[PATCH v2 8/9] samples/landlock: Add capability and namespace restriction support

["Mickaël Salaün" <mic@digikod.net>]

Extend the sandboxer sample to demonstrate the new Landlock capability
and namespace restriction features.  The LL_CAP environment variable
takes a colon-delimited list of allowed capabilities, parsed with
cap_from_name(3) from libcap.  Names (e.g. "cap_sys_chroot",
"CAP_SYS_ADMIN") are accepted; numeric strings (e.g. "18") work too via
cap_from_name's internal numeric fallback.  The LL_NS variable takes a
colon-delimited list of allowed namespace types by short name (e.g.
"user:uts:net").  Add best-effort degradation for older kernels that
predate the LANDLOCK_PERM_* features.

Allow creating user and UTS namespaces but deny network namespaces
(works as an unprivileged user).  All capabilities are available (LL_CAP
is not set), but namespace creation is still restricted to the types
listed in LL_NS.  The first command succeeds because user and UTS types
are in the allowed set, and sets the hostname inside the new UTS
namespace.  The second command fails because the network namespace type
is not allowed by the LANDLOCK_PERM_NAMESPACE_USE rule:

  LL_FS_RO=/ LL_FS_RW=/proc LL_NS="user:uts" \
    ./sandboxer /bin/sh -c \
    "unshare --user --uts --map-root-user hostname sandbox \
    && ! unshare --user --net true"

Allow only user namespace creation and CAP_SYS_CHROOT, denying all other
capabilities and namespace types (works as an unprivileged user).  An
unprivileged process creates a user namespace (no capability required)
and calls chroot inside it using the CAP_SYS_CHROOT granted within the
new namespace:

  LL_FS_RO=/ LL_FS_RW="" LL_NS="user" LL_CAP="cap_sys_chroot" \
    ./sandboxer /bin/sh -c \
    "unshare --user --keep-caps chroot / true"

Cc: Christian Brauner <brauner@kernel.org>
Cc: Günther Noack <gnoack@google.com>
Cc: Paul Moore <paul@paul-moore.com>
Cc: Serge E. Hallyn <serge@hallyn.com>
Cc: Tingmao Wang <m@maowtm.org>
Signed-off-by: Mickaël Salaün <mic@digikod.net>
---

Changes since v1:
https://lore.kernel.org/r/20260312100444.2609563-11-mic@digikod.net
- Rename LANDLOCK_PERM_NAMESPACE_ENTER references to
  LANDLOCK_PERM_NAMESPACE_USE (companion change to the introducing
  commit).
- Replace handled_perm = 0 with a per-bit mask in the ABI compat
  fall-through, mirroring the doc example so future ABI extensions
  adding new LANDLOCK_PERM_* bits do not get stripped.
- Parse LL_CAP values with cap_from_name(3) from libcap so users
  can pass capability names (e.g. "cap_sys_chroot") in addition to
  numbers.  cap_from_name accepts both: the canonical name lookup
  is case-insensitive, and a numeric-string fallback maps "18" to
  CAP_SYS_CHROOT identically to the previous numeric-only path.
  Drop the BITS_PER_TYPE workaround and the manual numeric bound
  check (cap_from_name does the right thing in both cases).  Link
  the sandboxer against libcap by adding userldlibs += -lcap in
  samples/landlock/Makefile.  Update help text and example command
  to show capability names (suggested by Günther Noack).
- Rename the LL_CAPS env var to LL_CAP for consistency with the
  singular form of all other sandboxer env vars (LL_NS, LL_FS_RO,
  LL_FS_RW, LL_TCP_BIND, LL_TCP_CONNECT, LL_SCOPED, LL_FORCE_LOG).
  Internal symbols renamed accordingly: ENV_CAPS_NAME -> ENV_CAP_NAME,
  populate_ruleset_caps() -> populate_ruleset_cap().
- Tingmao Wang's v1 Reviewed-by is not carried forward to v2: the
  cap_from_name() / libcap migration is a material implementation
  change requested by Günther Noack that was not part of his
  review.  Cc'd instead.
---
 samples/landlock/Makefile    |   1 +
 samples/landlock/sandboxer.c | 144 ++++++++++++++++++++++++++++++++++-
 2 files changed, 142 insertions(+), 3 deletions(-)

diff --git a/samples/landlock/Makefile b/samples/landlock/Makefile
index 5d601e51c2eb..b30239c8a281 100644
--- a/samples/landlock/Makefile
+++ b/samples/landlock/Makefile
@@ -3,6 +3,7 @@
 userprogs-always-y := sandboxer
 
 userccflags += -I usr/include
+userldlibs += -lcap
 
 .PHONY: all clean
 
diff --git a/samples/landlock/sandboxer.c b/samples/landlock/sandboxer.c
index 94e399e6b146..1582540f1a89 100644
--- a/samples/landlock/sandboxer.c
+++ b/samples/landlock/sandboxer.c
@@ -14,15 +14,17 @@
 #include <fcntl.h>
 #include <linux/landlock.h>
 #include <linux/socket.h>
+#include <sched.h>
+#include <stdbool.h>
 #include <stddef.h>
 #include <stdio.h>
 #include <stdlib.h>
 #include <string.h>
+#include <sys/capability.h>
 #include <sys/prctl.h>
 #include <sys/stat.h>
 #include <sys/syscall.h>
 #include <unistd.h>
-#include <stdbool.h>
 
