On Fri, Apr 11, 2014 at 3:46 PM, Serge E. Hallyn <[email protected]> wrote: > Quoting Andy Lutomirski ([email protected]): >> On Fri, Apr 11, 2014 at 3:29 PM, Serge E. Hallyn <[email protected]> wrote: >> > Quoting Andy Lutomirski ([email protected]): >> >> On Fri, Apr 11, 2014 at 2:52 PM, Serge E. Hallyn <[email protected]> wrote: >> >> > Quoting Andy Lutomirski ([email protected]): >> >> >> On Mon, Apr 7, 2014 at 11:13 AM, Serge E. Hallyn <[email protected]> >> >> >> wrote: >> >> >> > Quoting Andy Lutomirski ([email protected]): >> >> >> >> 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? > > I had expected it to succeed when I tried it, but it did in fact fail. > >> Isn't dumpable cleared on execve of a non-setuid >> program? Maybe I need to look this stuff up again. > > I guess this particular case was handled by setup_new_exec: > > if (uid_eq(current_euid(), current_uid()) && gid_eq(current_egid(), > current_gid())) > set_dumpable(current->mm, SUID_DUMP_USER); > else > set_dumpable(current->mm, suid_dumpable); > > since my euid was 0 and uid 1000, when I did the exec. >
Then we need to keep this working. I guess we can just keep the dumpable bit around. --Andy -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to [email protected] More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
