Re: [PATCH RFC 0/6 v2] block: Add config option to not allow writing to mounted devices

Mike Fleetwood
2023-07-05T12:27:39Z

On Tue, 4 Jul 2023 at 13:57, Jan Kara <jack@suse.cz> wrote:
>
> Hello!
>
> This is second version of the patches to add config option to not allow writing
> to mounted block devices. For motivation why this is interesting see patch 1/6.
> I've been testing the patches more extensively this time and I've found couple
> of things that get broken by disallowing writes to mounted block devices:
> 1) Bind mounts get broken because get_tree_bdev() / mount_bdev() first try to
> claim the bdev before searching whether it is already mounted. Patch 6
> reworks the mount code to avoid this problem.
> 2) btrfs mounting is likely having the same problem as 1). It should be fixable
> AFAICS but for now I've left it alone until we settle on the rest of the
> series.
> 3) "mount -o loop" gets broken because util-linux keeps the loop device open
> read-write when attempting to mount it. Hopefully fixable within util-linux.
> 4) resize2fs online resizing gets broken because it tries to open the block
> device read-write only to call resizing ioctl. Trivial to fix within
> e2fsprogs.
>
> Likely there will be other breakage I didn't find yet but overall the breakage
> looks minor enough that the option might be useful. Definitely good enough
> for syzbot fuzzing and likely good enough for hardening of systems with
> more tightened security.

5) Online e2label will break because it directly writes to the ext2/3/4
superblock while the FS is mounted to set the new label. Ext4 driver
will have to implement the SETFSLABEL ioctl() and e2label will have
to use it, matching what happens for online labelling of btrfs and
xfs.

Thanks,
Mike