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SubjectRe: [RFC][PATCH 00/10] Mount, FS, Block and Keyrings notifications [ver #3]
Date
Andy Lutomirski <luto@kernel.org> wrote:

> > > You are allowing arbitrary information flow between T and W above. Who
> > > cares about notifications?
> >
> > I do. If Watched object is /dev/null no data flow is possible.
> > There are many objects on a modern Linux system for which this
> > is true. Even if it's "just a file" the existence of one path
> > for data to flow does not justify ignoring the rules for other
> > data paths.
>
> Aha!
>
> Even ignoring security, writes to things like /dev/null should
> probably not trigger notifications to people who are watching
> /dev/null. (There are probably lots of things like this: /dev/zero,
> /dev/urandom, etc.)

Even writes to /dev/null might generate access notifications; leastways,
vfs_read() will call fsnotify_access() afterwards on success.

Whether or not you can set marks on open device files is another matter.

> David, are there any notification types that have this issue in your
> patchset? If so, is there a straightforward way to fix it?

I'm not sure what issue you're referring to specifically. Do you mean whether
writes to device files generate notifications?

> Generically, it seems like maybe writes to device nodes shouldn't trigger
> notifications since, despite the fact that different openers of a device
> node share an inode, there isn't necessarily any connection between them.

With the notification types I have currently implemented, I don't even notice
any accesses to a device file unless:

(1) Someone mounts over the top of one.

(2) The access triggers an I/O error or device reset or causes the device to
be attached or detached.

(3) Wangling the device causes some other superblock event.

(4) The driver calls request_key() and that creates a new key.

> Casey, if this is fixed in general, do you have another case where the
> right to write and the right to read do not imply the right to
> communicate?
>
> > An analogy is that two processes with different UIDs can open a file,
> > but still can't signal each other.
>
> What do you mean "signal"? If two processes with different UIDs can
> open the same file for read and write, then they can communicate with
> each other in many ways. For example, one can write to the file and
> the other can read it. One can take locks and the other can read the
> lock state. They can both map it and use any number of memory access
> side channels to communicate. But, of course, they can't send each
> other signals with kill().
>
> If, however, one of these processes is using some fancy mechanism
> (inotify, dnotify, kqueue, fanotify, whatever) to watch the file, and
> the other one writes it, then it seems inconsistent to lie to the
> watching process and say that the file wasn't written because some
> security policy has decided to allow the write, allow the read, but
> suppress this particular notification. Hence my request for a real
> example: when would it make sense to do this?

Note that fanotify requires CAP_SYS_ADMIN, but inotify and dnotify do not.

dnotify is applied to an open file, so it might be usable on a chardev such as
/dev/null, say.

David

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