keys: fix race with concurrent install_user_keyrings()

This fixes CVE-2013-1792.

There is a race in install_user_keyrings() that can cause a NULL pointer
dereference when called concurrently for the same user if the uid and
uid-session keyrings are not yet created.  It might be possible for an
unprivileged user to trigger this by calling keyctl() from userspace in
parallel immediately after logging in.

Assume that we have two threads both executing lookup_user_key(), both
looking for KEY_SPEC_USER_SESSION_KEYRING.

	THREAD A			THREAD B
	===============================	===============================
					==>call install_user_keyrings();
	if (!cred->user->session_keyring)
	==>call install_user_keyrings()
					...
					user->uid_keyring = uid_keyring;
	if (user->uid_keyring)
		return 0;
	<==
	key = cred->user->session_keyring [== NULL]
					user->session_keyring = session_keyring;
	atomic_inc(&key->usage); [oops]

At the point thread A dereferences cred->user->session_keyring, thread B
hasn't updated user->session_keyring yet, but thread A assumes it is
populated because install_user_keyrings() returned ok.

The race window is really small but can be exploited if, for example,
thread B is interrupted or preempted after initializing uid_keyring, but
before doing setting session_keyring.

This couldn't be reproduced on a stock kernel.  However, after placing
systemtap probe on 'user->session_keyring = session_keyring;' that
introduced some delay, the kernel could be crashed reliably.

Fix this by checking both pointers before deciding whether to return.
Alternatively, the test could be done away with entirely as it is checked
inside the mutex - but since the mutex is global, that may not be the best
way.

Signed-off-by: David Howells <dhowells@redhat.com>
Reported-by: Mateusz Guzik <mguzik@redhat.com>
Cc: <stable@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: James Morris <james.l.morris@oracle.com>
key_ref_t lookup_user_key(key_serial_t id, unsigned long lflags,
			  key_perm_t perm)
{
	struct request_key_auth *rka;
	const struct cred *cred;
	struct key *key;
	key_ref_t key_ref, skey_ref;
	int ret;

try_again:
	cred = get_current_cred();
	key_ref = ERR_PTR(-ENOKEY);

	switch (id) {
	case KEY_SPEC_THREAD_KEYRING:
		if (!cred->thread_keyring) {
			if (!(lflags & KEY_LOOKUP_CREATE))
				goto error;

			ret = install_thread_keyring();
			if (ret < 0) {
				key_ref = ERR_PTR(ret);
				goto error;
			}
			goto reget_creds;
		}

		key = cred->thread_keyring;
		atomic_inc(&key->usage);
		key_ref = make_key_ref(key, 1);
		break;

	case KEY_SPEC_PROCESS_KEYRING:
		if (!cred->process_keyring) {
			if (!(lflags & KEY_LOOKUP_CREATE))
				goto error;

			ret = install_process_keyring();
			if (ret < 0) {
				key_ref = ERR_PTR(ret);
				goto error;
			}
			goto reget_creds;
		}

		key = cred->process_keyring;
		atomic_inc(&key->usage);
		key_ref = make_key_ref(key, 1);
		break;

	case KEY_SPEC_SESSION_KEYRING:
		if (!cred->session_keyring) {
			/* always install a session keyring upon access if one
			 * doesn't exist yet */
			ret = install_user_keyrings();
			if (ret < 0)
				goto error;
			if (lflags & KEY_LOOKUP_CREATE)
				ret = join_session_keyring(NULL);
			else
				ret = install_session_keyring(
					cred->user->session_keyring);

			if (ret < 0)
				goto error;
			goto reget_creds;
		} else if (cred->session_keyring ==
			   cred->user->session_keyring &&
			   lflags & KEY_LOOKUP_CREATE) {
			ret = join_session_keyring(NULL);
			if (ret < 0)
				goto error;
			goto reget_creds;
		}

		rcu_read_lock();
		key = rcu_dereference(cred->session_keyring);
		atomic_inc(&key->usage);
		rcu_read_unlock();
		key_ref = make_key_ref(key, 1);
		break;

	case KEY_SPEC_USER_KEYRING:
		if (!cred->user->uid_keyring) {
			ret = install_user_keyrings();
			if (ret < 0)
				goto error;
		}

		key = cred->user->uid_keyring;
		atomic_inc(&key->usage);
		key_ref = make_key_ref(key, 1);
		break;

	case KEY_SPEC_USER_SESSION_KEYRING:
		if (!cred->user->session_keyring) {
			ret = install_user_keyrings();
			if (ret < 0)
				goto error;
		}

