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

[Mickaël Salaün]

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.

> 
> > diff --git a/fs/fcntl.c b/fs/fcntl.c
> > index 300e5d9ad913..c28dc6c005f1 100644
> > --- a/fs/fcntl.c
> > +++ b/fs/fcntl.c
> > @@ -87,8 +87,8 @@ static int setfl(int fd, struct file * filp, unsigned int arg)
> >         return error;
> >  }
> >
> > -static void f_modown(struct file *filp, struct pid *pid, enum pid_type type,
> > -                     int force)
> > +void __f_setown(struct file *filp, struct pid *pid, enum pid_type type,
> > +               int force)
> >  {
> >         write_lock_irq(&filp->f_owner.lock);
> >         if (force || !filp->f_owner.pid) {
> > @@ -98,19 +98,13 @@ static void f_modown(struct file *filp, struct pid *pid, enum pid_type type,
> >
> >                 if (pid) {
> >                         const struct cred *cred = current_cred();
> > +                       security_file_set_fowner(filp);
> >                         filp->f_owner.uid = cred->uid;
> >                         filp->f_owner.euid = cred->euid;
> >                 }
> >         }
> >         write_unlock_irq(&filp->f_owner.lock);
> >  }
> > -
> > -void __f_setown(struct file *filp, struct pid *pid, enum pid_type type,
> > -               int force)
> > -{
> > -       security_file_set_fowner(filp);
> > -       f_modown(filp, pid, type, force);
> > -}
> >  EXPORT_SYMBOL(__f_setown);
> >
> >  int f_setown(struct file *filp, int who, int force)
> > @@ -146,7 +140,7 @@ EXPORT_SYMBOL(f_setown);
> >
> >  void f_delown(struct file *filp)
> >  {
> > -       f_modown(filp, NULL, PIDTYPE_TGID, 1);
> > +       __f_setown(filp, NULL, PIDTYPE_TGID, 1);
> >  }
> >
> >  pid_t f_getown(struct file *filp)
> > --
> > 2.46.0
> 
> -- 
> paul-moore.com
>