posix capabilities inheritance
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Quoted 3 lines · Andy Lutomirski
1. Can a process have capabilities in its inheritable set and not in its permitted set? POSIX allows such processes to be created (pI = pP = full, then execute (fI = 0, fP = 0).
Sure, this is apparent to me in reading the spec. I'm not exactly sure if this is relavent though: if pP = pE = 0, it does not seem like it should be possible for pI=full to have any effect on exec, unless I am missing something.
Quoted 2 lines
Nevertheless, its pP evolution rule assumes that this never happens (pI capabilities can reappear).
Certainly with the current rule as implemented in 2.4, it looks as though you can regain permitted flags: pP' = (fP & X) | (fI & pI)
Is this what you mean when you say they can reappear? WRT the spec itself, I don't see this assumption. The rule could just as easily be: pP' = (fP & X) | (pP & fI & pI) (just an example) The rule in your patch seems like it should be compliant as well.
I'm not saying there aren't some problems to be worked out with the spec; I think there are. But I don't think this is a case of the spec being broken per se -- just too vague.
Quoted 3 lines
2. If a process has pE < pP (i.e. some caps disabled, e.g. uid=0, euid!=0), and exec's fE=full, then its capabilities get re-enabled. This seems like a pretty serious breakage of userspace.
How is this any different from traditional *nix setuid semantics? I suppose I can see your point somewhat if you are concerned specifically about the case where pE < pP execs fE=full && fP=0, but I am unconvinced this constitutes serious breakage. On the contrary, I think it seems most reasonable for those caps to be reenabled, especially for caps where fI=1, but perhaps even when fI=0.
I'm still looking over your patches to try to figure out how exactly you think this stuff should work. I'm glad to see someone out there is thinking about this. You're right, it's pretty complex and still far from useable.
--
Michael Glasgow < glasgow at beer dot net >
Quoted 5 lines · Michael Glasgow
Certainly with the current rule as implemented in 2.4, it looks as though you can regain permitted flags: pP' = (fP & X) | (fI & pI)
Is this what you mean when you say they can reappear?
yes. Intuitively, a process w/o some permitted capability should _never_ (unless fP != 0) get that capability back by calling exec. This happens in 2.4 and 2.6 in a worse way right now, though. Any uid=0 process that calls exec (if cap_bset is untouched) will regain all capabilities, making it (mostly) ineffective to restrict root processes. At the same time, non-root processes with extra caps can't usefully call helpers. I think this is the problem you originally noticed.
Quoted 4 lines
[...] WRT the
spec itself, I don't see this assumption. The rule could just as easily be: pP' = (fP & X) | (pP & fI & pI) (just an example) The rule in your patch seems like it should be compliant as well.
Maybe I misread the spec, but I thought it explicitly stated pP' = (fP & X) | (fI & pI)
(I can't find it right now, though...)
Quoted 5 lines
Quoted 3 lines
2. If a process has pE < pP (i.e. some caps disabled, e.g. uid=0, euid!=0), and exec's fE=full, then its capabilities get re-enabled. This seems like a pretty serious breakage of userspace.
Quoted 7 lines
How is this any different from traditional *nix setuid semantics? I suppose I can see your point somewhat if you are concerned specifically about the case where pE < pP execs fE=full && fP=0, but I am unconvinced this constitutes serious breakage. On the contrary, I think it seems most reasonable for those caps to be reenabled, especially for caps where fI=1, but perhaps even when fI=0.
I would hope that, on a system that supports file capabilities, a file w/o capabilities set and w/o setuid would behave exactly like a file with some "default" capabilities. In my patch, these capabilities are (=ei). In mainline Linux, there is no such capability set (witness the logic in cap_binprm_set_security).
As a test, this is IMHO correct: (-test-6 + my patch + both options on)
$ cp `which bash` .
$ chmod 4755 bash
$ su
Password:
# ./bash -p
$ dumpcap [a trivial program I wrote]
Real Eff
User 0 500
Group 0 0Caps: =ip
The bash -p process has uid = 0, euid = 500. When it execs dumpcap, neither its uid nor its euid change, so, in traditional POSIX, it should have no effective capabilities (as it acts like uid 500). (Should it have CAP_SETUID? My patch doesn't change this behavior, but I'm not sure it's correct right now.)
With the (POSIX) rule pE' = pP' & fE, the dumpcap process would have been uid=0, euid=500, and all caps effective, which is inconsistant with traditional semantics. Linux currently works correctly because fE and fP are dependent on initial uid and euid.
--Andy