[RFC PATCH v1 2/9] landlock: Support TCP listen access-control
Mickaël Salaün
mic at digikod.net
Wed Jul 31 18:30:01 UTC 2024
On Sun, Jul 28, 2024 at 08:25:55AM +0800, Mikhail Ivanov wrote:
> 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 at huawei-partners.com>
Thanks for this series!
I cannot apply this patch series though, could you please provide the
base commit? BTW, this can be automatically put in the cover letter
with the git format-patch's --base argument.
> ---
> include/uapi/linux/landlock.h | 23 +++--
> security/landlock/limits.h | 2 +-
> security/landlock/net.c | 90 ++++++++++++++++++++
> security/landlock/syscalls.c | 2 +-
> tools/testing/selftests/landlock/base_test.c | 2 +-
> 5 files changed, 108 insertions(+), 11 deletions(-)
>
> diff --git a/include/uapi/linux/landlock.h b/include/uapi/linux/landlock.h
> index 68625e728f43..6b8df3293eee 100644
> --- a/include/uapi/linux/landlock.h
> +++ b/include/uapi/linux/landlock.h
> @@ -104,13 +104,16 @@ 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 a specific port range. This can be
> - * configured thanks to the ``/proc/sys/net/ipv4/ip_local_port_range``
> - * sysctl (also used for IPv6). 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.
> + * It should be noted that some operations cause binding socket to a random
> + * available port from a specific port range. This can be configured thanks
> + * to the ``/proc/sys/net/ipv4/ip_local_port_range`` sysctl (also used for
> + * IPv6). Following operation requests are automatically translate to
> + * binding on the related port range:
> + *
> + * - A Landlock rule with port 0 and the ``LANDLOCK_ACCESS_NET_BIND_TCP``
> + * right means that binding on port 0 is allowed.
> + * - A Landlock rule with port 0 and the ``LANDLOCK_ACCESS_NET_LISTEN_TCP``
> + * right means listening without an explicit binding is allowed.
> */
> __u64 port;
> };
> @@ -251,7 +254,7 @@ 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.
> @@ -261,9 +264,13 @@ 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..a29cb27c3f14 100644
> --- a/security/landlock/net.c
> +++ b/security/landlock/net.c
> @@ -6,10 +6,12 @@
> * Copyright © 2022-2023 Microsoft Corporation
> */
>
> +#include "net/sock.h"
These should not be quotes.
> #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 +196,97 @@ static int hook_socket_connect(struct socket *const sock,
> LANDLOCK_ACCESS_NET_CONNECT_TCP);
> }
>
> +/*
> + * Checks that socket state and attributes are correct for listen.
> + * It is required to not wrongfully return -EACCES instead of -EINVAL.
> + *
> + * This checker requires sock->sk to be locked.
> + */
> +static int check_tcp_socket_can_listen(struct socket *const sock)
Is this function still useful with the listen LSM hook?
> +{
> + struct sock *sk = sock->sk;
> + unsigned char cur_sk_state = sk->sk_state;
> + const struct tcp_ulp_ops *icsk_ulp_ops;
> +
> + /* Allows only unconnected TCP socket to listen (cf. inet_listen). */
> + if (sock->state != SS_UNCONNECTED)
> + return -EINVAL;
> +
> + /*
> + * 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.
> + */
> + if (icsk_ulp_ops && !icsk_ulp_ops->clone)
> + return -EINVAL;
> + return 0;
> +}
> +
> +static int hook_socket_listen(struct socket *const sock, const int backlog)
> +{
Why can't we just call current_check_access_socket()?
> + 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;
> +
> + 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 03b470f5a85a..3752bcc033d4 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 3c1e9f35b531..52b00472a487 100644
> --- a/tools/testing/selftests/landlock/base_test.c
> +++ b/tools/testing/selftests/landlock/base_test.c
> @@ -75,7 +75,7 @@ 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
>
>
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