Re: [PATCH RFC v2] fhandle: expose u64 mount id to name_to_handle_at(2)

Aleksa Sarai
2024-06-01T08:12:51Z

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On 2024-05-28, Miklos Szeredi <miklos@szeredi.hu> wrote:
> On Tue, 28 May 2024 at 15:24, Christoph Hellwig <hch@infradead.org> wrote:
> >
> > On Tue, May 28, 2024 at 02:04:16PM +0200, Christian Brauner wrote:
> > > Can you please explain how opening an fd based on a handle returned from
> > > name_to_handle_at() and not using a mount file descriptor for
> > > open_by_handle_at() would work?
> >
> > Same as NFS file handles:
> >
> > name_to_handle_at returns a handle that includes a file system
> > identifier.
> >
> > open_by_handle_at looks up the superblock based on that identifier.
>
> The open file needs a specific mount, holding the superblock is not sufficient.

Not to mention that providing a mount fd is what allows for extensions
like Christian's proposed method of allowing restricted forms of
open_by_handle_at() to be used by unprivileged users.

If file handles really are going to end up being the "correct" mechanism
of referencing inodes by userspace, then future API designs really need
to stop assuming that the user is capable(CAP_DAC_READ_SEARCH). Being
able to open any file in any superblock the kernel knows about
(presumably using a kernel-internal mount if we are getting rid of the
mount fd) is also capable(CAP_SYS_ADMIN) territory.

Would the idea be to sign or MAC every file handle to avoid userspace
being able to brute-force the file handle of anything the system sees?
What happens if the key has to change? Then the handles aren't globally
unique anymore...

--
Aleksa Sarai
Senior Software Engineer (Containers)
SUSE Linux GmbH
<https://www.cyphar.com/>

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