[PATCH v8 1/4] Landlock: Add abstract unix socket connect restriction
Mickaël Salaün
mic at digikod.net
Fri Aug 2 16:47:04 UTC 2024
On Thu, Aug 01, 2024 at 10:02:33PM -0600, Tahera Fahimi wrote:
> This patch introduces a new "scoped" attribute to the landlock_ruleset_attr
> that can specify "LANDLOCK_SCOPED_ABSTRACT_UNIX_SOCKET" to scope
> abstract Unix sockets from connecting to a process outside of
> the same landlock domain. It implements two hooks, unix_stream_connect
> and unix_may_send to enforce this restriction.
>
> Closes: https://github.com/landlock-lsm/linux/issues/7
> Signed-off-by: Tahera Fahimi <fahimitahera at gmail.com>
>
> ---
> v8:
> - Code refactoring (improve code readability, renaming variable, etc.) based
> on reviews by Mickaël Salaün on version 7.
> - Adding warn_on_once to check (impossible) inconsistencies.
> - Adding inline comments.
> - Adding check_unix_address_format to check if the scoping socket is an abstract
> unix sockets.
> v7:
> - Using socket's file credentials for both connected(STREAM) and
> non-connected(DGRAM) sockets.
> - Adding "domain_sock_scope" instead of the domain scoping mechanism used in
> ptrace ensures that if a server's domain is accessible from the client's
> domain (where the client is more privileged than the server), the client
> can connect to the server in all edge cases.
> - Removing debug codes.
> v6:
> - Removing curr_ruleset from landlock_hierarchy, and switching back to use
> the same domain scoping as ptrace.
> - code clean up.
> v5:
> - Renaming "LANDLOCK_*_ACCESS_SCOPE" to "LANDLOCK_*_SCOPE"
> - Adding curr_ruleset to hierarachy_ruleset structure to have access from
> landlock_hierarchy to its respective landlock_ruleset.
> - Using curr_ruleset to check if a domain is scoped while walking in the
> hierarchy of domains.
> - Modifying inline comments.
> V4:
> - Rebased on Günther's Patch:
> https://lore.kernel.org/all/20240610082115.1693267-1-gnoack@google.com/
> so there is no need for "LANDLOCK_SHIFT_ACCESS_SCOPE", then it is removed.
> - Adding get_scope_accesses function to check all scoped access masks in a ruleset.
> - Using socket's file credentials instead of credentials stored in peer_cred
> for datagram sockets. (see discussion in [1])
> - Modifying inline comments.
> V3:
> - Improving commit description.
> - Introducing "scoped" attribute to landlock_ruleset_attr for IPC scoping
> purpose, and adding related functions.
> - Changing structure of ruleset based on "scoped".
> - Removing rcu lock and using unix_sk lock instead.
> - Introducing scoping for datagram sockets in unix_may_send.
> V2:
> - Removing wrapper functions
>
> [1]https://lore.kernel.org/all/20240610.Aifee5ingugh@digikod.net/
> ----
> ---
> include/uapi/linux/landlock.h | 30 +++++++
> security/landlock/limits.h | 3 +
> security/landlock/ruleset.c | 7 +-
> security/landlock/ruleset.h | 23 ++++-
> security/landlock/syscalls.c | 14 ++-
> security/landlock/task.c | 155 ++++++++++++++++++++++++++++++++++
> 6 files changed, 225 insertions(+), 7 deletions(-)
>
> diff --git a/include/uapi/linux/landlock.h b/include/uapi/linux/landlock.h
> index 68625e728f43..ab31e9f53e55 100644
> --- a/include/uapi/linux/landlock.h
> +++ b/include/uapi/linux/landlock.h
> @@ -37,6 +37,12 @@ struct landlock_ruleset_attr {
> * rule explicitly allow them.
> */
> __u64 handled_access_net;
> + /**
> + * @scoped: Bitmask of scopes (cf. `Scope flags`_)
> + * restricting a Landlock domain from accessing outside
> + * resources(e.g. IPCs).
