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

quoted

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

quoted

quoted

+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.

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