Re: [PATCH 3/7] vfs: Add a mount-notification facility

From: David Howells
Date: Mon Jun 03 2019 - 12:34:32 EST

Casey Schaufler <casey@xxxxxxxxxxxxxxxx> wrote:

> >> should be used. Someone or something caused the event. It can
> >> be important who it was.
> > The kernel's normal security model means that you should be able to
> > e.g. accept FDs that random processes send you and perform
> > read()/write() calls on them without acting as a subject in any
> > security checks; let alone close().
> Passed file descriptors are an anomaly in the security model
> that (in this developer's opinion) should have never been
> included. More than one of the "B" level UNIX systems disabled
> them outright.

Considering further on this, I think the only way to implement what you're
suggesting is to add a field to struct file to record the last fputter's creds
as the procedure of fputting is offloaded to a workqueue.

Note that's last fputter, not the last closer, as we don't track the number of
open fds linked to a file struct.

In the case of AF_UNIX sockets that contain in-the-process-of-being-passed fds
at the time of closure, this is further complicated by the socket fput being
achieved in the work item - thereby adding layers of indirection.

It might be possible to replace f_cred rather than adding a new field, but
that might get used somewhere after that point.

Note also that fsnotify_close() doesn't appear to use the last fputter's path
since it's not available if called from deferred fput.