Quoting Andy Lutomirski (l...@amacapital.net): > On Wed, Dec 5, 2012 at 1:05 PM, Serge Hallyn <serge.hal...@canonical.com> > wrote: > > Quoting Andy Lutomirski (l...@amacapital.net): > >> On Tue, Dec 4, 2012 at 5:54 AM, Serge E. Hallyn <se...@hallyn.com> wrote: > >> > Quoting Andy Lutomirski (l...@amacapital.net): > >> >> >> d) If I really wanted, I could emulate execve without actually doing > >> >> >> execve, and capabilities would be inherited. > >> >> > > >> >> > If you could modify the executable properties of the binary that has > >> >> > the privilege to wield a privilege then you are either exploiting an > >> >> > app bug, or doing something the privileged binary has been trusted to > >> >> > do. > >> >> > >> >> That's not what I mean. I would: > >> >> > >> >> fork() > >> >> munmap everything > >> >> mmap ld.so > >> >> set up a fake initial stack and the right fd or mapping or whatever > >> >> just to ld-linux.so > >> >> > >> >> That's almost execve, and privilege inheritance works. > >> > > >> > But of course that is why you only want to fill fI on programs you trust > >> > not to do that. What you are arguing is that you want to give fI on > >> > programs you don't trust anyway, and so heck why not just give it on > >> > everything. > >> > > >> > >> Huh? I'd set fP on a program I expect to do *exactly* that (or use > >> actual in-kernel capability inheritance, which I would find vastly > >> more pleasant). If I give a program a capability (via fP or fI & pI), > >> then I had better trust it not to abuse that capability. Having it > >> pass that capability on to a child helper process would be just fine > >> with me *because it already has that capability*. > >> > >> The problem with the current inheritance mechanism is that it's very > >> difficult to understand what it means for an fI bit or a pI bit to be > >> set. Saying "set a pI bit using pam if you want to grant permission > >> to that user to run a particular program with fI set" is crap -- it > >> only works if there is exactly one binary on the system with that bit > >> set. In any case, a different administrator or package might use it > >> for something different. > >> > >> Suppose I use the (apparently) current suggested approach: I install a > >> fI=cap_net_raw copy of tcpdump somewhere. Then I write a helper that > >> has fP=cap_new_raw and invokes that copy of tcpdump after appropriate > >> validation of parameters. All is well. > > > > Since you're writing a special helper, you can surely have it validate > > the userid and make it so the calling user doesn't have to have > > cap_net_raw in pI? > > I can and did.
Oh, oops, I mis-understood what you meant was the problem. Yup, that is a real limitation. Yes, with the posix file caps you will be disappointed unless you see pI=X as "this user may run any program which is Inh-trusted with X" and fI=X as "this program may be run with X by any user Inh-trusted with X". It almost makes me want to say that there should be an execve-analogue to prctl(PR_SET_KEEPCAPS), which says caps will remain unchanged for one execve. Or perhaps an intermediate securebits state between !SECBIT_NOROOT and SECBIT_NOROOT, which automatically transitions after the first execve to SECBIT_NOROOT. > The mere presence of a cap_net_raw+i tcpdump binary is more or less > equivalent to saying that users with cap_net_raw in pI can capture > packets. I've just prevented pI=cap_net_raw from meaning anything > less than "can capture packets". So I think we should bite the bullet > and just let programs opt in (via some appropriately careful > mechanism) to real capability inheritance. By real you mean more precise. I think it'd be very interesting to get together with Markku and learn more from the N9 experiment! Markku, are there any post-mortem analysis papers we can read for starters? Andy would not be trying to restrict root in general, so the ramification you cited may not necessarily be relevant. -serge -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