 #if defined(__GLIBC__)
 #include <linux/prctl.h>
@@ -60,6 +62,8 @@ static inline int landlock_restrict_self(const int ruleset_fd,
 #define ENV_FS_RW_NAME "LL_FS_RW"
 #define ENV_TCP_BIND_NAME "LL_TCP_BIND"
 #define ENV_TCP_CONNECT_NAME "LL_TCP_CONNECT"
+#define ENV_CAP_NAME "LL_CAP"
+#define ENV_NS_NAME "LL_NS"
 #define ENV_SCOPED_NAME "LL_SCOPED"
 #define ENV_FORCE_LOG_NAME "LL_FORCE_LOG"
 #define ENV_UDP_BIND_NAME "LL_UDP_BIND"
@@ -229,6 +233,117 @@ static int populate_ruleset_net(const char *const env_var, const int ruleset_fd,
 	return ret;
 }
 
+static __u64 str2ns(const char *const name)
+{
+	static const struct {
+		const char *name;
+		__u64 value;
+	} ns_map[] = {
+		/* clang-format off */
+		{ "cgroup",	CLONE_NEWCGROUP },
+		{ "ipc",	CLONE_NEWIPC },
+		{ "mnt",	CLONE_NEWNS },
+		{ "net",	CLONE_NEWNET },
+		{ "pid",	CLONE_NEWPID },
+		{ "time",	CLONE_NEWTIME },
+		{ "user",	CLONE_NEWUSER },
+		{ "uts",	CLONE_NEWUTS },
+		/* clang-format on */
+	};
+	size_t i;
+
+	for (i = 0; i < sizeof(ns_map) / sizeof(ns_map[0]); i++) {
+		if (strcmp(name, ns_map[i].name) == 0)
+			return ns_map[i].value;
+	}
+	return 0;
+}
+
+static int populate_ruleset_cap(const char *const env_var, const int ruleset_fd)
+{
+	int ret = 1;
+	char *env_cap_name, *env_cap_name_next, *strcap;
+	struct landlock_capability_attr cap_attr = {
+		.allowed_perm = LANDLOCK_PERM_CAPABILITY_USE,
+	};
+
+	env_cap_name = getenv(env_var);
+	if (!env_cap_name)
+		return 0;
+	env_cap_name = strdup(env_cap_name);
+	unsetenv(env_var);
+
+	env_cap_name_next = env_cap_name;
+	while ((strcap = strsep(&env_cap_name_next, ENV_DELIMITER))) {
+		cap_value_t cap;
+
+		if (strcmp(strcap, "") == 0)
+			continue;
+
+		if (cap_from_name(strcap, &cap)) {
+			fprintf(stderr, "Failed to parse capability \"%s\"\n",
+				strcap);
+			goto out_free_name;
+		}
+		cap_attr.capabilities = 1ULL << cap;
+		if (landlock_add_rule(ruleset_fd, LANDLOCK_RULE_CAPABILITY,
+				      &cap_attr, 0)) {
+			fprintf(stderr,
+				"Failed to update the ruleset with capability \"%s\": %s\n",
+				strcap, strerror(errno));
+			goto out_free_name;
+		}
+	}
+	ret = 0;
+
+out_free_name:
+	free(env_cap_name);
+	return ret;
+}
+
+static int populate_ruleset_ns(const char *const env_var, const int ruleset_fd)
+{
+	int ret = 1;
+	char *env_ns_name, *env_ns_name_next, *strns;
+	struct landlock_namespace_attr ns_attr = {
+		.allowed_perm = LANDLOCK_PERM_NAMESPACE_USE,
+	};
+
+	env_ns_name = getenv(env_var);
+	if (!env_ns_name)
+		return 0;
+	env_ns_name = strdup(env_ns_name);
+	unsetenv(env_var);
+
+	env_ns_name_next = env_ns_name;
+	while ((strns = strsep(&env_ns_name_next, ENV_DELIMITER))) {
+		__u64 ns_type;
+
+		if (strcmp(strns, "") == 0)
+			continue;
+
+		ns_type = str2ns(strns);
+		if (!ns_type) {
+			fprintf(stderr, "Unknown namespace type \"%s\"\n",
+				strns);
+			goto out_free_name;
+		}
+		ns_attr.namespace_types = ns_type;
+		if (landlock_add_rule(ruleset_fd, LANDLOCK_RULE_NAMESPACE,
+				      &ns_attr, 0)) {
+			fprintf(stderr,
+				"Failed to update the ruleset with namespace \"%s\": %s\n",
+				strns, strerror(errno));
+			goto out_free_name;
+		}
+	}
+	ret = 0;
+
