lsm sb_delete hook, was Re: [PATCH 4/7] vfs: Convert sb->s_inodes iteration to super_iter_inodes()

[Mickaël Salaün]

On Sat, Oct 05, 2024 at 08:57:32AM +1000, Dave Chinner wrote:
> On Fri, Oct 04, 2024 at 09:21:19AM +0200, Christian Brauner wrote:
> > On Fri, Oct 04, 2024 at 10:46:27AM GMT, Dave Chinner wrote:
> > > On Thu, Oct 03, 2024 at 06:17:31PM +0200, Jan Kara wrote:
> > > > On Thu 03-10-24 23:59:51, Dave Chinner wrote:
> > > > > As for the landlock code, I think it needs to have it's own internal
> > > > > tracking mechanism and not search the sb inode list for inodes that
> > > > > it holds references to. LSM cleanup should be run before before we
> > > > > get to tearing down the inode cache, not after....
> > > > 
> > > > Well, I think LSM cleanup could in principle be handled together with the
> > > > fsnotify cleanup but I didn't check the details.
> > > 
> > > I'm not sure how we tell if an inode potentially has a LSM related
> > > reference hanging off it. The landlock code looks to make an
> > > assumption in that the only referenced inodes it sees will have a
> > > valid inode->i_security pointer if landlock is enabled. i.e. it
> > > calls landlock_inode(inode) and dereferences the returned value
> > > without ever checking if inode->i_security is NULL or not.

Correct, i_security should always be valid when this hook is called
because it means that at least Landlock is enabled and then i_security
refers to a valid LSM blob.

> > > 
> > > I mean, we could do a check for inode->i_security when the refcount
> > > is elevated and replace the security_sb_delete hook with an
> > > security_evict_inode hook similar to the proposed fsnotify eviction
> > > from evict_inodes().

That would be nice.

> > > 
> > > But screwing with LSM instructure looks ....  obnoxiously complex
> > > from the outside...
> > 
> > Imho, please just focus on the immediate feedback and ignore all the
> > extra bells and whistles that we could or should do. I prefer all of
> > that to be done after this series lands.
> 
> Actually, it's not as bad as I thought it was going to be. I've
> already moved both fsnotify and LSM inode eviction to
> evict_inodes() as preparatory patches...

Good, please Cc me and Günther on related patch series.

FYI, we have the two release_inodes tests to check this hook in
tools/testing/selftests/landlock/fs_test.c

> 
> Dave Chinner (2):
>       vfs: move fsnotify inode eviction to evict_inodes()
>       vfs, lsm: rework lsm inode eviction at unmount
> 
>  fs/inode.c                    |  52 +++++++++++++---
>  fs/notify/fsnotify.c          |  60 -------------------
>  fs/super.c                    |   8 +--
>  include/linux/lsm_hook_defs.h |   2 +-
>  include/linux/security.h      |   2 +-
>  security/landlock/fs.c        | 134 ++++++++++--------------------------------
>  security/security.c           |  31 ++++++----
> 7 files changed, 99 insertions(+), 190 deletions(-)
> 
> -Dave.
> -- 
> Dave Chinner
> david at fromorbit.com
>