Does NFS support Linux Capabilities

[Jeff Layton]

On Thu, 2022-09-08 at 21:17 +0000, Chuck Lever III wrote:
> 
> > On Sep 8, 2022, at 5:03 PM, Jeff Layton <jlayton at kernel.org> wrote:
> > 
> > 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 No standards govern capabilities, but the Linux capability implementation is based on the withdrawn POSIX.1e draft standard; see ⟨https://archive.org/details/posix_1003.1e-990310⟩.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 trying to cite a withdrawn POSIX standard in an Internet RFC!)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.
> 
> That was the starting idea for accessing IMA metadata on NFS until
> we discovered that NFSv4 security labels are intended to enable only
> a single label per file. Capabilities are often present with SELinux
> labels.
> 
> It would work for a proof of concept, though.
> 

Yeah, that why I was saying "piggyback".

You'd need a combined SELinux+capabilities label (potentially with other
stuff in it as well). When you got one from the server, you'd have to
extract each piece and put in the right places in the inode.

But, like I said...I haven't thought through the implications here at
all (and am not looking for a project at the moment). ;)
-- 
Jeff Layton <jlayton at kernel.org>