Messages in this thread | | | Date | Tue, 29 Dec 2009 00:01:14 -0500 | From | Michael Stone <> | Subject | Re: RFC: disablenetwork facility. (v4) |
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Serge,
I think that Pavel's point, at its strongest and most general, could be rephrased as:
"Adding *any* interesting isolation facility to the kernel breaks backwards compatibility for *some* program [in a way that violates security goals]."
The reason is the one that I identified in my previous note:
"The purpose of isolation facilities is to create membranes inside which grievous security faults are converted into availability faults."
The question then is simply:
"How do we want to deal with the compatibility-breaking changes created by introducing new isolation facilities?"
So far, I've seen the following suggestions:
a) setuid restores pre-isolation semantics
- Doesn't work for me because it violates the security guarantee of the isolation primitive
b) setuid is an escape-hatch
- Probably the cleanest in the long-run
- Doesn't, by itself, suffice for Pavel since it violates backwards compatibility
c) signal to the kernel through a privileged mechanism that backwards-incompatible isolation may or may not be used
- No problems seen so far.
I would be happy with (c), assuming we can agree on an appropriate signalling mechanism and default.
So far, two defaults have been proposed:
default-deny incompatible isolation (Pavel) default-permit incompatible isolation (Michael)
So far, several signalling mechanisms have been proposed:
1) enabling a kernel config option implies default-permit
- My favorite; apparently insufficient for Pavel?
2) default-deny; disablesuid grants disablenetwork
- "disablesuid" is my name for the idea of dropping the privilege of exec'ing setuid binaries
- Suggested by Pavel and supported by several others.
- I think it has the same backwards-compatibility problem as disablenetwork: disablesuid is an isolation primitive.
3) default-deny; dropping a capability from the bounding set grants "permit"
- Suggested by Serge; seems nicely fine-grained but rather indirect
4) default-deny; setting a sysctl implies permit
- Suggested by Serge; works fine for me
5) default-deny; setting a kernel boot argument implies permit
- Suggested by Serge; I like the sysctl better.
I am happiest with (1) and, if (1) isn't good enough, with (4).
Pavel, what do you think of (4)?
Regards,
Michael
P.S. - I'd be happy to know more about existing precedent on introducing compatibility-breaking changes if any comes to mind. (For example, how were the Linux-specific rlimits handled?)
P.P.S. - On a completely unrelated note: imagine trying to use SELinux (or your favorite MAC framework) to restrict the use of prctl(PR_SET_NETWORK, PR_NETWORK_OFF). Am I right that sys_prctl() contains a time-of-check-to-time-of-use (TOCTTOU) race (with security_task_prctl() as the check and with prctl_set_network() as the use) as a result of the actual argument being passed by address rather than by value?
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