4 ms·

That's an easy thing to say after the fact indeed but yes. In fact after such a disastrous backdoor I wouldn't be surprised if OpenSSH moved all code calling ex

That's an easy thing to say after the fact indeed but yes. In fact after such a disastrous backdoor I wouldn't be surprised if OpenSSH moved all code calling external libraries to unprivileged processes to make sure such an attack can never have such a dramatic effect (an auth bypass would still likely be possible, but that's still way better than a root RCE…). At this point “All libraries could be malicious” is a threat model that must be considered for something as security critical as OpenSSH.

asveikau 3y ago
I don't think that's a threat model that OpenSSH should waste too much time on. Ultimately this is malicious code in the build machine compiling a critical system library. That's not reasonable to defend against. Keep in mind that upstream didn't even link to liblzma. Debian patched it to do so. OpenSSH should defend against that too?
- CanaryLayout 3y ago
  any one of us if we sat on the OSSH team would flip the middle finger. What code is the project supposed to write when nothing on main dyn loaded liblzma. It was brought in from a patch they don't have realistic control over. This is a Linux problem, and the problem is systemd, which is who brought the lib into memory and init'd it.
  - shzhdbi09gv8ioi 3y ago
    > This is a Linux problem, and the problem is systemd, which is who brought the lib into memory and init'd it. Not at all, it is a distro issue because a few distros such as Debian chose to patch openssh to bring in systemd support [1]. Other systemd-based distros like Arch Linux remains unaffected because they don't carry this patch. 1: https://sources.debian.org/src/openssh/1%3A9.7p1-2/debian/patches/systemd-readiness.patch/ https://sources.debian.org/src/openssh/1%3A9.7p1-2/debian/pa...
    - b112 3y ago
      Yet Redhat and others applied this patch, as systemd is so incapable of reliably launching processes, that it kept killing sshd without it. What a complete failure of an init system's job, and the patch was applied due to systemd not resolving the issue in another way. This is the problem with systemd. Way, way way too much complexity.
      - streb-lo 3y ago
        systemd is just fine on Arch without this patch. It's not a systemd issue, it's a larger cultural issue with packagers increasing attack surface to make their lives easier.
        b112 3y ago
        It absolutely, positively, 100% was added due to issues with systemd. Both Redhat and Debian and others applied this patch as a result. People didn't do it "just cause". https://bugzilla.redhat.com/show_bug.cgi?id=1381997 https://bugzilla.redhat.com/show_bug.cgi?id=1381997 Jakub Jelen 2016-10-12 08:40:44 UTC <snip> > Why do you want to avoid sd_notify()? Why to avoid this? Well, it is adding more systemd-specific bits and new build dependency to something that always worked well under other inits without any problems for years.
        streb-lo 3y ago
        They chose the worst solution to a problem that had multiple better solutions because of a pre-existing patch was the easiest path forward. That’s exactly what I’m talking about.
      - asveikau 3y ago
        I think the criticisms of systemd are valid but also tangential. I think Poettering himself is on one of the HN threads saying they didn't need to link to his library to accomplish what they sought to do. Lzma is also linked into a bunch of other critical stuff, including but not limited to distro package managers and the kernel itself, so if they didn't have sshd to compromise, they could have chosen another target.
  - jethro_tell 3y ago
    Brought in from a patch they rejected to accept because of this exact risk
- astrange 3y ago
  It is possible to prevent libraries from patching functions in other libraries; make those VM regions unwritable, don't let anyone make them writable, and adopt PAC or similar hardware protection so the kernel can't overwrite them either.
  - rwmj 3y ago
    That's already done, but in this case the attack happened in a glibc ifunc and those run before the patching protection is enabled (since an ifunc has to patch the PLT).
    - astrange 3y ago
      Sounds like libraries should only get to patch themselves. (Some difficulty with this one though. For instance you probably have to ban running arbitrary code at load time, but you should do this anyway because it will stop people from writing C++.)
      - rwmj 3y ago
        They're all in a single address space, there's nothing you can do to stop one part of a binary patching any other part.
        astrange 3y ago
        I already said the way to do it in my previous comment. Don't let arbitrary code run before page permissions are locked down.
        rwmj 3y ago
        If you're running in the binary you can call mprotect(2), and even if that is blocked you can cause all kinds of mischief. The original motivation for rings of protection on i286 was so that libraries could run in a different ring from the binary (usually library in ring 2 and program in ring 3), using a call gate (a very controlled type of call) to dispatch calls from the binary to the library, which stops the binary from modifying the library and IIRC libraries from touching each other. But x86-64 got rid of the middle rings.
jethro_tell 3y ago
Openssh has refused this patch that enables this in Debian multiple times as acrisk of preauth code execution as root. So maybe just don't patch sshd?

asveikau 3y ago

I don't think that's a threat model that OpenSSH should waste too much time on. Ultimately this is malicious code in the build machine compiling a critical system library. That's not reasonable to defend against. Keep in mind that upstream didn't even link to liblzma. Debian patched it to do so. OpenSSH should defend against that too?

