On 12/5/2012 2:20 PM, Serge Hallyn wrote: > 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
http://wt.tuxomania.net/publications/posix.1e/download.html Everyone should read the capabilities rationale. It answers most of the questions on this thread, and a bunch more. The capabilities mechanism has to support what are currently setuid-root programs without change and allow for new programs that use the mechanism wisely and fully. > -- > To unsubscribe from this list: send the line "unsubscribe > linux-security-module" in > the body of a message to majord...@vger.kernel.org > More majordomo info at http://vger.kernel.org/majordomo-info.html > -- 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/
