets, which don't provide you with the caller's pid or uid. Doing some googling I found there are indeed nonportable interfaces for this. First I found getpeerucred which is a Sun thing. Then I found Linux has a SO_PEERCRED socket option. [Edit: and there seems to exist getpeereid and LOCAL_CRED on some BSDs] Still seems like a weird or otherwise shaky thing to me. It feels like it breaks the abstraction of a socket.

spc476 13y ago
Unix domain sockets are the way to pass open file descriptors between processes, though (why? I don't know, it just is). I think that because of that, the mechanism was extended to also pass along the PID, UID and GID. I did use the open file descriptor with SO_PEERCRED options to implement a Linux-only daemon to open privileged sockets on behalf of a process: https://github.com/spc476/ipacld https://github.com/spc476/ipacld (something I'm surprised has never been written before).
- asveikau 13y ago
  I think the easier way is to create the socket with high privileges, then drop down to lower privilege.
  - spc476 13y ago
    But why have the keys to the entire kingdom when all you need is a single key to a single room? The daemon I wrote is very small, so there isn't much code to audit. The client that calls the daemon doesn't need to be root at all. The daemon can also make a lot of different checks (right UID, right GID, right executable image, etc.) before passing the privileged port back to the client. Edit: fixing the last sentence.
    - FooBarWidget 13y ago
      A lot of stuff is built on the "worse is better" philosophy. Introducing a daemon like yours will complicate the OS a little bit more. So instead, OS developers outsource the complexity to the apps, and only implement a quick hack to make this possible. In practice, the running-as-root-then-drop-privilege pattern works well enough so nobody bothered making a more complex, but potentially more secure, solution.