> + */
> + __u64 scoped;
> };
>
> /*
> @@ -266,4 +272,28 @@ struct landlock_net_port_attr {
> #define LANDLOCK_ACCESS_NET_BIND_TCP (1ULL << 0)
> #define LANDLOCK_ACCESS_NET_CONNECT_TCP (1ULL << 1)
> /* clang-format on */
> +
> +/**
> + * DOC: scope
> + *
> + * scoped attribute handles a set of restrictions on kernel IPCs through
> + * the following flags.
If present, this previous sentence should be after the "Scope flags"
section, otherwise it shows in the previous section. I don't think this
sentence is useful because of the next one, so you can remove it.
> + *
> + * Scope flags
> + * ~~~~~~~~~~~
> + *
> + * These flags enable to restrict a sandboxed process from a set of IPC
> + * actions. Setting a flag for a ruleset will isolate the Landlock domain
> + * to forbid connections to resources outside the domain.
> + *
> + * IPCs with scoped actions:
> + *
> + * - %LANDLOCK_SCOPED_ABSTRACT_UNIX_SOCKET: Restrict a sandboxed process
> + * from connecting to an abstract unix socket created by a process
> + * outside the related Landlock domain (e.g. a parent domain or a
> + * non-sandboxed process).
> + */
> +/* clang-format off */
> +#define LANDLOCK_SCOPED_ABSTRACT_UNIX_SOCKET (1ULL << 0)
> +/* clang-format on*/
> #endif /* _UAPI_LINUX_LANDLOCK_H */
> diff --git a/security/landlock/limits.h b/security/landlock/limits.h
> index 4eb643077a2a..eb01d0fb2165 100644
> --- a/security/landlock/limits.h
> +++ b/security/landlock/limits.h
> @@ -26,6 +26,9 @@
> #define LANDLOCK_MASK_ACCESS_NET ((LANDLOCK_LAST_ACCESS_NET << 1) - 1)
> #define LANDLOCK_NUM_ACCESS_NET __const_hweight64(LANDLOCK_MASK_ACCESS_NET)
>
> +#define LANDLOCK_LAST_SCOPE LANDLOCK_SCOPED_ABSTRACT_UNIX_SOCKET
> +#define LANDLOCK_MASK_SCOPE ((LANDLOCK_LAST_SCOPE << 1) - 1)
> +#define LANDLOCK_NUM_SCOPE __const_hweight64(LANDLOCK_MASK_SCOPE)
> /* clang-format on */
>
> #endif /* _SECURITY_LANDLOCK_LIMITS_H */
> diff --git a/security/landlock/ruleset.c b/security/landlock/ruleset.c
> index 6ff232f58618..a93bdbf52fff 100644
> --- a/security/landlock/ruleset.c
> +++ b/security/landlock/ruleset.c
> @@ -52,12 +52,13 @@ static struct landlock_ruleset *create_ruleset(const u32 num_layers)
>
> struct landlock_ruleset *
> landlock_create_ruleset(const access_mask_t fs_access_mask,
> - const access_mask_t net_access_mask)
> + const access_mask_t net_access_mask,
> + const access_mask_t scope_mask)
> {
> struct landlock_ruleset *new_ruleset;
>
> /* Informs about useless ruleset. */
> - if (!fs_access_mask && !net_access_mask)
> + if (!fs_access_mask && !net_access_mask && !scope_mask)
> return ERR_PTR(-ENOMSG);
> new_ruleset = create_ruleset(1);
> if (IS_ERR(new_ruleset))
> @@ -66,6 +67,8 @@ landlock_create_ruleset(const access_mask_t fs_access_mask,
> landlock_add_fs_access_mask(new_ruleset, fs_access_mask, 0);
> if (net_access_mask)
> landlock_add_net_access_mask(new_ruleset, net_access_mask, 0);
> + if (scope_mask)
> + landlock_add_scope_mask(new_ruleset, scope_mask, 0);
> return new_ruleset;
> }
>
> diff --git a/security/landlock/ruleset.h b/security/landlock/ruleset.h
> index 0f1b5b4c8f6b..c749fa0b3ecd 100644
> --- a/security/landlock/ruleset.h
> +++ b/security/landlock/ruleset.h
> @@ -35,6 +35,8 @@ typedef u16 access_mask_t;
> static_assert(BITS_PER_TYPE(access_mask_t) >= LANDLOCK_NUM_ACCESS_FS);
> /* Makes sure all network access rights can be stored. */
