[poettering]

Uh. systemd documents the protocol at various places and the protocol is trivial: a single text datagram sent to am AF_UNIX socket whose path you get via the NOTIFY_SOCKET. That's trivial to implement for any one with some basic unix programming knowledge. And i tell pretty much anyone who wants to listen that they should just implement the proto on their own if thats rhe only reason for a libsystemd dep otherwise. In particular non-C environments really should do their own native impl and not botjer wrapping libsystemd just for this.

But let me stress two other things:

Libselinux pulls in liblzma too and gets linked into tons more programs than libsystemd. And will end up in sshd too (at the very least via libpam/pam_selinux). And most of the really big distros tend do support selinux at least to some level. Hence systemd or not, sshd remains vulnerable by this specific attack.

With that in mind libsystemd git dropped the dep on liblzma actually, all compressors are now dlopen deps and thus only pulled in when needed.

[agwa]

> And i tell pretty much anyone who wants to listen that they should just implement the proto on their own if thats rhe only reason for a libsystemd dep otherwise

Could you point out where the man page (https://www.freedesktop.org/software/systemd/man/latest/sd_n...) says this?

[chadcatlett]

The notes section has a brief description of the protocol and the different kinds of sockets involved.

[NekkoDroid]

If you are talking about the stability of that interface: https://systemd.io/PORTABILITY_AND_STABILITY/

[oklischat]

That site states "using libsystemd is a good choice"

:-D

[o11c]

Deferring the load of the library often just makes things harder to analyze, not necessarily more secure. I imagine many of the comments quoting `ldd` are wrongly forgetting about `dlopen`.

(I really wish there were a way to link such that the library isn't actually loaded but it still shows in the metadata, so you can get the performance benefits of doing less work but can still analyze the dependency DAG easily)

[poettering]

It would make things more secure in this specific backdooring case, since sshd only calls a single function of libsystemd (sd_notify) and that one would not trigger the dlopen of liblzma, hence the specific path chosen by the backdoor would not work (unless libselinux fucks it up fter all, see other comments)

Dlopen has drawbacks but also major benefits. We decided the benefits relatively clearly outweigh the drawbacks, but of course people may disagree.

I have proposed a mechanism before, that would expose the list of libs we potentially load via dlopen into an ELF section or ELF note. This could be consumed by things such as packagae managers (for auto-dep generation) and ldd. However there was no interest in getting this landed from anyone else, so I dropped it.

Note that there are various cases where people use dlopen not on hardcoded lib names, but dynamically configured ones, where this would not help. I.e. things like glibc nss or pam or anything else plugin based. But in particular pam kinda matters since that tends to be loaded into almost any kind of security relavant software, including sshd.

[o11c]

The plugin-based case can covered by the notion of multiple "entry points": every library that is intended to be `dlopen`ed is tagged with the name of the interface it provides, and every library that does such `dlopen`ing mentions the names of such interfaces rather than the names of libraries directly. Of course your `ldd` tool has to scan all the libraries on the system to know what might be loaded, but `ldconfig` already does that for libraries not in a private directory.

This might sound like a lot of work for a package-manager-less-language ecosystem at first, but if you consider "tag" as "exports symbol with name", it is in fact already how most C plugin systems work (a few use an incompatible per-library computed name though, or rely entirely on global constructors). So really only the loading programs need to be modified, just like the fixed-name `dlopen`.

[pabs3]

I note that Solaris had runtime-optional but compile-time linked shared libraries, I always wondered why Linux/glibc never adopted them.

[iforgotpassword]

> And i tell pretty much anyone who wants to listen that they should just implement the proto on their own if thats rhe only reason for a libsystemd dep otherwise.

That's what I think too. Do the relevant docs point this out too? Ages ago they didn't. I think we should try to avoid that people just google "implement systemd notify daemon" and end up on a page that says "link to libsystemd and call sd_notify()".

