Messages in this thread | | | Date | Mon, 24 May 2004 17:23:54 -0700 | From | Andy Lutomirski <> | Subject | Re: [PATCH] caps, compromise version (was Re: [PATCH] scaled-back caps, take 4) |
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Chris Wright wrote:
> * Andy Lutomirski (luto@myrealbox.com) wrote: > >>>Hehe, arm wrestling could be entertaining ;-) I'm in favor of the most >>>conservative change, which I feel is in my patch. But I'm game to >>>continue to pick on each. >> >> >>I like your legacy mode. I don't like making processes inherit >>non-legacyness. (With your patch, some daemon might be secure >>when started from initscripts but insecure when started from the >>command line, if root ended up in non-legacy mode.) > > > Hmm, that was intentional (my very first cut at this thing cleared it, > but that patch had many other broken behaviours). Specifically because > it goes through pI, which POSIX draft says is untouched through exec.
Not in IRIX, though. And I'm afraid of:
cap -c all-i <some setuid binary> versus cap -c all+i <some setuid binary>
Suddently the binary's behavior might be different. This isn't inheritantly bad, but it seems like a pointless gotcha.
I like my version of using inheritable for legaciness, but only because my inheritable semantics make sense. Your version would worry me a lot less if you just added a new field. But mine doesn't actually need the new field ;)
>> >>"Legacy mode" is controlled by a new bit in task_struct called >>keep_all_caps (controlled by PRCTL_SET_KEEPALLCAPS). This bit turns >>off setuid emulation completely (except for setfsuid). > > > I had same idea. I wished we could hijack keep_capabilities as a > bit vector.
It's a bitfield. Just add fields -- no cost in memory. Fairly large cost in compile time, though...
> > >>The evolution rules are: >>pP' = (fP & X) | (pI & pP) [with the setuid-nonroot fix] >>pE' = (pE | fP) & pP' >>pI' = full >> >>This time around, I haven't touched the unsafeness rules. >> >>The magic is in the setuid emulation: >> if (current->uid == 0 || current->euid == 0) >> cap_set_full(current->cap_inheritable); >> else >> cap_clear(current->cap_inheritable); >> >>So, unless a program plays with it's inheritable mask, >>root will not pick up caps on exec (which is good -- it >>means it's safe to chroot somewhere, disable all caps >>except CAP_SETUID, and let untrusted code play around.) >>But, if you start as root and setuid away, _even with >>keepcaps_, you lose the caps on exec. Which is the broken >>behavior we want to preserve. >> >>So, to avoid this, new code can either set keep_all_caps >>or just explicitly enable inheritance after setuid, in >>which case it just works. >> >>I have pI' = full because otherwise it's just one more >>(partially) user-controlled variable that programs need >>to worry about. (And because anything else would break >>root.) > > > How do you keep passing down the same caps through multiple execs?
This only takes effect when set*uid is called successfully. It bites programs that start as non-root with CAP_SETUID and change their uid, but these programs either don't exist or don't work at all right now.
[root@luto andy]# cap -c all+i -u andy bash [andy@luto andy]$ dumpcap [note second exec] Real Eff User 500 500 Group 500 500
Caps: =ip cap_setpcap-p
> > >>As for the rest of the changes: >> >>The code no longer assumes that pI<pP, so I yanked all checks >>on the inheritable mask. On the other hand, it makes no >>sense to me for capset when changing lots of processes' >>masks to affect the inheritable mask. So I made it leave >>it alone, except when changing current. >> >>keep_all_caps is clearly not entirely necessary. I can take >>it out if anyone objects. >> >>I yanked all capset sanity checks from kernel/capability.c -- >>they were duplicates anyway. >> >>And I left the old (IMHO pointless) behavior that one needs to hack >>init in order to use CAP_SETPCAP. >> >>[Side note: for cap_bset to be useful, I think there needs to be >>an operation "atomically remove these caps from all tasks." I >>don't see one.] > > > Yeah. It depends on the definition of useful. Get a couple privileged > tasks running (which may fork/exec from time to time), then clamp down > the machine is one form of useful. In general, I don't cap_bset is that > useful though.
Especially with CAP_SYS_ADMIN... SELinux is clearly the way to go here.
I just discovered a patch (http://www.kernel.org/pub/linux/libs/security/linux-privs/kernel-2.4-fcap/README) that claims to implement per-process-tree maximum cap masks (like I did for awhile). It hasn't been maintained, though.
If one of our patches hits -mm or -linus, I may try and add a feature like that. It'll (rightly) annoy the SELinux folks, though.
> > >>This patch also should work fine if VFS capabilities are >>introduced (there's an fP mask which defaults to (setuid- >>root ? full : 0). >> >>Patch against 2.6.6-mm4 (-mm5 didn't like my filesystem...). >>It's not as well tested as it should be. The old cap.cc >>tool still works (but remember to set inheritable). I >>don't have a tool yet to play with keep_all_caps. > > > I can add this to the test stuff to play with it.
Except that I fail a lot of your tests because of inheritable mask differences. Oh, well.
I may revive my ext3 caps patch sometime. Is there a way to make that work with your patch?
--Andy - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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