5 ms·

> Liblzma ends up dynamically linked to sshd because of a systemd-related extension added by many Linux packagers that pulls in liblzma as an unrelated dependen

> Liblzma ends up dynamically linked to sshd because of a systemd-related extension added by many Linux packagers that pulls in liblzma as an unrelated dependency kinda wish this was unpacked a bit more, why exactly is a service executable dynamically linking to a library without using any of its symbols or functions, because of systemd. and a follow up if some openbsd folks can comment. over the years i've read about various unique security capabilities in openbsd, it seems natural to ask, what kernel or OS capabilities does openbsd provide to thwart the stage 2 efforts for this class of injection techniques?

MrStonedOne 3y ago
[dead]
zokier 3y ago
Libsystemd is a grabbag utility library, and completely optional: > The libsystemd library provides functions that allow interacting with various interfaces provided by the systemd(1) service manager, as well as various other functions and constants useful for implementing services in general. https://www.freedesktop.org/software/systemd/man/latest/libsystemd.html https://www.freedesktop.org/software/systemd/man/latest/libs... E.g. the service readiness protocol is pretty trivial to implement yourself if you don't want to pull libsystemd as dep.
logicprog 3y ago
> kinda wish this was unpacked a bit more, why exactly is a service executable dynamically linking to a library without using any of its symbols or functions, because of systemd. If I recall correctly based on what I've read about this — I believe from the original mailing list post that noticed the vulnerability — it's because under certain circumstances in order to enable certain functionality you might want sshd to be able to talk to systemd, so distros often patch sshd with code to do that. But obviously you need a library to implement actually speaking system's protocol, and as it happens, the easiest way to do that is to include the entirety of libsystemd, since it has functions for doing that, even though there are at least two other libraries that implement just the communication functionality and are actually designed for non-systemd programs to use. The problem with that idea being that libsystemd, being a whole standard library for all systemd-related functionality that is probably mostly designed for use by programs in the tightly integrated systemd family and as a reference imenentation, also includes a lot of other code, including code that has to deal with compression that depends on liblzma, even though all of that is never used by sshd, because it only uses the small subsection of the library it needs.
- o11c 3y ago
  > even though there are at least two other libraries that implement just the communication functionality and are actually designed for non-systemd programs to use. The important question is: if one of those libraries is used, and then something else pulls in `libsystemd`, will they conflict?
  - zokier 3y ago
    Why would they? There isn't any magic in libsystemd, its just a normal c lib
    - o11c 3y ago
      A lot of systemd-replacement shims try to be transparent, which means exporting the same symbols as the real systemd libs and thus causing weird conflicts if you link to the real systemd libs too.
- zokier 3y ago
  > But obviously you need a library to implement actually speaking system's protocol That is overstatement. The docs have basic self-contained example how to implement the notification without libraries, its 50 lines, majority of which is just error handling: https://www.freedesktop.org/software/systemd/man/devel/sd_notify.html#Notes https://www.freedesktop.org/software/systemd/man/devel/sd_no...
  - NekkoDroid 3y ago
    The example was only recently added (like 1-2 days ago). Before that it was only really explained and said it was stable (and guarnteed as stable API).
  - logicprog 3y ago
    Oh, cool! I was just trying to give the maximum benefit of the doubt, but this is good to know. The systemd hate never seems as justified as it'd like to be...
MarkSweep 3y ago
The problem was that SSH linked to libsystemd. Libsystemd linked to xz. So the library got pulled in transitvly. Here are two recent commits that fix the problem by reducing dependencies: https://salsa.debian.org/ssh-team/openssh/-/commit/cc5f37cb8405cba624a133f4b8f464fbe381c5c8 https://salsa.debian.org/ssh-team/openssh/-/commit/cc5f37cb8... https://github.com/systemd/systemd/pull/31550 https://github.com/systemd/systemd/pull/31550
amiga386 3y ago
