[PATCH v3 1/2] fs: Fix file_set_fowner LSM hook inconsistencies

[Paul Moore]

On Sun, Sep 8, 2024 at 2:11 PM Mickaël Salaün <mic at digikod.net> wrote:
>
> On Wed, Aug 21, 2024 at 12:32:17PM -0400, Paul Moore wrote:
> > On Wed, Aug 21, 2024 at 5:56 AM Mickaël Salaün <mic at digikod.net> wrote:
> > >
> > > The fcntl's F_SETOWN command sets the process that handle SIGIO/SIGURG
> > > for the related file descriptor.  Before this change, the
> > > file_set_fowner LSM hook was always called, ignoring the VFS logic which
> > > may not actually change the process that handles SIGIO (e.g. TUN, TTY,
> > > dnotify), nor update the related UID/EUID.
> > >
> > > Moreover, because security_file_set_fowner() was called without lock
> > > (e.g. f_owner.lock), concurrent F_SETOWN commands could result to a race
> > > condition and inconsistent LSM states (e.g. SELinux's fown_sid) compared
> > > to struct fown_struct's UID/EUID.
> > >
> > > This change makes sure the LSM states are always in sync with the VFS
> > > state by moving the security_file_set_fowner() call close to the
> > > UID/EUID updates and using the same f_owner.lock .
> > >
> > > Rename f_modown() to __f_setown() to simplify code.
> > >
> > > Cc: Al Viro <viro at zeniv.linux.org.uk>
> > > Cc: Casey Schaufler <casey at schaufler-ca.com>
> > > Cc: Christian Brauner <brauner at kernel.org>
> > > Cc: James Morris <jmorris at namei.org>
> > > Cc: Jann Horn <jannh at google.com>
> > > Cc: Ondrej Mosnacek <omosnace at redhat.com>
> > > Cc: Paul Moore <paul at paul-moore.com>
> > > Cc: Serge E. Hallyn <serge at hallyn.com>
> > > Cc: Stephen Smalley <stephen.smalley.work at gmail.com>
> > > Fixes: 1da177e4c3f4 ("Linux-2.6.12-rc2")
> > > Signed-off-by: Mickaël Salaün <mic at digikod.net>
> > > ---
> > >
> > > Changes since v2:
> > > https://lore.kernel.org/r/20240812174421.1636724-1-mic@digikod.net
> > > - Only keep the LSM hook move.
> > >
> > > Changes since v1:
> > > https://lore.kernel.org/r/20240812144936.1616628-1-mic@digikod.net
> > > - Add back the file_set_fowner hook (but without user) as
> > >   requested by Paul, but move it for consistency.
> > > ---
> > >  fs/fcntl.c | 14 ++++----------
> > >  1 file changed, 4 insertions(+), 10 deletions(-)
> >
> > This looks reasonable to me, and fixes a potential problem with
> > existing LSMs.  Unless I hear any strong objections I'll plan to merge
> > this, and patch 2/2, into the LSM tree tomorrow.
>
> I didn't see these patches in -next, did I miss something?
> Landlock will use this hook really soon and it would make it much easier
> if these patches where upstream before.

Ah!  My apologies, I'll do that right now and send another update once
it's done.  FWIW, I'm going to tag 1/2 for stable, but since we are at
-rc7 presently I'll just plan to send it during the next merge window.

-- 
paul-moore.com