Messages in this thread | | | Date | Fri, 11 Apr 2014 23:52:28 +0200 | From | "Serge E. Hallyn" <> | Subject | Re: [lxc-devel] Kernel bug? Setuid apps and user namespaces |
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Quoting Andy Lutomirski (luto@amacapital.net): > On Mon, Apr 7, 2014 at 11:13 AM, Serge E. Hallyn <serge@hallyn.com> wrote: > > Quoting Andy Lutomirski (luto@amacapital.net): > >> I'm starting to think that we need to extend dumpable to something > >> much more general like a list of struct creds that someone needs to be > >> able to ptrace, *in addition to current creds* in order to access > >> sensitive /proc files, coredumps, etc. If you get started as setuid, > > > > Hm, yeah, this sort of makes sense. > > > >> then you start with two struct creds in the list (or maybe just your > >> euid and uid). If you get started !setuid, then your initial creds > >> are in the list. It's possible that few or no things will need to > >> change that list after execve. > >> > >> If all of the entries and current->cred are in the same user_ns, then > >> we can dump as userns root. If they're in different usernses, then we > >> dump as global root or maybe the common ancestor root. > >> setuid(getuid()) and other such nastiness may have to empty the list, > >> or maybe we can just use a prctl for that. > > > > A few questions, > > > > 1. is there any other action which would trigger adding a new cred to > > the ist? > > I don't think so. Anyone who can ptrace you from the start can > corrupt you such that you leak rights even if some future action > prevents new ptracers from attaching. > > OTOH, it might be nice for something like an HTTPS server to be able > to fork and shove its private key into the child, while preventing > anyone from ptracing the child. But doing this securely without help > from someone with a different uid might be impossible anyway. > > > > > 2. would execve clear (and re-init) the list of creds? > > Probably. Thoughts?
Yeah it seems to me it should be re-initialized, with a cred added to the list for every open fd.
> We could have a way to ask execve not to reinit the list. Such a > mechanism would have to require no_new_privs to prevent a > straightforward attack on any setuid binary.
If we don't add a cred for every open fd, then I think we need this. If we do, then I'm not sure this makes sense.
> We's also want PR_SET_DUMPABLE or a new prctl to be able reset the > list to contain just current-.cred, I think.
Yeah PR_SET_DUMPABLE should reset it, agreed.
-serge
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