		key = cred->user->session_keyring;
		atomic_inc(&key->usage);
		key_ref = make_key_ref(key, 1);
		break;

	case KEY_SPEC_GROUP_KEYRING:
		/* group keyrings are not yet supported */
		key_ref = ERR_PTR(-EINVAL);
		goto error;

	case KEY_SPEC_REQKEY_AUTH_KEY:
		key = cred->request_key_auth;
		if (!key)
			goto error;

		atomic_inc(&key->usage);
		key_ref = make_key_ref(key, 1);
		break;

	case KEY_SPEC_REQUESTOR_KEYRING:
		if (!cred->request_key_auth)
			goto error;

		down_read(&cred->request_key_auth->sem);
		if (test_bit(KEY_FLAG_REVOKED,
			     &cred->request_key_auth->flags)) {
			key_ref = ERR_PTR(-EKEYREVOKED);
			key = NULL;
		} else {
			rka = cred->request_key_auth->payload.data;
			key = rka->dest_keyring;
			atomic_inc(&key->usage);
		}
		up_read(&cred->request_key_auth->sem);
		if (!key)
			goto error;
		key_ref = make_key_ref(key, 1);
		break;

	default:
		key_ref = ERR_PTR(-EINVAL);
		if (id < 1)
			goto error;

		key = key_lookup(id);
		if (IS_ERR(key)) {
			key_ref = ERR_CAST(key);
			goto error;
		}

		key_ref = make_key_ref(key, 0);

		/* check to see if we possess the key */
		skey_ref = search_process_keyrings(key->type, key,
						   lookup_user_key_possessed,
						   cred);

		if (!IS_ERR(skey_ref)) {
			key_put(key);
			key_ref = skey_ref;
		}

		break;
	}

	/* unlink does not use the nominated key in any way, so can skip all
	 * the permission checks as it is only concerned with the keyring */
	if (lflags & KEY_LOOKUP_FOR_UNLINK) {
		ret = 0;
		goto error;
	}

	if (!(lflags & KEY_LOOKUP_PARTIAL)) {
		ret = wait_for_key_construction(key, true);
		switch (ret) {
		case -ERESTARTSYS:
			goto invalid_key;
		default:
			if (perm)
				goto invalid_key;
		case 0:
			break;
		}
	} else if (perm) {
		ret = key_validate(key);
		if (ret < 0)
			goto invalid_key;
	}

	ret = -EIO;
	if (!(lflags & KEY_LOOKUP_PARTIAL) &&
	    !test_bit(KEY_FLAG_INSTANTIATED, &key->flags))
		goto invalid_key;

	/* check the permissions */
	ret = key_task_permission(key_ref, cred, perm);
	if (ret < 0)
		goto invalid_key;

	key->last_used_at = current_kernel_time().tv_sec;

error:
	put_cred(cred);
	return key_ref;

invalid_key:
	key_ref_put(key_ref);
	key_ref = ERR_PTR(ret);
	goto error;

	/* if we attempted to install a keyring, then it may have caused new
	 * creds to be installed */
reget_creds:
	put_cred(cred);
	goto try_again;
}

key_ref_t lookup_user_key(key_serial_t id, unsigned long lflags,
			  key_perm_t perm)
{
	struct request_key_auth *rka;
	const struct cred *cred;
	struct key *key;
	key_ref_t key_ref, skey_ref;
	int ret;

try_again:
	cred = get_current_cred();
	key_ref = ERR_PTR(-ENOKEY);

	switch (id) {
	case KEY_SPEC_THREAD_KEYRING:
		if (!cred->thread_keyring) {
			if (!(lflags & KEY_LOOKUP_CREATE))
				goto error;

			ret = install_thread_keyring();
			if (ret < 0) {
				key_ref = ERR_PTR(ret);
				goto error;
			}
			goto reget_creds;
		}

		key = cred->thread_keyring;
		atomic_inc(&key->usage);
		key_ref = make_key_ref(key, 1);
		break;

	case KEY_SPEC_PROCESS_KEYRING:
		if (!cred->process_keyring) {
			if (!(lflags & KEY_LOOKUP_CREATE))
				goto error;

			ret = install_process_keyring();
			if (ret < 0) {
				key_ref = ERR_PTR(ret);
				goto error;
			}
			goto reget_creds;
		}

		key = cred->process_keyring;
		atomic_inc(&key->usage);
		key_ref = make_key_ref(key, 1);
		break;

