Re: [PATCH] sys_getppid: add missing rcu_dereference

[Kees Cook]

On Wed, Dec 7, 2011 at 4:52 PM, Paul E. McKenney
<paulmck@xxxxxxxxxxxxxxxxxx> wrote:
> On Wed, Dec 07, 2011 at 04:24:20PM -0800, Mandeep Singh Baines wrote:
>> Kees Cook (keescook@xxxxxxxxxxxx) wrote:
>> > On Tue, Dec 6, 2011 at 6:45 PM, Mandeep Singh Baines <msb@xxxxxxxxxxxx> 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@xxxxxxxxxxxx>
>> > > Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
>> > > Cc: Pavel Emelyanov <xemul@xxxxxxxxxx>
>> > > Cc: Oleg Nesterov <oleg@xxxxxxxxxx>
>> > > Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
>> > > Cc: Kees Cook <keescook@xxxxxxxxxxxx>
>> > > ---
>> > >  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.
>
> Indeed there is!  There is a CONFIG_SPARSE_RCU_POINTER kernel parameter
> that tells sparse to check for proper use of RCU-protected pointers.
> For this to work, the RCU-protected pointer in question must be marked
> with "__rcu".  Sparse will then complain if the pointer is accessed
> without using one of the rcu_dereference() family of functions.
>
>> 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));
>> }
>
> The above is useful for common code that might be called from both
> RCU readers (where rcu_read_lock() is in effect) and updaters
> (which hold tasklist_lock).  But although a task_real_parent()-style
> function can be extremely useful, it will not catch cases where
> rcu_dereference() was not used but should have been.  For that,
> as noted above, we have CONFIG_SPARSE_RCU_POINTER. __rcu, and sparse.
>
>                                                        Thanx, Paul
>
>> > 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.

I gave this a shot, and added __rcu to sched.h's real_parent
definition. Got warnings out of apparmor and tomoyo, but not
kernel/sched.c. Is there some reason sched_show_task doesn't need the
rcu_dereference() around its use of real_parent?

void sched_show_task(struct task_struct *p)
{
...
        printk(KERN_CONT "%5lu %5d %6d 0x%08lx\n", free,
                task_pid_nr(p), task_pid_nr(p->real_parent),
                (unsigned long)task_thread_info(p)->flags);
...
}

void show_state_filter(unsigned long state_filter)
{
...
        rcu_read_lock();
        do_each_thread(g, p) {
...
                if (!state_filter || (p->state & state_filter))
                        sched_show_task(p);
...
        } while_each_thread(g, p);
...
        rcu_read_unlock();
...
}

-- 
Kees Cook
ChromeOS Security
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