> static_assert(BITS_PER_TYPE(access_mask_t) >= LANDLOCK_NUM_ACCESS_NET);
> +/* Makes sure all scoped rights can be stored*/
> +static_assert(BITS_PER_TYPE(access_mask_t) >= LANDLOCK_NUM_SCOPE);
> /* Makes sure for_each_set_bit() and for_each_clear_bit() calls are OK. */
> static_assert(sizeof(unsigned long) >= sizeof(access_mask_t));
>
> @@ -42,6 +44,7 @@ static_assert(sizeof(unsigned long) >= sizeof(access_mask_t));
> struct access_masks {
> access_mask_t fs : LANDLOCK_NUM_ACCESS_FS;
> access_mask_t net : LANDLOCK_NUM_ACCESS_NET;
> + access_mask_t scoped : LANDLOCK_NUM_SCOPE;
> };
>
> typedef u16 layer_mask_t;
> @@ -233,7 +236,8 @@ struct landlock_ruleset {
>
> struct landlock_ruleset *
> landlock_create_ruleset(const access_mask_t access_mask_fs,
> - const access_mask_t access_mask_net);
> + const access_mask_t access_mask_net,
> + const access_mask_t scope_mask);
>
> void landlock_put_ruleset(struct landlock_ruleset *const ruleset);
> void landlock_put_ruleset_deferred(struct landlock_ruleset *const ruleset);
> @@ -280,6 +284,16 @@ landlock_add_net_access_mask(struct landlock_ruleset *const ruleset,
> ruleset->access_masks[layer_level].net |= net_mask;
> }
>
> +static inline void
> +landlock_add_scope_mask(struct landlock_ruleset *const ruleset,
> + const access_mask_t scope_mask, const u16 layer_level)
> +{
> + access_mask_t scoped_mask = scope_mask & LANDLOCK_MASK_SCOPE;
> +
> + WARN_ON_ONCE(scope_mask != scoped_mask);
> + ruleset->access_masks[layer_level].scoped |= scoped_mask;
> +}
> +
> static inline access_mask_t
> landlock_get_raw_fs_access_mask(const struct landlock_ruleset *const ruleset,
> const u16 layer_level)
> @@ -303,6 +317,13 @@ landlock_get_net_access_mask(const struct landlock_ruleset *const ruleset,
> return ruleset->access_masks[layer_level].net;
> }
>
> +static inline access_mask_t
> +landlock_get_scope_mask(const struct landlock_ruleset *const ruleset,
> + const u16 layer_level)
> +{
> + return ruleset->access_masks[layer_level].scoped;
> +}
> +
> bool landlock_unmask_layers(const struct landlock_rule *const rule,
> const access_mask_t access_request,
> layer_mask_t (*const layer_masks)[],
> diff --git a/security/landlock/syscalls.c b/security/landlock/syscalls.c
> index 03b470f5a85a..f51b29521d6b 100644
> --- a/security/landlock/syscalls.c
> +++ b/security/landlock/syscalls.c
> @@ -97,8 +97,9 @@ static void build_check_abi(void)
> */
> ruleset_size = sizeof(ruleset_attr.handled_access_fs);
> ruleset_size += sizeof(ruleset_attr.handled_access_net);
> + ruleset_size += sizeof(ruleset_attr.scoped);
> BUILD_BUG_ON(sizeof(ruleset_attr) != ruleset_size);
> - BUILD_BUG_ON(sizeof(ruleset_attr) != 16);
> + BUILD_BUG_ON(sizeof(ruleset_attr) != 24);
>
> path_beneath_size = sizeof(path_beneath_attr.allowed_access);
> path_beneath_size += sizeof(path_beneath_attr.parent_fd);
> @@ -149,7 +150,7 @@ 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
> @@ -170,7 +171,7 @@ static const struct file_operations ruleset_fops = {
> * Possible returned errors are:
> *
> * - %EOPNOTSUPP: Landlock is supported by the kernel but disabled at boot time;
> - * - %EINVAL: unknown @flags, or unknown access, or too small @size;
> + * - %EINVAL: unknown @flags, or unknown access, or unknown scope, or too small @size;
I think you missed one of my previous review.
