On May 29, 2019, at 10:46 AM, Casey Schaufler [off-list ref] wrote:

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On 5/29/2019 10:13 AM, Andy Lutomirski wrote:

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On May 29, 2019, at 8:53 AM, Casey Schaufler [off-list ref] wrote:

On 5/29/2019 4:00 AM, David Howells wrote:
Jann Horn [off-list ref] wrote:

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+void post_mount_notification(struct mount *changed,
+                            struct mount_notification *notify)
+{
+       const struct cred *cred = current_cred();

This current_cred() looks bogus to me. Can't mount topology changes
come from all sorts of places? For example, umount_mnt() from
umount_tree() from dissolve_on_fput() from __fput(), which could
happen pretty much anywhere depending on where the last reference gets
dropped?

IIRC, that's what Casey argued is the right thing to do from a security PoV.
Casey?

You need to identify the credential of the subject that triggered
the event. If it isn't current_cred(), the cred needs to be passed
in to post_mount_notification(), or derived by some other means.

Taking a step back, why do we care who triggered the event?  It seems to me that we should care whether the event happened and whether the *receiver* is permitted to know that.

There are two filesystems, "dot" and "dash". I am not allowed
to communicate with Fred on the system, and all precautions have
been taken to ensure I cannot. Fred asks for notifications on
all mount activity. I perform actions that result in notifications
on "dot" and "dash". Fred receives notifications and interprets
them using Morse code. This is not OK. If Wilma, who *is* allowed
to communicate with Fred, does the same actions, he should be
allowed to get the messages via Morse.

Under this scenario, Fred should not be allowed to enable these watches. If you give yourself and Fred unconstrained access to the same FS, then can communicate.

Other security modelers may disagree. The models they produce
are going to be *very* complicated and will introduce agents and
intermediate objects to justify Fred's reception of an event as
a read operation.

I disagree. They’ll model the watch as something to prevent if they want to restrict communication.

quoted

(And receiver means whoever subscribed, presumably, not whoever called read() or mmap().)

The receiver is the process that gets the event. There may
be more than one receiver, and the receivers may have different
credentials. Each needs to be checked separately.

I think it’s a bit crazy to have the same event queue with two readers who read different things.