Re: subvolumes as partitions and mount options

[Graham Cobb <g.btrfs@cobb.uk.net>]

On 27/03/2023 20:50, Matt Zagrabelny wrote:
> On Mon, Mar 27, 2023 at 2:25 PM Andrei Borzenkov <arvidjaar@gmail.com> wrote:
>>
>> On 27.03.2023 21:48, Matt Zagrabelny wrote:
>>> Greetings,
>>>
>>> I have a root partition btrfs file system.
>>>
>>> I need to have /tmp, /var, /var/tmp, /var/log, and other directories
>>> under separate partitions so that certain mount options can be set for
>>> those partitions/directories.
>>>
>>> I'm testing out a subvolume mount with the subvolume /subv_content
>>> mounted at /subv_mnt.
>>>
>>> For instance, the noexec mount option can be circumvented:
>>
>> "exec/noexec" option applies to mount instance, it is not persistent
>> property of underlying filesystem. It is not specific to btrfs at all.
> 
> Agreed. My email was more about subvolumes and the mount point has the
> "noexec", but the actual subvolume doesn't - so there exists a path on
> disk where folks can exec the same file by circumventing the mount
> option by directly invoking the full path under the subvolume.

So, create the subvolume inside a non-world-readable directory? In fact,
I always create all the subvolumes inside top level (subvolid=5)
subvolume but that subvolume is not normally mounted. /, /tmp, /var, etc
are all subvolumes and subvolid=5 is not mounted at all (or can be
mounted with a mount point somewhere not world accessible).

Don't make the mistake of thinking that subvolumes have to be visible
anywhere in the filesystem except the place you mount them.

Graham