	case KEY_SPEC_SESSION_KEYRING:
		if (!cred->session_keyring) {
			/* always install a session keyring upon access if one
			 * doesn't exist yet */
			ret = install_user_keyrings();
			if (ret < 0)
				goto error;
			if (lflags & KEY_LOOKUP_CREATE)
				ret = join_session_keyring(NULL);
			else
				ret = install_session_keyring(
					cred->user->session_keyring);

			if (ret < 0)
				goto error;
			goto reget_creds;
		} else if (cred->session_keyring ==
			   cred->user->session_keyring &&
			   lflags & KEY_LOOKUP_CREATE) {
			ret = join_session_keyring(NULL);
			if (ret < 0)
				goto error;
			goto reget_creds;
		}

		rcu_read_lock();
		key = rcu_dereference(cred->session_keyring);
		atomic_inc(&key->usage);
		rcu_read_unlock();
		key_ref = make_key_ref(key, 1);
		break;

	case KEY_SPEC_USER_KEYRING:
		if (!cred->user->uid_keyring) {
			ret = install_user_keyrings();
			if (ret < 0)
				goto error;
		}

		key = cred->user->uid_keyring;
		atomic_inc(&key->usage);
		key_ref = make_key_ref(key, 1);
		break;

	case KEY_SPEC_USER_SESSION_KEYRING:
		if (!cred->user->session_keyring) {
			ret = install_user_keyrings();
			if (ret < 0)
				goto error;
		}

		key = cred->user->session_keyring;
		atomic_inc(&key->usage);
		key_ref = make_key_ref(key, 1);
		break;

	case KEY_SPEC_GROUP_KEYRING:
		/* group keyrings are not yet supported */
		key_ref = ERR_PTR(-EINVAL);
		goto error;

	case KEY_SPEC_REQKEY_AUTH_KEY:
		key = cred->request_key_auth;
		if (!key)
			goto error;

		atomic_inc(&key->usage);
		key_ref = make_key_ref(key, 1);
		break;

	case KEY_SPEC_REQUESTOR_KEYRING:
		if (!cred->request_key_auth)
			goto error;

		down_read(&cred->request_key_auth->sem);
		if (test_bit(KEY_FLAG_REVOKED,
			     &cred->request_key_auth->flags)) {
			key_ref = ERR_PTR(-EKEYREVOKED);
			key = NULL;
		} else {
			rka = cred->request_key_auth->payload.data;
			key = rka->dest_keyring;
			atomic_inc(&key->usage);
		}
		up_read(&cred->request_key_auth->sem);
		if (!key)
			goto error;
		key_ref = make_key_ref(key, 1);
		break;

	default:
		key_ref = ERR_PTR(-EINVAL);
		if (id < 1)
			goto error;

		key = key_lookup(id);
		if (IS_ERR(key)) {
			key_ref = ERR_CAST(key);
			goto error;
		}

		key_ref = make_key_ref(key, 0);

		/* check to see if we possess the key */
		skey_ref = search_process_keyrings(key->type, key,
						   lookup_user_key_possessed,
						   cred);

		if (!IS_ERR(skey_ref)) {
			key_put(key);
			key_ref = skey_ref;
		}

		break;
	}

	/* unlink does not use the nominated key in any way, so can skip all
	 * the permission checks as it is only concerned with the keyring */
	if (lflags & KEY_LOOKUP_FOR_UNLINK) {
		ret = 0;
		goto error;
	}

	if (!(lflags & KEY_LOOKUP_PARTIAL)) {
		ret = wait_for_key_construction(key, true);
		switch (ret) {
		case -ERESTARTSYS:
			goto invalid_key;
		default:
			if (perm)
				goto invalid_key;
		case 0:
			break;
		}
	} else if (perm) {
		ret = key_validate(key);
		if (ret < 0)
			goto invalid_key;
	}

	ret = -EIO;
	if (!(lflags & KEY_LOOKUP_PARTIAL) &&
	    !test_bit(KEY_FLAG_INSTANTIATED, &key->flags))
		goto invalid_key;

	/* check the permissions */
	ret = key_task_permission(key_ref, cred, perm);
	if (ret < 0)
		goto invalid_key;

	key->last_used_at = current_kernel_time().tv_sec;

error:
	put_cred(cred);
	return key_ref;

invalid_key:
	key_ref_put(key_ref);
	key_ref = ERR_PTR(ret);
	goto error;

	/* if we attempted to install a keyring, then it may have caused new
	 * creds to be installed */
reget_creds:
	put_cred(cred);
	goto try_again;
}