> * - %E2BIG or %EFAULT: @attr or @size inconsistencies;
> * - %ENOMSG: empty &landlock_ruleset_attr.handled_access_fs.
> */
> @@ -213,9 +214,14 @@ SYSCALL_DEFINE3(landlock_create_ruleset,
> LANDLOCK_MASK_ACCESS_NET)
> return -EINVAL;
>
> + /* Checks IPC scoping content (and 32-bits cast). */
> + if ((ruleset_attr.scoped | LANDLOCK_MASK_SCOPE) != LANDLOCK_MASK_SCOPE)
> + return -EINVAL;
> +
> /* Checks arguments and transforms to kernel struct. */
> ruleset = landlock_create_ruleset(ruleset_attr.handled_access_fs,
> - ruleset_attr.handled_access_net);
> + ruleset_attr.handled_access_net,
> + ruleset_attr.scoped);
> if (IS_ERR(ruleset))
> return PTR_ERR(ruleset);
>
> diff --git a/security/landlock/task.c b/security/landlock/task.c
> index 849f5123610b..7e8579ebae83 100644
> --- a/security/landlock/task.c
> +++ b/security/landlock/task.c
> @@ -13,6 +13,8 @@
> #include <linux/lsm_hooks.h>
> #include <linux/rcupdate.h>
> #include <linux/sched.h>
> +#include <net/sock.h>
> +#include <net/af_unix.h>
Please sort the included files.
>
> #include "common.h"
> #include "cred.h"
> @@ -108,9 +110,162 @@ static int hook_ptrace_traceme(struct task_struct *const parent)
> return task_ptrace(parent, current);
> }
>
> +static bool walk_and_check(const struct landlock_ruleset *const child,
> + struct landlock_hierarchy **walker,
> + size_t base_layer, size_t deep_layer,
> + access_mask_t check_scoping)
> +{
> + if (!child || base_layer < 0 || !(*walker))
> + return false;
> +
> + for (deep_layer; base_layer < deep_layer; deep_layer--) {
> + if (check_scoping & landlock_get_scope_mask(child, deep_layer))
> + return false;
> + *walker = (*walker)->parent;
> + if (WARN_ON_ONCE(!*walker))
> + /* there is an inconsistency between num_layers
> + * and landlock_hierarchy in the ruleset
> + */
> + return false;
> + }
> + return true;
> +}
> +
> +/**
> + * domain_IPC_scope - Checks if the client domain is scoped in the same
Please don't use uppercase (IPC) in function or variable's names, it is
reserved to "#define" statements only.
> + * domain as the server.
> + *
> + * @client: IPC sender domain.
> + * @server: IPC receiver domain.
> + *
> + * Check if the @client domain is scoped to access the @server; the @server
> + * must be scoped in the same domain.
> + */
> +static bool domain_IPC_scope(const struct landlock_ruleset *const client,
> + const struct landlock_ruleset *const server,
> + access_mask_t ipc_type)
> +{
> + size_t client_layer, server_layer = 0;
> + int base_layer;
> + struct landlock_hierarchy *client_walker, *server_walker;
> + bool is_scoped;
> +
> + /* Quick return if client has no domain */
> + if (!client)
> + return true;
> +
> + client_layer = client->num_layers - 1;
> + client_walker = client->hierarchy;
> + if (server) {
> + server_layer = server->num_layers - 1;
> + server_walker = server->hierarchy;
> + }
> + base_layer = (client_layer > server_layer) ? server_layer :
> + client_layer;
> +
> + /* For client domain, walk_and_check ensures the client domain is
> + * not scoped until gets to base_layer.