[DPA2]

The correct thing to do would be to put different unrelated APIs into their own library, instead of everything into libsystemd0. This has always been one of my biggest issues with it. It makes it hard to replace just one API from that library, because on a binary distribution, only one package can provide it. And as a nice side effect, surprises like this one could then be avoided.

[pabs3]

systemd developers have already rejected that approach, so I guess we will end up with lots of reimplementations, both in individual projects and third-party libsystemd-notify style libraries.

[eijk]

I see that different clients implemented in different languages will need different client-libraries and maintaining all that is not something, a core project is going to do but if using the raw protocol instead of the convenience of libsystemd is a (commonly ignored) recommendation which makes a lot of sense in terms of segmentation, providing at least one reference implementation would point all systemd users into the right direction. Recommending that each client should just implement the (trivial) protocol access itself does not make so much sense to me.

[3v1n0]

IIRC sshd loads libpam only if specifically configured for it. So while it's not wrong, it's also a more edge case for the backdoor to work.

[b112]

And will end up in sshd too (at the very least via libpam/pam_selinux).

Inaccurate.

It's not pulled in on any sysvinit Debian system I run. It is on stable, oldstable, and oldoldstable systems via systemd.

Not systemd:

# ldd $(which sshd) linux-vdso.so.1 (0x00007ffcb57f5000)

        libcrypt.so.1 => /lib/x86_64-linux-gnu/libcrypt.so.1 (0x00007fbad13c9000)

        libwrap.so.0 => /lib/x86_64-linux-gnu/libwrap.so.0 (0x00007fbad13bd000)

        libaudit.so.1 => /lib/x86_64-linux-gnu/libaudit.so.1 (0x00007fbad138c000)

        libpam.so.0 => /lib/x86_64-linux-gnu/libpam.so.0 (0x00007fbad137a000)

        libsystemd.so.0 => /lib/x86_64-linux-gnu/libsystemd.so.0 (0x00007fbad12d5000)

        libselinux.so.1 => /lib/x86_64-linux-gnu/libselinux.so.1 (0x00007fbad12a5000)

        libgssapi_krb5.so.2 => /lib/x86_64-linux-gnu/libgssapi_krb5.so.2 (0x00007fbad1253000)

        libkrb5.so.3 => /lib/x86_64-linux-gnu/libkrb5.so.3 (0x00007fbad1179000)

        libcom_err.so.2 => /lib/x86_64-linux-gnu/libcom_err.so.2 (0x00007fbad1173000)

        libcrypto.so.3 => /lib/x86_64-linux-gnu/libcrypto.so.3 (0x00007fbad0c00000)

        libz.so.1 => /lib/x86_64-linux-gnu/libz.so.1 (0x00007fbad1154000)

        libc.so.6 => /lib/x86_64-linux-gnu/libc.so.6 (0x00007fbad0a1f000)

        libnsl.so.2 => /lib/x86_64-linux-gnu/libnsl.so.2 (0x00007fbad1137000)

        libcap-ng.so.0 => /lib/x86_64-linux-gnu/libcap-ng.so.0 (0x00007fbad112f000)

        libcap.so.2 => /lib/x86_64-linux-gnu/libcap.so.2 (0x00007fbad1123000)

        /lib64/ld-linux-x86-64.so.2 (0x00007fbad156a000)

        libpcre2-8.so.0 => /lib/x86_64-linux-gnu/libpcre2-8.so.0 (0x00007fbad1089000)

        libk5crypto.so.3 => /lib/x86_64-linux-gnu/libk5crypto.so.3 (0x00007fbad09f2000)

        libkrb5support.so.0 => /lib/x86_64-linux-gnu/libkrb5support.so.0 (0x00007fbad09e4000)

        libkeyutils.so.1 => /lib/x86_64-linux-gnu/libkeyutils.so.1 (0x00007fbad09dd000)

        libresolv.so.2 => /lib/x86_64-linux-gnu/libresolv.so.2 (0x00007fbad09cc000)

        libtirpc.so.3 => /lib/x86_64-linux-gnu/libtirpc.so.3 (0x00007fbad099e000)

systemd:

