Re: [PATCH] sys_getppid: add missing rcu_dereference

[Mandeep Singh Baines <msb@chromium.org>]

Kees Cook (keescook@chromium.org) wrote:
> On Tue, Dec 6, 2011 at 6:45 PM, Mandeep Singh Baines <msb@chromium.org> wrote:
> > In order to safely dereference current->real_parent inside an
> > rcu_read_lock, we need an rcu_dereference.
> >
> > Signed-off-by: Mandeep Singh Baines <msb@chromium.org>
> > Cc: Thomas Gleixner <tglx@linutronix.de>
> > Cc: Pavel Emelyanov <xemul@openvz.org>
> > Cc: Oleg Nesterov <oleg@redhat.com>
> > Cc: Andrew Morton <akpm@linux-foundation.org>
> > Cc: Kees Cook <keescook@chromium.org>
> > ---
> >  kernel/timer.c |    2 +-
> >  1 files changed, 1 insertions(+), 1 deletions(-)
> >
> > diff --git a/kernel/timer.c b/kernel/timer.c
> > index dbaa624..9c3c62b 100644
> > --- a/kernel/timer.c
> > +++ b/kernel/timer.c
> > @@ -1368,7 +1368,7 @@ SYSCALL_DEFINE0(getppid)
> >        int pid;
> >
> >        rcu_read_lock();
> > -       pid = task_tgid_vnr(current->real_parent);
> > +       pid = task_tgid_vnr(rcu_dereference(current->real_parent));
> >        rcu_read_unlock();
> >
> >        return pid;
> 
> Should parent and real_parent also be marked in sched.h with __rcu so
> sparse can find other missing rcu_dereference()s? And if not, why?
> (tasklist lock?)
> 

Good idea. I was thinking the same thing. There's gotta be some way of
finding these bugs via lockdep or sparse.

One idea I had was to create a ->real_parent accessor and insert an
rcu_dereference_check there. That way lockdep could catch these bugs.

static inline struct task_struct *task_real_parent(struct task_struct *task)
{
        return rcu_dereference_check(task->real_parent,
			             lockdep_is_held(&tasklist_lock));
}


> I think I see at least are few other users (security/apparmor/audit.c,
> security/tomoyo/common.h, kernel/sched.c) that need rcu_dereference()
> when accessing real_parent, there are probably more.
> 
> -Kees
> 
> -- 
> Kees Cook
> ChromeOS Security