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

From: Jann Horn
Date: Wed May 29 2019 - 12:16:14 EST

On Wed, May 29, 2019 at 5:53 PM Casey Schaufler <casey@xxxxxxxxxxxxxxxx> wrote:
> On 5/29/2019 4:00 AM, David Howells wrote:
> > Jann Horn <jannh@xxxxxxxxxx> wrote:
> >
> >>> +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.

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(). If you send a file descriptor over
a unix domain socket and the unix domain socket is garbage collected,
for example, I think the close() will just come from some random,
completely unrelated task that happens to trigger the garbage

Also, I think if someone does I/O via io_uring, I think the caller's
credentials for read/write operations will probably just be normal
kernel creds?

Here the checks probably aren't all that important, but in other
places, when people try to use an LSM as the primary line of defense,
checks that don't align with the kernel's normal security model might
lead to a bunch of problems.