sshd is started by systemd. systemd has several ways of starting programs and waiting until they're "ready" before starting other programs that depend on them: Type=oneshot, simple, exec, forking, dbus, notify, ... A while back, several distro maintainers found problems with using Type=exec (?) and chose Type=notify instead. When sshd is ready, it notifies systemd. How do you notify systemd? You send a datagram to systemd's unix domain socket. That's about 10 lines of C code. But to make life even simpler, systemd's developers also provided the one-line sd_notify() call, which is in libsystemd.so. This library is so other programmers can easily integrate with systemd. So the distro maintainers patched sshd to use the sd_notify() function from libsystemd.so What else is in libsystemd.so? That's right, systemd also does logging. All the logging functions are in there, so user programs can do logging the systemd way. You can even _read_ logs, using the functions in libsystemd.so. For example, sd_journal_open_files(). By the way... systemd supports the environment variable SYSTEMD_JOURNAL_COMPRESS which can be LZ4, XZ or ZSTD, to allow systemd log files to be compressed. So, if you're a client program, that needs to read systemd logs, you'll call sd_journal_open_files() in libsystemd.so, which may then need liblz4, liblzma or libzstd functions. These compression libraries could be dynamically loaded, should sd_journal_open_files() need them - which is what https://github.com/systemd/systemd/pull/31550 https://github.com/systemd/systemd/pull/31550 submitted on the 29th February this year did. But clearly that's not in common use. No, right now, most libsystemd.so libraries have headers saying "you'll need to load liblz4.so, liblzma.so and libzstd before you can load me!", so liblzma.so gets loaded for the logging functions that sshd doesn't use, so the distro maintainers of sshd can add 1 line instead of 10 to notify systemd that sshd is ready.
- xorcist 3y ago
  This seems like a clear case of premature optimization. During the three decades sshd has existed I have never seen a real world situation where the equivalent of Type=exec was not enough. The time window where sshd is started and not yet ready to receive connections is short, and clients will have a connection timeout orders of magnitude larger. The notify functionality is more relevant for things like Java middleware processes and clients that lack the functionality to poll and wait. Under most situations none of this is relevant for sshd. These patches solve a problem very few people have. If you really have this problem, the systemd readiness is far from enough to solve the problem. The readiness is sent too early and there could still be permission problems that would cause sshd to be ready but the connection to fail. Even more relevant is local firewall rules that are completely out of scope for a readiness check! Polling for readiness is the only robust way.
mrspuratic 3y ago
AFAICT libsystemd is the "blessed" way of a daemon interacting with systemd and its notification protocol. libsystemd is something of a kitchen sink library now, it wasn't always so: there used to be separate libsystemd-daemon that had the functions a daemon might use (and not the ones it won't, like compression wrappers for functions in external libraries). https://lwn.net/Articles/587373/ https://lwn.net/Articles/587373/ A lot of things (e.g. OpenSSH, Apache) don't use it natively, you won't tend to see it in the official sources of non-Linux specific software. Package maintainers who choose to foist^H^H^H^H^Huse systemd add it to those packages. It's quite fun working out why Apache hangs when you port a config and don't realise it needs mod_systemd loaded to stop systemd stamping on it. There's an extra security wrinkle too: lazy symbol resolution is not the done thing now, it's preferred that symbols are resolved at loading time so important chunks of memory can be protected from other types of attacks (more clarity in one of the preceding articles: https://research.swtch.com/xz-script https://research.swtch.com/xz-script ).
tedunangst 3y ago
There's very little you can do to limit the scope of a backdoor in sshd. If sshd can't do anything, you wouldn't be able to do anything after logging in either, which wouldn't be very useful.
fmajid 3y ago
Systemd provides via libsystemd a call sd_notify() that tells systemd the daemon is not only started but ready to accept connections. libsystemd, being a kitchen-sink like everything else systemd, has a bunch of unrelated functionality including one that pulled liblzma as a dependency. The backdoored liblzma relies on a misfeature of glibc called ifuncs, where a library can override a function by calling a special init function in the library. This is so for instance if you have a version of a function optimized for AVX512, one ofor AVX2 and one not optimized for those at all, the init function would check which features the CPU supports and picks the best one. Seemingly ifuncs doesn't check the function being overriden is in the same library, and it was replacing OpenSSH's RSA auth function. OpenSSH added a clean-room libsystemd-free implementation of sd_notify() after this fiasco, in the hope Linux distros will stop linking against libsystemd, this will appear in OpenSSH 9.8: https://bugzilla.mindrot.org/show_bug.cgi?id=2641 https://bugzilla.mindrot.org/show_bug.cgi?id=2641

