Re: user_subvol_rm_allowed vs rmdir_subvol

[Chris Murphy <lists@colorremedies.com>]

Super. Thanks for the reply!

On Tue, Sep 10, 2019 at 3:29 AM misono.tomohiro@fujitsu.com
<misono.tomohiro@fujitsu.com> wrote:
>
> Hello,
>
> (It seems that you already have answers but anyway...)
>
> > Came across this podman issue yesterday
> > https://github.com/containers/libpod/issues/3963
> >
> >
> > Question 1: For unprivileged use case, is it intentional that the user creates a subvolume/snapshot using 'btrfs sub
> > create' and that the user delete it with 'rm -rf' ?
>
> Yes. The problem with "btrfs sub delete" is that the permission check is only performed to the top directory(subvolume).
> Terefore unless user_subvol_rm_allowed mount option is used, "bttrfs sub delete" command is restricted for unprivileged user.
>
> >
> > And is the consequence of this performance? Because I see rm -rf must individually remove all files and dirs from the
> > subvolume first, before rmdir() is called to remove the subvolume. Where as 'btrfs sub del' calls BTRFS_IOC_SNAP_DESTROY
> > ioctl which is pretty much immediate, with cleanup happening in the background.
>
> Yes.
>
> >
> >
> > Question 2:
> >
> > As it relates to the podman issue, what do Btrfs developers recommend?
> > If kernel > 4.18, and if unprivileged, then use 'rm -rf' to delete subvolumes? Otherwise use 'btrfs sub del' with root
> > privilege?
>
> - "btrfs sub delete" if mounted with subvol_rm_allowed
> - "rm -r" if not mounted with subvol_rm_allowed
>
> > Question 3:
> > man 5 btrfs has a confusing note for user_subvol_rm_allowed mount option:
> >
> >                Note
> >                historically, any user could create a snapshot even if he was not owner of the source subvolume, the
> > subvolume deletion has been restricted
> >                for that reason. The subvolume creation has been restricted but this mount option is still required.
> > This is a usability issue.
> >
> > 2nd sentence "subvolume creation has been restricted"  I can't parse that. Is it an error, or can it be worded differently?
>
> You cannot create a snapshot of a subvolume which is owned by other user now (apparently old btrfs allowed this).
>
> In summary, subvolume deletion by unprivileged user is restricted by default because:
>  1. a user could create a snapshot which was not owned by the user in old btrfs.
>  2. BTRFS_IOC_SNAP_DESTROY ioctl only performs permission check to the top directory.
>
> I think 1 is not a problem anymore, byt 2 still remains.
>
> Thanks.



-- 
Chris Murphy