On Mon, 2008-10-20 at 15:25 -0400, Daniel J Walsh wrote: > -----BEGIN PGP SIGNED MESSAGE----- > Hash: SHA1 > > Stephen Smalley wrote: > > On Fri, 2008-10-17 at 17:18 -0400, Daniel J Walsh wrote: > >> -----BEGIN PGP SIGNED MESSAGE----- > >> Hash: SHA1 > >> > >> Stephen Smalley wrote: > >>> On Fri, 2008-10-17 at 16:04 -0400, Eric Paris wrote: > >>>> On Fri, 2008-10-17 at 15:39 -0400, Stephen Smalley wrote: > >>>> > >>>>> I don't follow the above. First, the CAP_SETUID capability controls the > >>>>> ability to use set*uid() system calls, not to execute setuid binaries > >>>>> (aside from a special case for shared state). Second, if some other > >>>>> confined domain executes a setuid binary created by this user, it is > >>>>> still limited by the permissions granted to that original confined > >>>>> domain as far as SELinux is concerned. > >>>> He's saying there are almost 200 domains that can run setuid apps. > >>> As I said, CAP_SETUID isn't about whether or not you can run a setuid > >>> app. You can do that without CAP_SETUID. Obviously since a normal > >>> unprivileged user shell can run a setuid program in the first place. > >>> > >>>> Limiting the number of domains that can create new setuid apps limits > >>>> the number of places that these domains can go. Clearly they are all > >>>> still confined to their domain and whatever it allows, but allowing them > >>>> to gain root priveledges may give them the ability to attack other parts > >>> Wait - how did they gain root privileges? root privileges are > >>> capabilities, and we control capabilities based on domain. > >>> > >>>> of the system normally controlled by dac. This certainly doesn't lessen > >>>> the MAC confinement. Lets assume an audited system in which we are > >>>> certain the only suid app untrusted users are allowed to run is ping. > >>>> So the users have the right to run suid apps. They are protected from > >>>> each other by DAC. > >>>> > >>>> Confined webadmin writes a program which is clears out other users > >>>> public_html when they get a spurious DMCA takedown notice. He then > >>>> (because he is a lazy bumbling idiot) makes his script suid so he does > >>>> not have to go into his confined webadmin account constantly to delete > >>>> users webpages. > >>> He could also make a daemon that runs under his uid and accepts commands > >>> via local socket. > >>> > >>>> Normal DAC protects user2 from being attacked by user1. Because of the > >>>> bumbling incompetance of confined webadmin user1 can now use this setuid > >>>> app to do things which he is allow by selinux policy but denied by > >>>> normal DAC permissions. > >>>> > >>>> Why did webadmin need to make a setuid app to begin with? file caps are > >>>> already protected by CAP_SETFCAP. Lets assume system policy says that > >>>> su should not be setuid. Should the webadmin really be allowed to > >>>> easily override that system policy because he wants to use su - to get > >>>> to his confined domain instead of sudo? > >>> He can't get to his confined domain via some other program unless policy > >>> says he can. He can only get to some other uid that way. > >>> > >>>> She bumbling idiot really be > >>>> allowed to say add o+x to su - when the system policy only really wanted > >>>> group wheel to be able to run su? Should the bumbling idiot be able to > >>>> remove the suid flag from a program and not be able to fix it? > >>>> > >>>> I think there are some real gains that can be made by limiting how > >>>> confined admins or untrusted users can deal with suid apps. > >>>> > >>>> I'd probably be inclined to add changing the uid/gid/other things that > >>>> clear setuid to be things which require this permission. I don't see a > >>>> reason to allow my webadmin to chown su to himself... > >>> He can't do that unless he owns su (or that itself is subject to > >>> fowner). And chown'ing or writing to a suid app clears the suid bit > >>> forcibly. > >>> > >>> Come on, guys, you can do better. > >>> > >> This hole discussion has actually opened me up to a new understanding. > >> SELinux right now does not prevent the execution of setuid applications > >> if you do not have setuid capability, it only prevents you from running > >> apps that actually execute the setuid() call. > >> > >> I think this is a problem. One of the things I have been saying in my > >> presentations is that running as staff_t will prevent you from running > >> any setuid application unless a transition is defined. Turns out this > >> is wrong and I was lying. > >> > >> # cp /usr/bin/id /usr/bin/myid > >> # chmod 4755 /usr/bin/myid > >> > >> $ myid > >> uid=3267(dwalsh) gid=3267(dwalsh) euid=0(root) groups=3267(dwalsh) > >> context=staff_u:staff_r:staff_t:s0 > >> > >> So running a chmod o+s file as staff_t will run as EUID 0. I think this > >> is something SELinux should block. At least treat this the same way a > >> file system mounted nosetuid would. > > > > That would require a control on the exec path instead of chmod/setattr - > > which is what I suggested a year ago in response to the original RFC. > > > Sounds like a good idea, I guess you should have pushed it. :^) > > > But note that gaining EUID 0 does not automatically grant privilege > > under SELinux - your capabilities are still limited based on domain and > > your ability to read/write even root-owned files is controlled based on > > the (domain, type, file class) triple. > > > > Understood. But we still have the risc of having a fairly wide open > user type like staff_t tripping on a setuid app that could exculate > privs. Or the ability for two staff_t users to attach each other > through setuid apps. Ok, then I think the path forward is: - a setsuid file permission check only applied when setting suid/sgid in selinux_inode_setattr (and maybe when adding wider execute access, although that's harder if you want to take ACLs into account), - a new process permission check in selinux_bprm_set_security() when bprm->e_uid != current->euid or bprm->e_gid != current->egid or ! cap_issubset(bprm->cap_post_exec_permitted, current->cap_permitted). The first one controls what processes can create new entrypoints into a uid/gid. The second one controls the ability to invoke a suid/sgid or file-caps program at all. -- Stephen Smalley National Security Agency -- This message was distributed to subscribers of the selinux mailing list. 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