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? > Now I want to grant only a subset of users permission to run ping. So > I modify ping so it's cap_net_raw=i (not p) and grant those users > pI=cap_net_raw. > > The end result: I introduced a security hole: the users with > cap_net_raw=i can run tcpdump *without validation via the helper*. > Oops. -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/