MrStonedOne 3y ago

[dead]

zokier 3y ago

Libsystemd is a grabbag utility library, and completely optional: > The libsystemd library provides functions that allow interacting with various interfaces provided by the systemd(1) service manager, as well as various other functions and constants useful for implementing services in general. https://www.freedesktop.org/software/systemd/man/latest/libsystemd.html https://www.freedesktop.org/software/systemd/man/latest/libs... E.g. the service readiness protocol is pretty trivial to implement yourself if you don't want to pull libsystemd as dep.

logicprog 3y ago

> kinda wish this was unpacked a bit more, why exactly is a service executable dynamically linking to a library without using any of its symbols or functions, because of systemd. If I recall correctly based on what I've read about this — I believe from the original mailing list post that noticed the vulnerability — it's because under certain circumstances in order to enable certain functionality you might want sshd to be able to talk to systemd, so distros often patch sshd with code to do that. But obviously you need a library to implement actually speaking system's protocol, and as it happens, the easiest way to do that is to include the entirety of libsystemd, since it has functions for doing that, even though there are at least two other libraries that implement just the communication functionality and are actually designed for non-systemd programs to use. The problem with that idea being that libsystemd, being a whole standard library for all systemd-related functionality that is probably mostly designed for use by programs in the tightly integrated systemd family and as a reference imenentation, also includes a lot of other code, including code that has to deal with compression that depends on liblzma, even though all of that is never used by sshd, because it only uses the small subsection of the library it needs.

o11c 3y ago
> even though there are at least two other libraries that implement just the communication functionality and are actually designed for non-systemd programs to use. The important question is: if one of those libraries is used, and then something else pulls in `libsystemd`, will they conflict?
- zokier 3y ago
  Why would they? There isn't any magic in libsystemd, its just a normal c lib
  - o11c 3y ago
    A lot of systemd-replacement shims try to be transparent, which means exporting the same symbols as the real systemd libs and thus causing weird conflicts if you link to the real systemd libs too.
zokier 3y ago
> But obviously you need a library to implement actually speaking system's protocol That is overstatement. The docs have basic self-contained example how to implement the notification without libraries, its 50 lines, majority of which is just error handling: https://www.freedesktop.org/software/systemd/man/devel/sd_notify.html#Notes https://www.freedesktop.org/software/systemd/man/devel/sd_no...
- NekkoDroid 3y ago
  The example was only recently added (like 1-2 days ago). Before that it was only really explained and said it was stable (and guarnteed as stable API).
- logicprog 3y ago
  Oh, cool! I was just trying to give the maximum benefit of the doubt, but this is good to know. The systemd hate never seems as justified as it'd like to be...

MarkSweep 3y ago

The problem was that SSH linked to libsystemd. Libsystemd linked to xz. So the library got pulled in transitvly. Here are two recent commits that fix the problem by reducing dependencies: https://salsa.debian.org/ssh-team/openssh/-/commit/cc5f37cb8405cba624a133f4b8f464fbe381c5c8 https://salsa.debian.org/ssh-team/openssh/-/commit/cc5f37cb8... https://github.com/systemd/systemd/pull/31550 https://github.com/systemd/systemd/pull/31550