CanaryLayout 3y ago
any one of us if we sat on the OSSH team would flip the middle finger. What code is the project supposed to write when nothing on main dyn loaded liblzma. It was brought in from a patch they don't have realistic control over. This is a Linux problem, and the problem is systemd, which is who brought the lib into memory and init'd it.
- shzhdbi09gv8ioi 3y ago
  > This is a Linux problem, and the problem is systemd, which is who brought the lib into memory and init'd it. Not at all, it is a distro issue because a few distros such as Debian chose to patch openssh to bring in systemd support [1]. Other systemd-based distros like Arch Linux remains unaffected because they don't carry this patch. 1: https://sources.debian.org/src/openssh/1%3A9.7p1-2/debian/patches/systemd-readiness.patch/ https://sources.debian.org/src/openssh/1%3A9.7p1-2/debian/pa...
  - b112 3y ago
    Yet Redhat and others applied this patch, as systemd is so incapable of reliably launching processes, that it kept killing sshd without it. What a complete failure of an init system's job, and the patch was applied due to systemd not resolving the issue in another way. This is the problem with systemd. Way, way way too much complexity.
    - streb-lo 3y ago
      systemd is just fine on Arch without this patch. It's not a systemd issue, it's a larger cultural issue with packagers increasing attack surface to make their lives easier.
      - b112 3y ago
        It absolutely, positively, 100% was added due to issues with systemd. Both Redhat and Debian and others applied this patch as a result. People didn't do it "just cause". https://bugzilla.redhat.com/show_bug.cgi?id=1381997 https://bugzilla.redhat.com/show_bug.cgi?id=1381997 Jakub Jelen 2016-10-12 08:40:44 UTC <snip> > Why do you want to avoid sd_notify()? Why to avoid this? Well, it is adding more systemd-specific bits and new build dependency to something that always worked well under other inits without any problems for years.
        streb-lo 3y ago
        They chose the worst solution to a problem that had multiple better solutions because of a pre-existing patch was the easiest path forward. That’s exactly what I’m talking about.
    - asveikau 3y ago
      I think the criticisms of systemd are valid but also tangential. I think Poettering himself is on one of the HN threads saying they didn't need to link to his library to accomplish what they sought to do. Lzma is also linked into a bunch of other critical stuff, including but not limited to distro package managers and the kernel itself, so if they didn't have sshd to compromise, they could have chosen another target.
- jethro_tell 3y ago
  Brought in from a patch they rejected to accept because of this exact risk
astrange 3y ago
It is possible to prevent libraries from patching functions in other libraries; make those VM regions unwritable, don't let anyone make them writable, and adopt PAC or similar hardware protection so the kernel can't overwrite them either.
- rwmj 3y ago
  That's already done, but in this case the attack happened in a glibc ifunc and those run before the patching protection is enabled (since an ifunc has to patch the PLT).
  - astrange 3y ago
    Sounds like libraries should only get to patch themselves. (Some difficulty with this one though. For instance you probably have to ban running arbitrary code at load time, but you should do this anyway because it will stop people from writing C++.)
    - rwmj 3y ago
      They're all in a single address space, there's nothing you can do to stop one part of a binary patching any other part.
      - astrange 3y ago
        I already said the way to do it in my previous comment. Don't let arbitrary code run before page permissions are locked down.
        rwmj 3y ago
        If you're running in the binary you can call mprotect(2), and even if that is blocked you can cause all kinds of mischief. The original motivation for rings of protection on i286 was so that libraries could run in a different ring from the binary (usually library in ring 2 and program in ring 3), using a call gate (a very controlled type of call) to dispatch calls from the binary to the library, which stops the binary from modifying the library and IIRC libraries from touching each other. But x86-64 got rid of the middle rings.

jethro_tell 3y ago

Openssh has refused this patch that enables this in Debian multiple times as acrisk of preauth code execution as root. So maybe just don't patch sshd?