# ldd $(which sshd) linux-vdso.so.1 (0x00007ffc4d3eb000)

        libcrypt.so.1 => /lib/x86_64-linux-gnu/libcrypt.so.1 (0x00007feb8aa35000)

        libwrap.so.0 => /lib/x86_64-linux-gnu/libwrap.so.0 (0x00007feb8aa29000)

        libaudit.so.1 => /lib/x86_64-linux-gnu/libaudit.so.1 (0x00007feb8a9f8000)

        libpam.so.0 => /lib/x86_64-linux-gnu/libpam.so.0 (0x00007feb8a9e6000)

        libsystemd.so.0 => /lib/x86_64-linux-gnu/libsystemd.so.0 (0x00007feb8a916000)

        libselinux.so.1 => /lib/x86_64-linux-gnu/libselinux.so.1 (0x00007feb8a8e6000)

        libgssapi_krb5.so.2 => /lib/x86_64-linux-gnu/libgssapi_krb5.so.2 (0x00007feb8a894000)

        libkrb5.so.3 => /lib/x86_64-linux-gnu/libkrb5.so.3 (0x00007feb8a7ba000)

        libcom_err.so.2 => /lib/x86_64-linux-gnu/libcom_err.so.2 (0x00007feb8a7b4000)

        libcrypto.so.3 => /lib/x86_64-linux-gnu/libcrypto.so.3 (0x00007feb8a200000)

        libz.so.1 => /lib/x86_64-linux-gnu/libz.so.1 (0x00007feb8a795000)

        libc.so.6 => /lib/x86_64-linux-gnu/libc.so.6 (0x00007feb8a01f000)

        libnsl.so.2 => /lib/x86_64-linux-gnu/libnsl.so.2 (0x00007feb8a778000)

        libcap-ng.so.0 => /lib/x86_64-linux-gnu/libcap-ng.so.0 (0x00007feb8a770000)

        libcap.so.2 => /lib/x86_64-linux-gnu/libcap.so.2 (0x00007feb8a764000)

        libgcrypt.so.20 => /lib/x86_64-linux-gnu/libgcrypt.so.20 (0x00007feb89ed8000)

        liblzma.so.5 => /lib/x86_64-linux-gnu/liblzma.so.5 (0x00007feb8a735000)

        libzstd.so.1 => /lib/x86_64-linux-gnu/libzstd.so.1 (0x00007feb89e1c000)

        liblz4.so.1 => /lib/x86_64-linux-gnu/liblz4.so.1 (0x00007feb8a70d000)

        /lib64/ld-linux-x86-64.so.2 (0x00007feb8abb5000)

        libpcre2-8.so.0 => /lib/x86_64-linux-gnu/libpcre2-8.so.0 (0x00007feb89d82000)

        libk5crypto.so.3 => /lib/x86_64-linux-gnu/libk5crypto.so.3 (0x00007feb8a6e0000)

        libkrb5support.so.0 => /lib/x86_64-linux-gnu/libkrb5support.so.0 (0x00007feb8a6d2000)

        libkeyutils.so.1 => /lib/x86_64-linux-gnu/libkeyutils.so.1 (0x00007feb8a6c9000)

        libresolv.so.2 => /lib/x86_64-linux-gnu/libresolv.so.2 (0x00007feb8a6b8000)

        libtirpc.so.3 => /lib/x86_64-linux-gnu/libtirpc.so.3 (0x00007feb8a68a000)

        libgpg-error.so.0 => /lib/x86_64-linux-gnu/libgpg-error.so.0 (0x00007feb89d5a000)

EG

# ldd $(which sshd) | grep liblz

        liblzma.so.5 => /lib/x86_64-linux-gnu/liblzma.so.5 (0x00007fd1e647a000)

        liblz4.so.1 => /lib/x86_64-linux-gnu/liblz4.so.1 (0x00007fd1e6398000)

[3v1n0]

You also need to have sshd enabled to use PAM and your sshd pam stack should include pam_selinux. Then it will be dynamically loaded only when sshd starts a PAM session.