amiga386 3y ago

sshd is started by systemd. systemd has several ways of starting programs and waiting until they're "ready" before starting other programs that depend on them: Type=oneshot, simple, exec, forking, dbus, notify, ... A while back, several distro maintainers found problems with using Type=exec (?) and chose Type=notify instead. When sshd is ready, it notifies systemd. How do you notify systemd? You send a datagram to systemd's unix domain socket. That's about 10 lines of C code. But to make life even simpler, systemd's developers also provided the one-line sd_notify() call, which is in libsystemd.so. This library is so other programmers can easily integrate with systemd. So the distro maintainers patched sshd to use the sd_notify() function from libsystemd.so What else is in libsystemd.so? That's right, systemd also does logging. All the logging functions are in there, so user programs can do logging the systemd way. You can even _read_ logs, using the functions in libsystemd.so. For example, sd_journal_open_files(). By the way... systemd supports the environment variable SYSTEMD_JOURNAL_COMPRESS which can be LZ4, XZ or ZSTD, to allow systemd log files to be compressed. So, if you're a client program, that needs to read systemd logs, you'll call sd_journal_open_files() in libsystemd.so, which may then need liblz4, liblzma or libzstd functions. These compression libraries could be dynamically loaded, should sd_journal_open_files() need them - which is what https://github.com/systemd/systemd/pull/31550 https://github.com/systemd/systemd/pull/31550 submitted on the 29th February this year did. But clearly that's not in common use. No, right now, most libsystemd.so libraries have headers saying "you'll need to load liblz4.so, liblzma.so and libzstd before you can load me!", so liblzma.so gets loaded for the logging functions that sshd doesn't use, so the distro maintainers of sshd can add 1 line instead of 10 to notify systemd that sshd is ready.

xorcist 3y ago
This seems like a clear case of premature optimization. During the three decades sshd has existed I have never seen a real world situation where the equivalent of Type=exec was not enough. The time window where sshd is started and not yet ready to receive connections is short, and clients will have a connection timeout orders of magnitude larger. The notify functionality is more relevant for things like Java middleware processes and clients that lack the functionality to poll and wait. Under most situations none of this is relevant for sshd. These patches solve a problem very few people have. If you really have this problem, the systemd readiness is far from enough to solve the problem. The readiness is sent too early and there could still be permission problems that would cause sshd to be ready but the connection to fail. Even more relevant is local firewall rules that are completely out of scope for a readiness check! Polling for readiness is the only robust way.

mrspuratic 3y ago

AFAICT libsystemd is the "blessed" way of a daemon interacting with systemd and its notification protocol. libsystemd is something of a kitchen sink library now, it wasn't always so: there used to be separate libsystemd-daemon that had the functions a daemon might use (and not the ones it won't, like compression wrappers for functions in external libraries). https://lwn.net/Articles/587373/ https://lwn.net/Articles/587373/ A lot of things (e.g. OpenSSH, Apache) don't use it natively, you won't tend to see it in the official sources of non-Linux specific software. Package maintainers who choose to foist^H^H^H^H^Huse systemd add it to those packages. It's quite fun working out why Apache hangs when you port a config and don't realise it needs mod_systemd loaded to stop systemd stamping on it. There's an extra security wrinkle too: lazy symbol resolution is not the done thing now, it's preferred that symbols are resolved at loading time so important chunks of memory can be protected from other types of attacks (more clarity in one of the preceding articles: https://research.swtch.com/xz-script https://research.swtch.com/xz-script ).

tedunangst 3y ago

There's very little you can do to limit the scope of a backdoor in sshd. If sshd can't do anything, you wouldn't be able to do anything after logging in either, which wouldn't be very useful.

fmajid 3y ago

Systemd provides via libsystemd a call sd_notify() that tells systemd the daemon is not only started but ready to accept connections. libsystemd, being a kitchen-sink like everything else systemd, has a bunch of unrelated functionality including one that pulled liblzma as a dependency. The backdoored liblzma relies on a misfeature of glibc called ifuncs, where a library can override a function by calling a special init function in the library. This is so for instance if you have a version of a function optimized for AVX512, one ofor AVX2 and one not optimized for those at all, the init function would check which features the CPU supports and picks the best one. Seemingly ifuncs doesn't check the function being overriden is in the same library, and it was replacing OpenSSH's RSA auth function. OpenSSH added a clean-room libsystemd-free implementation of sd_notify() after this fiasco, in the hope Linux distros will stop linking against libsystemd, this will appear in OpenSSH 9.8: https://bugzilla.mindrot.org/show_bug.cgi?id=2641 https://bugzilla.mindrot.org/show_bug.cgi?id=2641