Messages in this thread | | | From | (Eric W. Biederman) | Date | Tue, 03 May 2016 00:54:40 -0500 | Subject | Re: [PATCH 1/1] simplified security.nscapability xattr |
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"Serge E. Hallyn" <serge@hallyn.com> writes:
> Quoting Andrew G. Morgan (morgan@kernel.org): >> >> I guess I'm confused how we have strayed so far that this isn't an obvious >> requirement. Uid=0 as being the root of privilege was the basic problem >> that capabilities were designed to change. > > The task executing the file can be any uid mapped into the namespace. The > file only has to be owned by the root of the user_ns. Which I agree is > unfortunate. We can work around it by putting the root uid into the xattr > itself (which still isn't orthogonal but allows the file to at least by > owned by non-root), but the problem then is that a task needs to know its > global root k_uid just to write the xattr.
The root kuid is just make_kuids(user_ns, 0) so it is easy to find.
It might be a hair better to use the userns->owner instead of the root uid. That would allow user namespaces without a mapped root to still use file capabilities.
>> Uid is an acl concept. Capabilities are supposed to be independent of that. >> >> If we want to support NS file capabilities I would look at replacing the >> xattr syscall with a dedicated file capabilities modification syscall. Then > > That was one ofthe possibilities I'd mentioned in my earlier proposal, > fwiw. The problem is if we want tar to still work unmodified then > simple xattr operations still have to work. > > Maybe there's workable semantics there though. Worth thinking about.
If the problem is compatibilty please look at posix_acl_fix_xattr_from_user. With something similar for the security.capability attribute we can perform whatever transformation makes sense. I admit adding 4 bytes is a bit of a pain in that context but not a big one.
Eric
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