Re: RFC: disablenetwork facility. (v4)

[Bryan Donlan]

On Tue, Dec 29, 2009 at 1:36 PM, Eric W. Biederman
<ebiederm@xxxxxxxxxxxx> wrote:
> Bryan Donlan <bdonlan@xxxxxxxxx> writes:
>
>> On Tue, Dec 29, 2009 at 11:39 AM, Serge E. Hallyn <serue@xxxxxxxxxx> wrote:
>>> Quoting Bryan Donlan (bdonlan@xxxxxxxxx):
>>>> On Tue, Dec 29, 2009 at 10:11 AM, Serge E. Hallyn <serue@xxxxxxxxxx> wrote:
>>>> > Eric, let me specifically point out a 'disable setuid-root'
>>>> > problem on linux: root still owns most of the system even when
>>>> > it's not privileged.  So does "disable setuid-root" mean
>>>> > we don't allow exec of setuid-root binaries at all, or that
>>>> > we don't setuid to root, or that we just don't raise privileges
>>>> > for setuid-root?
>>>>
>>>> I, for one, think it would be best to handle it exactly like the
>>>> nosuid mount option - that is, pretend the file doesn't have any
>>>> setuid bits set. There's no reason to deny execution; if the process
>>>> would otherwise be able to execute it, it can also copy the file to
>>>> make a non-suid version and execute that instead. And some programs
>>>> can operate with reduced function without setuid. For example, screen
>>>> comes to mind; it needs root to share screen sessions between multiple
>>>> users, but can operate for a single user just fine without root, and
>>>> indeed the latter is usually the default configuration.
>>>
>>> That's fine with me, seems safe for a fully unprivileged program to
>>> use, and would make sense to do through one of the securebits set
>>> with prctl(PR_SET_SECUREBITS).
>>>
>>> In addition, I assume we would also refuse to honor file capabilities?
>>
>> Yes - essentially a one-time switch saying "never allow me to gain
>> capabilities again".
>
> That is what I was thinking.  Does setresuid case problems?  Assuming
> the application that drop permissions could have successfully
> called setresuid?

It's probably reasonable to require that real == effective == saved ==
fs UID (and same for GID); anything else brings up sticky issues of
"which UID is a higher capability?"
If a process does this call, it's effectively saying that the only way
it's going to be accessing resources beyond its current UID and
capabilities is by talking to another process over a (unix domain)
socket.
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