> + * For server_domain, it only ensures that the server domain exist.
> + */
> + if (client_layer != server_layer) {
> + if (client_layer > server_layer)
> + is_scoped = walk_and_check(client, &client_walker,
> + server_layer, client_layer,
> + ipc_type);
> + else
> + is_scoped = walk_and_check(server, &server_walker,
> + client_layer, server_layer,
> + ipc_type & 0);
> + if (!is_scoped)
> + return false;
> + }
> + /* client and server are at the same level in hierarchy. If client is
> + * scoped, the server must be scoped in the same domain
> + */
> + for (base_layer; base_layer >= 0; base_layer--) {
> + if (landlock_get_scope_mask(client, base_layer) & ipc_type) {
> + /* This check must be here since access would be denied only if
> + * the client is scoped and the server has no domain, so
> + * if the client has a domain but is not scoped and the server
> + * has no domain, access is guaranteed.
> + */
> + if (!server)
> + return false;
> +
> + if (server_walker == client_walker)
> + return true;
> +
> + return false;
> + }
> + client_walker = client_walker->parent;
> + server_walker = server_walker->parent;
> + /* Warn if there is an incosistenncy between num_layers and
> + * landlock_hierarchy in each of rulesets
> + */
> + if (WARN_ON_ONCE(base_layer > 0 &&
> + (!server_walker || !client_walker)))
> + return false;
> + }
> + return true;
> +}
> +
> +static bool sock_is_scoped(struct sock *const other)
> +{
> + const struct landlock_ruleset *dom_other;
> + const struct landlock_ruleset *const dom =
> + landlock_get_current_domain();
> +
> + /* the credentials will not change */
> + lockdep_assert_held(&unix_sk(other)->lock);
> + dom_other = landlock_cred(other->sk_socket->file->f_cred)->domain;
> + return domain_IPC_scope(dom, dom_other,
> + LANDLOCK_SCOPED_ABSTRACT_UNIX_SOCKET);
> +}
> +
> +static bool check_unix_address_format(struct sock *const sock)
> +{
> + struct unix_address *addr = unix_sk(sock)->addr;
> +
> + if (!addr)
> + return true;
> +
> + if (addr->len > sizeof(AF_UNIX)) {
AF_UNIX is the family, not a length.
I'm wondering if we cannot also check the "path" field or something
else.
> + /* handling unspec sockets */
> + if (!addr->name[0].sun_path)
> + return true;
> +
> + if (addr->name[0].sun_path[0] == '\0')
> + if (!sock_is_scoped(sock))
This function should only check the address format. sock_is_scoped()
can be called in each LSM hook.
> + return false;
> + }
> +
> + return true;
> +}
> +
> +static int hook_unix_stream_connect(struct sock *const sock,
> + struct sock *const other,
> + struct sock *const newsk)
> +{
To not impact unsandboxed tasks, we need to first check if current is
sandboxed with Landlock, and then pass this dom to the following
functions.
> + if (check_unix_address_format(other))
> + return 0;
> +
> + return -EPERM;
> +}
> +
> +static int hook_unix_may_send(struct socket *const sock,
> + struct socket *const other)
> +{
> + if (check_unix_address_format(other->sk))
> + return 0;
> +
> + return -EPERM;
> +}
> +
> static struct security_hook_list landlock_hooks[] __ro_after_init = {
> LSM_HOOK_INIT(ptrace_access_check, hook_ptrace_access_check),
> LSM_HOOK_INIT(ptrace_traceme, hook_ptrace_traceme),
> + LSM_HOOK_INIT(unix_stream_connect, hook_unix_stream_connect),
> + LSM_HOOK_INIT(unix_may_send, hook_unix_may_send),
> };
>
> __init void landlock_add_task_hooks(void)
> --
> 2.34.1
>
>
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