[RFC PATCH] fs: Add vfs_masks_device_ioctl*() helpers

[Dave Chinner]

On Fri, Mar 08, 2024 at 12:03:01PM +0100, Günther Noack wrote:
> On Fri, Mar 08, 2024 at 08:02:13AM +0100, Arnd Bergmann wrote:
> > On Fri, Mar 8, 2024, at 00:09, Dave Chinner wrote:
> > > On Thu, Mar 07, 2024 at 03:40:44PM -0500, Paul Moore wrote:
> > >> On Thu, Mar 7, 2024 at 7:57 AM Günther Noack <gnoack at google.com> wrote:
> > >> I need some more convincing as to why we need to introduce these new
> > >> hooks, or even the vfs_masked_device_ioctl() classifier as originally
> > >> proposed at the top of this thread.  I believe I understand why
> > >> Landlock wants this, but I worry that we all might have different
> > >> definitions of a "safe" ioctl list, and encoding a definition into the
> > >> LSM hooks seems like a bad idea to me.
> > >
> > > I have no idea what a "safe" ioctl means here. Subsystems already
> > > restrict ioctls that can do damage if misused to CAP_SYS_ADMIN, so
> > > "safe" clearly means something different here.
> > 
> > That was my problem with the first version as well, but I think
> > drawing the line between "implemented in fs/ioctl.c" and
> > "implemented in a random device driver fops->unlock_ioctl()"
> > seems like a more helpful definition.
> 
> Yes, sorry for the confusion - that is exactly what I meant to say with "safe".:
> 
> Those are the IOCTL commands implemented in fs/ioctl.c which do not go through
> f_ops->unlocked_ioctl (or the compat equivalent).

Which means all the ioctls we wrequire for to manage filesystems are
going to be considered "unsafe" and barred, yes?

That means you'll break basic commands like 'xfs_info' that tell you
the configuration of the filesystem. It will prevent things like
online growing and shrinking, online defrag, fstrim, online
scrubbing and repair, etc will not worki anymore. It will break
backup utilities like xfsdump, and break -all- the device management
of btrfs and bcachefs filesystems.

Further, all the setup and management of -VFS functionality- like
fsverity and fscrypt is actually done at the filesystem level (i.e
through ->unlocked_ioctl, no do_vfs_ioctl()) so those are all going
to get broken as well despite them being "vfs features".

Hence from a filesystem perspective, this is a fundamentally
unworkable definition of "safe".

> We want to give people a way with Landlock so that they can restrict the use of
> device-driver implemented IOCTLs, but where they can keep using the bulk of
> more harmless IOCTLs in fs/ioctl.c.

Hah! There's plenty of "harm" that can be done through those ioctls.
It's the entry point for things like filesystem freeze/thaw, FIEMAP
(returns physical data location information), file cloning,
deduplication and per-inode feature manipulation. Lots of this stuff
is under CAP_SYS_ADMIN because they aren't safe for to be exposed to
general users...

So, yeah, I don't think this definition of "safe" is actually useful
in any way. It's arbitrary, and will require both widespread
whitelisting of ioctls to maintain a useful working system and
widespread blacklisting to create a secure system....

-Dave.
-- 
Dave Chinner
david at fromorbit.com