[apparmor] [PATCH v2 1/7] lsm: Add granular mount hooks to replace security_sb_mount

[Song Liu]

On Fri, May 8, 2026 at 1:10 PM Paul Moore <paul at paul-moore.com> wrote:
>
> On Wed, Apr 29, 2026 at 8:03 PM Song Liu <song at kernel.org> wrote:
> >
> > Add six new LSM hooks for mount operations:
> >
> > - mount_bind(from, to, recurse): bind mount with pre-resolved
> >   struct path for source and destination.
> > - mount_new(fc, mp, mnt_flags, flags, data): new mount, called after
> >   mount options are parsed. The flags and data parameters carry the
> >   original mount(2) flags and data for LSMs that need them (AppArmor,
> >   Tomoyo).
> > - mount_remount(fc, mp, mnt_flags, flags, data): filesystem remount,
> >   called after mount options are parsed into the fs_context.
> > - mount_reconfigure(mp, mnt_flags, flags): mount flag reconfiguration
> >   (MS_REMOUNT|MS_BIND path).
> > - mount_move(from, to): move mount with pre-resolved paths.
> > - mount_change_type(mp, ms_flags): propagation type changes.
> >
> > These replace the monolithic security_sb_mount() which conflates
> > multiple distinct operations into a single hook, and suffers from
> > TOCTOU issues where LSMs re-resolve string-based dev_name via
> > kern_path().
> >
> > The mount_move hook is added alongside the existing move_mount hook.
> > During the transition, LSMs register for both hooks. The move_mount
> > hook will be removed once all LSMs have been converted.
> >
> > Some LSMs, such as apparmor and tomoyo, audit the original input passed
> > in the mount syscall. To keep the same behavior, argument data and flags
> > are passed in do_* functions. These can be removed if these LSMs no
> > longer need these information.
> >
> > All new hooks are registered as sleepable BPF LSM hooks.
> >
> > Code generated with the assistance of Claude, reviewed by human.
> >
> > Reviewed-by: Stephen Smalley <stephen.smalley.work at gmail.com>
> > Tested-by: Stephen Smalley <stephen.smalley.work at gmail.com> # for selinux only
> > Signed-off-by: Song Liu <song at kernel.org>
> > ---
> >  fs/namespace.c                |  35 ++++++++++--
> >  include/linux/lsm_hook_defs.h |  12 ++++
> >  include/linux/security.h      |  50 +++++++++++++++++
> >  kernel/bpf/bpf_lsm.c          |   7 +++
> >  security/security.c           | 101 ++++++++++++++++++++++++++++++++++
> >  5 files changed, 199 insertions(+), 6 deletions(-)
>
> ...
>
> > @@ -3708,6 +3724,10 @@ static int do_move_mount_old(const struct path *path, const char *old_name)
> >         if (err)
> >                 return err;
> >
> > +       err = security_mount_move(&old_path, path);
> > +       if (err)
> > +               return err;
> > +
> >         return do_move_mount(&old_path, path, 0);
> >  }
>
> While the security_sb_mount() hook calls into do_move_mount_old(), the
> security_move_mount() hook calls into do_mount_mount().  As you remove
> both of these LSM hooks in patch 7/7, should we consider moving the
> new security_mount_move() into do_move_mount()?  If not, how do we
> ensure that we don't lose coverage when removing the
> security_move_mount() hook, or can you explain why it is not needed?

Patch 7/7 _replaces_ security_move_mount() with security_mount_move()
in vfs_move_mount().  IOW, security_mount_move() is called from both
vfs_move_mount() and do_move_mount_old(), so we are not losing any
coverage. Did I miss something?

vfs_move_mount() has a special case (MNT_TREE_PROPAGATION).
If we move the hook to do_move_mount(), we are missing the coverage
for this case. Therefore, I think current code as-is is the best design at
this point.

Does this make sense?

Thanks,
Song