[RFC] install_session_keyring

[Suzanne Wood]

Hello,
In a study of the control flow graph dumps to check that 
an rcu_assign_pointer() with a given argument type has 
preceded a call to rcu_dereference(), I've come across 
install_session_keyring() of security/keys/process_keys.c.  
We note that although no rcu_read_lock() is in place 
locally or in the function's kernel callers, siglock 
likely addresses that.  While the rcu_dereference() would 
indicate a desire for 'old' to persist, synchronize_rcu() 
is called prior to key_put(old) which "disposes of 
reference to a key."  The order of events with a use of 
the copy of the pointer following synchronize_rcu() is 
what I question.

Thanks.
Suzanne

/******************************************************/
/*
 * install a session keyring, discarding the old one
 * - if a keyring is not supplied, an empty one is invented
 */
static int install_session_keyring(struct task_struct *tsk,
                                   struct key *keyring)
{
        unsigned long flags;
        struct key *old;
        char buf[20];
        int ret;

        /* create an empty session keyring */
        if (!keyring) {
                sprintf(buf, "_ses.%u", tsk->tgid);

                keyring = keyring_alloc(buf, tsk->uid, tsk->gid, 1, NULL);
                if (IS_ERR(keyring)) {
                        ret = PTR_ERR(keyring);
                        goto error;
                }
        }
        else {
                atomic_inc(&keyring->usage);
        }

        /* install the keyring */
        spin_lock_irqsave(&tsk->sighand->siglock, flags);
        old = rcu_dereference(tsk->signal->session_keyring);
        rcu_assign_pointer(tsk->signal->session_keyring, keyring);
        spin_unlock_irqrestore(&tsk->sighand->siglock, flags);

        ret = 0;

        /* we're using RCU on the pointer */
        synchronize_rcu();
        key_put(old);
error:
        return ret;

} /* end install_session_keyring() */

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