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.

Maybe I should pass in NULL creds in the case that an event is being generated
because an object is being destroyed due to the last usage[*] being removed.

You should pass the cred of the process that removed the
last usage. If the last usage was removed by something like
the power being turned off on a disk drive a system cred
should be used. Someone or something caused the event. It can
be important who it was.

 [*] Usage, not ref - Superblocks are a bit weird in their accounting.

David