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

From: Casey Schaufler
Date: Wed May 29 2019 - 11:57:23 EST

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.

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

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