I think we have the regressions under control now.

https://salsa.debian.org/debian/flatpak/-/commits/wip/1.10.x/ is
packaging of 1.10.5 aimed at inclusion in Debian 11, including one
post-1.10.5 bug fix https://github.com/flatpak/flatpak/pull/4461 which
will hopefully be included in 1.10.6. I'm waiting for an opinion from
the Debian security team. For release series that are already based on
1.10.x, I'd recommend basing your releases on that version.

For full effectiveness, you'll want libseccomp 2.5.2, with which we can
block all the syscalls we identified as undesired, including
`mount_setattr()`.

Failing that, libseccomp 2.5.0 is sufficient to be able to block
`clone3()`, which I think should prevent a successful exploit: by
preventing creation of new user namespaces, it stops a malicious or
compromised Flatpak app from getting CAP_SYS_ADMIN in a new user
namespace, which it would need if it wanted to be able to invoke
`mount_setattr()`.

For release series that use 1.6.x or 1.0.x, Flatpak upstream does not
support those branches any more and will not make new releases. If
someone wants to get involved upstream, I'd accept MRs against those
branches as a coordination point for "if you're stuck on this branch,
here's what other distros are doing...", similar to what I'm doing for
1.2.x on https://github.com/flatpak/flatpak/pull/4455.

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https://bugs.launchpad.net/bugs/1946578

Title:
  Placeholder for CVE-2021-41133

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