Commit message: KEYS: Fix RCU no-lock warning in keyctl_session_to_parent()

There's an protected access to the parent process's credentials in the middle
of keyctl_session_to_parent().  This results in the following RCU warning:

  ===================================================
  [ INFO: suspicious rcu_dereference_check() usage. ]
  ---------------------------------------------------
  security/keys/keyctl.c:1291 invoked rcu_dereference_check() without protection!

  other info that might help us debug this:

  rcu_scheduler_active = 1, debug_locks = 0
  1 lock held by keyctl-session-/2137:
   #0:  (tasklist_lock){.+.+..}, at: [<ffffffff811ae2ec>] keyctl_session_to_parent+0x60/0x236

  stack backtrace:
  Pid: 2137, comm: keyctl-session- Not tainted 2.6.36-rc2-cachefs+ #1
  Call Trace:
   [<ffffffff8105606a>] lockdep_rcu_dereference+0xaa/0xb3
   [<ffffffff811ae379>] keyctl_session_to_parent+0xed/0x236
   [<ffffffff811af77e>] sys_keyctl+0xb4/0xb6
   [<ffffffff81001eab>] system_call_fastpath+0x16/0x1b

The code should take the RCU read lock to make sure the parents credentials
don't go away, even though it's holding a spinlock and has IRQ disabled.

Signed-off-by: David Howells <dhowells@redhat.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
CVE: CVE-2010-2960
CWE: CWE-476
Vulnerability description: The keyctl_session_to_parent function in security/keys/keyctl.c in the Linux kernel 2.6.35.4 and earlier expects that a certain parent session keyring exists, which allows local users to cause a denial of service (NULL pointer dereference and system crash) or possibly have unspecified other impact via a KEYCTL_SESSION_TO_PARENT argument to the keyctl function.

Project: linux-2.6

long keyctl_session_to_parent(void)
{
#ifdef TIF_NOTIFY_RESUME
	struct task_struct *me, *parent;
	const struct cred *mycred, *pcred;
	struct cred *cred, *oldcred;
	key_ref_t keyring_r;
	int ret;

	keyring_r = lookup_user_key(KEY_SPEC_SESSION_KEYRING, 0, KEY_LINK);
	if (IS_ERR(keyring_r))
		return PTR_ERR(keyring_r);

	/* our parent is going to need a new cred struct, a new tgcred struct
	 * and new security data, so we allocate them here to prevent ENOMEM in
	 * our parent */
	ret = -ENOMEM;
	cred = cred_alloc_blank();
	if (!cred)
		goto error_keyring;

	cred->tgcred->session_keyring = key_ref_to_ptr(keyring_r);
	keyring_r = NULL;

	me = current;
	write_lock_irq(&tasklist_lock);

	parent = me->real_parent;
	ret = -EPERM;

	/* the parent mustn't be init and mustn't be a kernel thread */
	if (parent->pid <= 1 || !parent->mm)
		goto not_permitted;

	/* the parent must be single threaded */
	if (!thread_group_empty(parent))
		goto not_permitted;

	/* the parent and the child must have different session keyrings or
	 * there's no point */
	mycred = current_cred();
	pcred = __task_cred(parent);
	if (mycred == pcred ||
	    mycred->tgcred->session_keyring == pcred->tgcred->session_keyring)
		goto already_same;

	/* the parent must have the same effective ownership and mustn't be
	 * SUID/SGID */
	if (pcred->uid	!= mycred->euid	||
	    pcred->euid	!= mycred->euid	||
	    pcred->suid	!= mycred->euid	||
	    pcred->gid	!= mycred->egid	||
	    pcred->egid	!= mycred->egid	||
	    pcred->sgid	!= mycred->egid)
		goto not_permitted;

	/* the keyrings must have the same UID */
	if (pcred->tgcred->session_keyring->uid != mycred->euid ||
	    mycred->tgcred->session_keyring->uid != mycred->euid)
		goto not_permitted;

	/* if there's an already pending keyring replacement, then we replace
	 * that */
	oldcred = parent->replacement_session_keyring;

	/* the replacement session keyring is applied just prior to userspace
	 * restarting */
	parent->replacement_session_keyring = cred;
	cred = NULL;
	set_ti_thread_flag(task_thread_info(parent), TIF_NOTIFY_RESUME);

	write_unlock_irq(&tasklist_lock);
	if (oldcred)
		put_cred(oldcred);
	return 0;

already_same:
	ret = 0;
not_permitted:
	write_unlock_irq(&tasklist_lock);
	put_cred(cred);
	return ret;

error_keyring:
	key_ref_put(keyring_r);
	return ret;

#else /* !TIF_NOTIFY_RESUME */
	/*
	 * To be removed when TIF_NOTIFY_RESUME has been implemented on
	 * m68k/xtensa
	 */
#warning TIF_NOTIFY_RESUME not implemented
	return -EOPNOTSUPP;
#endif /* !TIF_NOTIFY_RESUME */
}