-----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. -----BEGIN PGP SIGNATURE----- Version: GnuPG v1.4.9 (GNU/Linux) Comment: Using GnuPG with Fedora - http://enigmail.mozdev.org iEYEARECAAYFAkj820UACgkQrlYvE4MpobMpBgCg6bgdShjBt9SNMibqrUos+Wqt UHsAnjKornOi6HKExM5ktKNZbPRSz1tC =rsXR -----END PGP SIGNATURE----- -- This message was distributed to subscribers of the selinux mailing list. If you no longer wish to subscribe, send mail to majordomo@xxxxxxxxxxxxx with the words "unsubscribe selinux" without quotes as the message.
