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SubjectRe: [PATCH] caps, compromise version (was Re: [PATCH] scaled-back caps, take 4)
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
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