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Message-ID: <CALCETrUAdOo0v7hw9jU65spaY2VUqMj=rXqR2_iTdZX9OKqVDw@mail.gmail.com> Date: Fri, 11 Apr 2014 15:32:35 -0700 From: Andy Lutomirski <luto@...capital.net> To: "Serge E. Hallyn" <serge@...lyn.com> Cc: Serge Hallyn <serge.hallyn@...ntu.com>, "Eric W. Biederman" <ebiederm@...ssion.com>, Sean Pajot <sean.pajot@...culink.com>, lxc-devel@...ts.linuxcontainers.org, "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org> Subject: Re: [lxc-devel] Kernel bug? Setuid apps and user namespaces On Fri, Apr 11, 2014 at 3:29 PM, Serge E. Hallyn <serge@...lyn.com> wrote: > Quoting Andy Lutomirski (luto@...capital.net): >> On Fri, Apr 11, 2014 at 2:52 PM, Serge E. Hallyn <serge@...lyn.com> wrote: >> > Quoting Andy Lutomirski (luto@...capital.net): >> >> On Mon, Apr 7, 2014 at 11:13 AM, Serge E. Hallyn <serge@...lyn.com> wrote: >> >> > Quoting Andy Lutomirski (luto@...capital.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. >> >> What do you mean "every fd"? >> >> It seems odd to me that execve of anything that isn't setuid would add >> anything to the list -- attackers can always ptrace before the execve >> happens. > > Maybe you're right. Maybe I shouldn't reason about this on a friday > afternoon. > > My *thought* was setuid-root program opens /etc/shadow, then execs a > regular program keeping that open. Attaching to that fails now though, > presumably due to dumpable. > Why would it fail? Isn't dumpable cleared on execve of a non-setuid program? Maybe I need to look this stuff up again. --Andy -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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