Re: Linux capabilities to Capsicum
- In reply to: David Chisnall : "Re: Linux capabilities to Capsicum"
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Date: Sun, 17 Apr 2022 16:58:46 UTC
Hello David, Thank you for the detailed response. George On Sun, Apr 17, 2022 at 6:26 PM David Chisnall <firstname.lastname@example.org> wrote: > Hi, > > I don’t think you’ll have much luck trying to map Linux capabilities to > Capsicum. Although they have similar names, they are very different. > > Linux capabilities, confusingly, are not a capability system. A > capability is an unforgeable token of authority that can be delegated and > must be presented to perform an action. Linux ‘capabilities’ are > permissions that relate to the ambient authority of a process: simply > having the permission is sufficient to perform any of the privileged > actions. There is no namable object that you are able to present to the > relevant system calls to be explicitly choose to exert the authority. > > In contrast, Capsicum makes file descriptors into capabilities. Once you > enter capability mode, you have no ambient authority. You many not access > any global namespace except by presenting a capability (file descriptor) > with the relevant authority to a system call. > > Linux capabilities are intended to allow programs to have some subset of > root privileges. This is very difficult to do well because the privileges > that root holds on *NIX systems were never intended to be decomposed. The > set that you list add up to complete root power, in several ways. For > example: > > - If you have CAP_SYS_PTRACE then you can attach to init (or any other > unrestricted daemon), inject arbitrary code, and tell it to execute on your > behalf. > - If you have CAP_DAC_OVERRIDE then you can (unless running with some > code-signing checks) modify bits of the filesystem that unrestricted > programs running as root will trust as containing system binaries and have > them exec code that you’ve injected. > - If you have CAP_SYS_ADMIN, can do pretty much anything that root can do > even without additional elevation steps, including any `ioctl` on block > devices. > > I don’t think that you’d lose anything other than a tiny bit of defence in > depth that costs an attacker several seconds to bypass by simply skipping > the privilege separation that this kind of use of Linux capabilities buys > you. > > Similar restrictions could be imposed by a MAC policy but that is a lot > of work to implement. It would be a nice project for someone to look at > Linux Capabilities and the Solaris equivalents and build something that > exposed this kind of functionality. > > The more traditional UNIX way of doing what you need is to have a separate > process that runs as root and exposes an RPC interface to the Python code > that performs these trusted actions on its behalf. That would be a lot > less effort to implement, though again the security benefits are negligible > if the set of privileged actions includes the full set authorised by those > Linux permissions since they equate to giving the unprivileged process > complete control over your system. > > David > >  > https://www.freebsd.org/cgi/man.cgi?query=mac&sektion=9&apropos=0&manpath=FreeBSD+13.0-RELEASE+and+Ports > > > On 16 Apr 2022, at 18:17, George Diaconu <email@example.com> wrote: > > > > Hello, > > > > Together with my colleagues we are trying to port OpenStack to FreeBSD. > As part of the process we need to modify a python package used by OpenStack > called oslo_privsep. This package uses linux capabilities to give OpenStack > services the least permissions they need. > > Now as part of porting to FreeBSD we want to replace the linux > capabilities with Capsicum. We found a list of Capsicum capabilities at > . So far we found that the package uses at least the following 5 > capabilities described in : > > - CAP_DAC_OVERRIDE > > - CAP_DAC_READ_SEARCH > > - CAP_NET_ADMIN > > - CAP_SYS_PTRACE > > - CAP_SYS_ADMIN > > > > What would be the respective capabilities in Capsicum? > > > > Thank you, > > George > > > >  > https://www.freebsd.org/cgi/man.cgi?query=rights&sektion=4&apropos=0&manpath=FreeBSD+13.0-RELEASE+and+Ports > >  https://man7.org/linux/man-pages/man7/capabilities.7.html > >