Does NFS support Linux Capabilities

[Jeff Layton]

On Thu, 2022-09-08 at 20:24 +0000, Chuck Lever III wrote:
> [ This question comes up on occasion, so I've added a few interested
>   parties to the Cc: list ]
> 
> > On Sep 8, 2022, at 8:27 AM, battery dude <jyf007 at gmail.com> wrote:
> > 
> > According to https://access.redhat.com/solutions/2117321 this article,
> > I want to ask, how to make NFS support the penetration of Linux
> > Capabilities
> 
> That link is access-limited, so I was able to view only the top
> few paragraphs of it. Not very open, Red Hat.
> 
> TL;DR: I looked into this while trying to figure out how to enable
> IMA on NFS files. It's difficult for many reasons.
> 
> 
> A few of these reasons include:
> 
> The NFS protocol is a standard, and is implemented on a wide variety
> of OS platforms. Each OS implements its own flavor of capabilities.
> There's no way to translate amongst the variations to ensure
> interoperation. On Linux, capabilities(7) says:
> 
> > No standards govern capabilities, but the Linux capability implementation is based on the withdrawn POSIX.1e draft standard; see ⟨https://archive.org/details/posix_1003.1e-990310⟩.
> 
> I'm not sure how closely other implementations come to implementing
> POSIX.1e, but there are enough differences that interoperability
> could be a nightmare. Anything Linux has done differently than
> POSIX.1e would be encumbered by GPL, making it nearly impossible to
> standardize those differences. (Let alone the possible problems
> trying to cite a withdrawn POSIX standard in an Internet RFC!)
> 
> The NFSv4 WG could invent our own capabilities scheme, just as was
> done with NFSv4 ACLs. I'm not sure everyone would agree that effort
> was 100% successful.
> 
> 
> Currently, an NFS server bases its access control choices on the
> RPC user that makes each request. We'd have to figure out a way to
> enable NFS clients and servers to communicate more than just user
> identity to enable access control via capabilities.
> 
> When sending an NFS request, a client would have to provide a set
> of capabilities to the server so the server can make appropriate
> access control choices for that request.
> 
> The server would have to report the updated capset when a client
> accesses and executes a file with capabilities, and the server
> would have to trust that its clients all respect those capsets
> correctly.
> 
> 
> Because capabilities are security-related, setting and retrieving
> capabilities should be done only over networks that ensure
> integrity of communication. So, protection via RPC-with-TLS or
> RPCSEC GSS with an integrity service ought to be a requirement
> both for setting and updating capabilities and for transmitting
> any protected file content. We have implementations, but there
> is always an option of not deploying this kind of protection
> when NFS is actually in use, making capabilities just a bit of
> security theater in those cases.
> 
> 
> Given these enormous challenges, who would be willing to pay for
> standardization and implementation? I'm not saying it can't or
> shouldn't be done, just that it would be a mighty heavy lift.
> But maybe other folks on the Cc: list have ideas that could
> make this easier than I believe it to be.
> 
> 

I'm not disputing anything you wrote above, and I clearly haven't
thought through the security implications, but I wonder if we could
piggyback this info onto security label support somehow? That already
requires a (semi-opaque) per-inode attribute, which is mostly what's
required for file capabilities.
-- 
Jeff Layton <jlayton at kernel.org>