LANDLOCK_ACCESS_NET_BIND_TCP is useful to limit the scope of "bindable" ports to forbid a malicious sandboxed process to impersonate a legitimate server process. However, bind(2) might be used by (TCP) clients to set the source port to a (legitimate) value. Controlling the ports that can be used for listening would allow (TCP) clients to explicitly bind to ports that are forbidden for listening. Such control is implemented with a new LANDLOCK_ACCESS_NET_LISTEN_TCP access right that restricts listening on undesired ports with listen(2). It's worth noticing that this access right doesn't affect changing backlog value using listen(2) on already listening socket. * Create new LANDLOCK_ACCESS_NET_LISTEN_TCP flag. * Add hook to socket_listen(), which checks whether the socket is allowed to listen on a binded local port. * Add check_tcp_socket_can_listen() helper, which validates socket attributes before the actual access right check. * Update `struct landlock_net_port_attr` documentation with control of binding to ephemeral port with listen(2) description. * Change ABI version to 6. Closes: https://github.com/landlock-lsm/linux/issues/15 Signed-off-by: Mikhail Ivanov <ivanov.mikhail1@xxxxxxxxxxxxxxxxxxx> --- Changes since v1: * Refactors 'struct landlock_net_port_attr' documentation. * Fixes check_tcp_socket_can_listen() description. * Adds lockdep_assert_held() into check_tcp_socket_can_listen(). * Minor fixes. --- include/uapi/linux/landlock.h | 26 ++++-- security/landlock/limits.h | 2 +- security/landlock/net.c | 98 ++++++++++++++++++++ security/landlock/syscalls.c | 2 +- tools/testing/selftests/landlock/base_test.c | 2 +- 5 files changed, 119 insertions(+), 11 deletions(-) diff --git a/include/uapi/linux/landlock.h b/include/uapi/linux/landlock.h index 2c8dbc74b955..f7dd6949c50b 100644 --- a/include/uapi/linux/landlock.h +++ b/include/uapi/linux/landlock.h @@ -111,14 +111,20 @@ struct landlock_net_port_attr { /** * @port: Network port in host endianness. * - * It should be noted that port 0 passed to :manpage:`bind(2)` will bind - * to an available port from the ephemeral port range. This can be - * configured with the ``/proc/sys/net/ipv4/ip_local_port_range`` sysctl - * (also used for IPv6). + * Some socket operations will fall back to using a port from the ephemeral port + * range, if no specific port is requested by the caller. Among others, this + * happens in the following cases: * - * A Landlock rule with port 0 and the ``LANDLOCK_ACCESS_NET_BIND_TCP`` - * right means that requesting to bind on port 0 is allowed and it will - * automatically translate to binding on the related port range. + * - :manpage:`bind(2)` is invoked with a socket address that uses port 0. + * - :manpage:`listen(2)` is invoked on a socket without previously calling + * :manpage:`bind(2)`. + * + * These two actions, which implicitly use an ephemeral port, can be allowed with + * a Landlock rule with port 0 and the ``LANDLOCK_ACCESS_NET_BIND_TCP`` / + * ``LANDLOCK_ACCESS_NET_LISTEN_TCP`` right. + * + * The ephemeral port range is configured in the + * ``/proc/sys/net/ipv4/ip_local_port_range`` sysctl (also used for IPv6). */ __u64 port; }; @@ -259,7 +265,7 @@ struct landlock_net_port_attr { * DOC: net_access * * Network flags - * ~~~~~~~~~~~~~~~~ + * ~~~~~~~~~~~~~ * * These flags enable to restrict a sandboxed process to a set of network * actions. This is supported since the Landlock ABI version 4. @@ -269,9 +275,13 @@ struct landlock_net_port_attr { * - %LANDLOCK_ACCESS_NET_BIND_TCP: Bind a TCP socket to a local port. * - %LANDLOCK_ACCESS_NET_CONNECT_TCP: Connect an active TCP socket to * a remote port. + * - %LANDLOCK_ACCESS_NET_LISTEN_TCP: Listen for TCP socket connections on + * a local port. This access right is available since the sixth version + * of the Landlock ABI. */ /* clang-format off */ #define LANDLOCK_ACCESS_NET_BIND_TCP (1ULL << 0) #define LANDLOCK_ACCESS_NET_CONNECT_TCP (1ULL << 1) +#define LANDLOCK_ACCESS_NET_LISTEN_TCP (1ULL << 2) /* clang-format on */ #endif /* _UAPI_LINUX_LANDLOCK_H */ diff --git a/security/landlock/limits.h b/security/landlock/limits.h index 4eb643077a2a..2ef147389474 100644 --- a/security/landlock/limits.h +++ b/security/landlock/limits.h @@ -22,7 +22,7 @@ #define LANDLOCK_MASK_ACCESS_FS ((LANDLOCK_LAST_ACCESS_FS << 1) - 1) #define LANDLOCK_NUM_ACCESS_FS __const_hweight64(LANDLOCK_MASK_ACCESS_FS) -#define LANDLOCK_LAST_ACCESS_NET LANDLOCK_ACCESS_NET_CONNECT_TCP +#define LANDLOCK_LAST_ACCESS_NET