+out_free_name:
+	free(env_ns_name);
+	return ret;
+}
+
 /* Returns true on error, false otherwise. */
 static bool check_ruleset_scope(const char *const env_var,
 				struct landlock_ruleset_attr *ruleset_attr)
@@ -330,6 +445,10 @@ static const char help[] =
 	"prepare to receive on port / client: set as source port)\n"
 	"* " ENV_UDP_CONNECT_SEND_NAME ": remote UDP ports allowed to connect "
 	"or sendmsg (client: use as destination port / server: receive only from it)\n"
+	"* " ENV_CAP_NAME ": capabilities allowed to use, as names "
+	"or numbers (e.g. cap_net_bind_service, cap_sys_admin, 18)\n"
+	"* " ENV_NS_NAME ": namespace types allowed to use "
+	"(cgroup, ipc, mnt, net, pid, time, user, uts)\n"
 	"* " ENV_SCOPED_NAME ": actions denied on the outside of the landlock domain\n"
 	"  - \"a\" to restrict opening abstract unix sockets\n"
 	"  - \"s\" to restrict sending signals\n"
@@ -343,6 +462,8 @@ static const char help[] =
 	ENV_TCP_BIND_NAME "=\"9418\" "
 	ENV_TCP_CONNECT_NAME "=\"80:443\" "
 	ENV_UDP_CONNECT_SEND_NAME "=\"53\" "
+	ENV_CAP_NAME "=\"cap_sys_admin\" "
+	ENV_NS_NAME "=\"user:uts:net\" "
 	ENV_SCOPED_NAME "=\"a:s\" "
 	"%1$s bash -i\n"
 	"\n"
@@ -368,6 +489,8 @@ int main(const int argc, char *const argv[], char *const *const envp)
 				      LANDLOCK_ACCESS_NET_CONNECT_SEND_UDP,
 		.scoped = LANDLOCK_SCOPE_ABSTRACT_UNIX_SOCKET |
 			  LANDLOCK_SCOPE_SIGNAL,
+		.handled_perm = LANDLOCK_PERM_CAPABILITY_USE |
+				LANDLOCK_PERM_NAMESPACE_USE,
 	};
 	int supported_restrict_flags = LANDLOCK_RESTRICT_SELF_LOG_NEW_EXEC_ON;
 	int set_restrict_flags = 0;
@@ -455,11 +578,12 @@ int main(const int argc, char *const argv[], char *const *const envp)
 			~LANDLOCK_ACCESS_FS_RESOLVE_UNIX;
 		__attribute__((fallthrough));
 	case 9:
-		/* Removes UDP support for ABI < 10 */
+		/* Removes UDP support and LANDLOCK_PERM_* for ABI < 10 */
 		ruleset_attr.handled_access_net &=
 			~(LANDLOCK_ACCESS_NET_BIND_UDP |
 			  LANDLOCK_ACCESS_NET_CONNECT_SEND_UDP);
-
+		ruleset_attr.handled_perm &= ~(LANDLOCK_PERM_NAMESPACE_USE |
+					       LANDLOCK_PERM_CAPABILITY_USE);
 		/* Must be printed for any ABI < LANDLOCK_ABI_LAST. */
 		fprintf(stderr,
 			"Hint: You should update the running kernel "
@@ -504,6 +628,14 @@ int main(const int argc, char *const argv[], char *const *const envp)
 			~LANDLOCK_ACCESS_NET_CONNECT_SEND_UDP;
 	}
 
+	/* Removes capability handling if not set by a user. */
+	if (!getenv(ENV_CAP_NAME))
+		ruleset_attr.handled_perm &= ~LANDLOCK_PERM_CAPABILITY_USE;
+
+	/* Removes namespace handling if not set by a user. */
+	if (!getenv(ENV_NS_NAME))
+		ruleset_attr.handled_perm &= ~LANDLOCK_PERM_NAMESPACE_USE;
+
 	if (check_ruleset_scope(ENV_SCOPED_NAME, &ruleset_attr))
 		return 1;
 
@@ -556,6 +688,12 @@ int main(const int argc, char *const argv[], char *const *const envp)
 		goto err_close_ruleset;
 	}
 
+	if (populate_ruleset_cap(ENV_CAP_NAME, ruleset_fd))
+		goto err_close_ruleset;
+
+	if (populate_ruleset_ns(ENV_NS_NAME, ruleset_fd))
+		goto err_close_ruleset;
+
 	if (prctl(PR_SET_NO_NEW_PRIVS, 1, 0, 0, 0)) {
 		perror("Failed to restrict privileges");
 		goto err_close_ruleset;
-- 
2.54.0