LANDLOCK_ACCESS_NET_LISTEN_TCP #define LANDLOCK_MASK_ACCESS_NET ((LANDLOCK_LAST_ACCESS_NET << 1) - 1) #define LANDLOCK_NUM_ACCESS_NET __const_hweight64(LANDLOCK_MASK_ACCESS_NET) diff --git a/security/landlock/net.c b/security/landlock/net.c index 669ba260342f..0e494b46d086 100644 --- a/security/landlock/net.c +++ b/security/landlock/net.c @@ -6,10 +6,12 @@ * Copyright © 2022-2023 Microsoft Corporation */ +#include <net/sock.h> #include <linux/in.h> #include <linux/net.h> #include <linux/socket.h> #include <net/ipv6.h> +#include <net/tcp.h> #include "common.h" #include "cred.h" @@ -194,9 +196,105 @@ static int hook_socket_connect(struct socket *const sock, LANDLOCK_ACCESS_NET_CONNECT_TCP); } +/* + * Checks that socket state and attributes are correct for listen. + * Returns 0 on success and -EINVAL otherwise. + * + * This checker requires sock->sk to be locked. + */ +static int check_tcp_socket_can_listen(struct socket *const sock) +{ + struct sock *sk = sock->sk; + unsigned char cur_sk_state; + const struct tcp_ulp_ops *icsk_ulp_ops; + + lockdep_assert_held(&sk->sk_lock.slock); + + /* Allows only unconnected TCP socket to listen (cf. inet_listen). */ + if (sock->state != SS_UNCONNECTED) + return -EINVAL; + + cur_sk_state = sk->sk_state; + /* + * Checks sock state. This is needed to ensure consistency with inet stack + * error handling (cf. __inet_listen_sk). + */ + if (WARN_ON_ONCE(!((1 << cur_sk_state) & (TCPF_CLOSE | TCPF_LISTEN)))) + return -EINVAL; + + icsk_ulp_ops = inet_csk(sk)->icsk_ulp_ops; + + /* + * ULP (Upper Layer Protocol) stands for protocols which are higher than + * transport protocol in OSI model. Linux has an infrastructure that + * allows TCP sockets to support logic of some ULP (e.g. TLS ULP). + * + * Sockets can listen only if ULP control hook has clone method + * (cf. inet_csk_listen_start) + */ + if (icsk_ulp_ops && !icsk_ulp_ops->clone) + return -EINVAL; + return 0; +} + +static int hook_socket_listen(struct socket *const sock, const int backlog) +{ + int err = 0; + int family; + __be16 port; + struct sock *sk; + const struct landlock_ruleset *const dom = get_current_net_domain(); + + if (!dom) + return 0; + if (WARN_ON_ONCE(dom->num_layers < 1)) + return -EACCES; + + /* Checks if it's a (potential) TCP socket. */ + if (sock->type != SOCK_STREAM) + return 0; + + sk = sock->sk; + family = sk->__sk_common.skc_family; + /* + * Socket cannot be assigned AF_UNSPEC because this type is used only + * in the context of addresses. + * + * Doesn't restrict listening for non-TCP sockets. + */ + if (family != AF_INET && family != AF_INET6) + return 0; + + lock_sock(sk); + /* + * Calling listen(2) for a listening socket does nothing with its state and + * only changes backlog value (cf. __inet_listen_sk). Checking of listen + * access right is not required. + */ + if (sk->sk_state == TCP_LISTEN) + goto release_nocheck; + + /* + * Checks socket state to not wrongfully return -EACCES instead + * of -EINVAL. + */ + err = check_tcp_socket_can_listen(sock); + if (unlikely(err)) + goto release_nocheck; + + port = htons(inet_sk(sk)->inet_num); + release_sock(sk); + return check_access_socket(dom, port, LANDLOCK_ACCESS_NET_LISTEN_TCP); + +release_nocheck: + release_sock(sk); + return err; +} + static struct security_hook_list landlock_hooks[] __ro_after_init = { LSM_HOOK_INIT(socket_bind, hook_socket_bind), LSM_HOOK_INIT(socket_connect, hook_socket_connect), + LSM_HOOK_INIT(socket_listen, hook_socket_listen), }; __init void landlock_add_net_hooks(void) diff --git a/security/landlock/syscalls.c b/security/landlock/syscalls.c index ccc8bc6c1584..328198e8a9f5 100644 --- a/security/landlock/syscalls.c +++ b/security/landlock/syscalls.c @@ -149,7 +149,7 @@ static const struct file_operations ruleset_fops = { .write = fop_dummy_write, }; -#define LANDLOCK_ABI_VERSION 5 +#define LANDLOCK_ABI_VERSION 6 /** * sys_landlock_create_ruleset - Create a new ruleset diff --git a/tools/testing/selftests/landlock/base_test.c b/tools/testing/selftests/landlock/base_test.c index 3b26bf3cf5b9..1bc16fde2e8a 100644 --- a/tools/testing/selftests/landlock/base_test.c +++ b/tools/testing/selftests/landlock/base_test.c @@ -76,7 +76,7 @@ TEST(abi_version) const struct landlock_ruleset_attr ruleset_attr = { .handled_access_fs = LANDLOCK_ACCESS_FS_READ_FILE, }; - ASSERT_EQ(5, landlock_create_ruleset(NULL, 0, + ASSERT_EQ(6, landlock_create_ruleset(NULL, 0, LANDLOCK_CREATE_RULESET_VERSION)); ASSERT_EQ(-1, landlock_create_ruleset(&ruleset_attr, 0, -- 2.34.1
