samples/landlock: Add support for abstract UNIX socket scoping

The sandboxer can receive the character "a" as input from the
environment variable LL_SCOPE to restrict sandboxed processes from
connecting to an abstract UNIX socket created by a process outside of
the sandbox.

Example
=======

Create an abstract UNIX socket to listen with socat(1):
socat abstract-listen:mysocket -

Create a sandboxed shell and pass the character "a" to LL_SCOPED:
LL_FS_RO=/ LL_FS_RW=. LL_SCOPED="a" ./sandboxer /bin/bash

Note that any other form of input (e.g. "a:a", "aa", etc) is not
acceptable.

If the sandboxed process tries to connect to the listening socket, the
connection will fail:
socat - abstract-connect:mysocket

Signed-off-by: Tahera Fahimi <[e-mail]>
Link: https://lore.kernel.org/r/d8af[e-mail]
[mic: Improve commit message, simplify check_ruleset_scope() with
inverted error code and only one scoped change, always unset environment
variable]
Signed-off-by: Mickaël Salaün <[e-mail]>

samples/landlock/sandboxer.c

···
 #include <fcntl.h>
 #include <linux/landlock.h>
 #include <linux/prctl.h>
+#include <linux/socket.h>
 #include <stddef.h>
 #include <stdio.h>
 #include <stdlib.h>
···
 #include <sys/stat.h>
 #include <sys/syscall.h>
 #include <unistd.h>
+#include <stdbool.h>
 
 #ifndef landlock_create_ruleset
 static inline int
···
 #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_SCOPED_NAME "LL_SCOPED"
 #define ENV_DELIMITER ":"
 
 static int parse_path(char *env_path, const char ***const path_list)
···
 	return ret;
 }
 
+/* Returns true on error, false otherwise. */
+static bool check_ruleset_scope(const char *const env_var,
+				struct landlock_ruleset_attr *ruleset_attr)
+{
+	char *env_type_scope, *env_type_scope_next, *ipc_scoping_name;
+	bool error = false;
+	bool abstract_scoping = false;
+
+	/* Scoping is not supported by Landlock ABI */
+	if (!(ruleset_attr->scoped & LANDLOCK_SCOPE_ABSTRACT_UNIX_SOCKET))
+		goto out_unset;
+
+	env_type_scope = getenv(env_var);
+	/* Scoping is not supported by the user */
+	if (!env_type_scope || strcmp("", env_type_scope) == 0)
+		goto out_unset;
+
+	env_type_scope = strdup(env_type_scope);
+	env_type_scope_next = env_type_scope;
+	while ((ipc_scoping_name =
+			strsep(&env_type_scope_next, ENV_DELIMITER))) {
+		if (strcmp("a", ipc_scoping_name) == 0 && !abstract_scoping) {
+			abstract_scoping = true;
+		} else {
+			fprintf(stderr, "Unknown or duplicate scope \"%s\"\n",
+				ipc_scoping_name);
+			error = true;
+			goto out_free_name;
+		}
+	}
+
+out_free_name:
+	free(env_type_scope);
+
+out_unset:
+	if (!abstract_scoping)
+		ruleset_attr->scoped &= ~LANDLOCK_SCOPE_ABSTRACT_UNIX_SOCKET;
+
+	unsetenv(env_var);
+	return error;
+}
+
 /* clang-format off */
 
 #define ACCESS_FS_ROUGHLY_READ ( \
···
 
 /* clang-format on */
 
-#define LANDLOCK_ABI_LAST 5
+#define LANDLOCK_ABI_LAST 6
 
 int main(const int argc, char *const argv[], char *const *const envp)
 {
···
 		.handled_access_fs = access_fs_rw,
 		.handled_access_net = LANDLOCK_ACCESS_NET_BIND_TCP |
 				      LANDLOCK_ACCESS_NET_CONNECT_TCP,
+		.scoped = LANDLOCK_SCOPE_ABSTRACT_UNIX_SOCKET,
 	};
 
 	if (argc < 2) {
 		fprintf(stderr,
-			"usage: %s=\"...\" %s=\"...\" %s=\"...\" %s=\"...\"%s "
+			"usage: %s=\"...\" %s=\"...\" %s=\"...\" %s=\"...\" %s=\"...\" %s "
 			"<cmd> [args]...\n\n",
 			ENV_FS_RO_NAME, ENV_FS_RW_NAME, ENV_TCP_BIND_NAME,
-			ENV_TCP_CONNECT_NAME, argv[0]);
+			ENV_TCP_CONNECT_NAME, ENV_SCOPED_NAME, argv[0]);
 		fprintf(stderr,
 			"Execute a command in a restricted environment.\n\n");
 		fprintf(stderr,
···
 		fprintf(stderr,
 			"* %s: list of ports allowed to connect (client).\n",
 			ENV_TCP_CONNECT_NAME);
+		fprintf(stderr, "* %s: list of scoped IPCs.\n",
+			ENV_SCOPED_NAME);
 		fprintf(stderr,
 			"\nexample:\n"
 			"%s=\"${PATH}:/lib:/usr:/proc:/etc:/dev/urandom\" "
 			"%s=\"/dev/null:/dev/full:/dev/zero:/dev/pts:/tmp\" "
 			"%s=\"9418\" "
 			"%s=\"80:443\" "
+			"%s=\"a\" "
 			"%s bash -i\n\n",
 			ENV_FS_RO_NAME, ENV_FS_RW_NAME, ENV_TCP_BIND_NAME,
-			ENV_TCP_CONNECT_NAME, argv[0]);
+			ENV_TCP_CONNECT_NAME, ENV_SCOPED_NAME, argv[0]);
 		fprintf(stderr,
 			"This sandboxer can use Landlock features "
 			"up to ABI version %d.\n",
···
 		/* Removes LANDLOCK_ACCESS_FS_IOCTL_DEV for ABI < 5 */
 		ruleset_attr.handled_access_fs &= ~LANDLOCK_ACCESS_FS_IOCTL_DEV;
 
+		__attribute__((fallthrough));
+	case 5:
+		/* Removes LANDLOCK_SCOPE_ABSTRACT_UNIX_SOCKET for ABI < 6 */
+		ruleset_attr.scoped &= ~LANDLOCK_SCOPE_ABSTRACT_UNIX_SOCKET;
 		fprintf(stderr,
 			"Hint: You should update the running kernel "
 			"to leverage Landlock features "
···
 		ruleset_attr.handled_access_net &=
 			~LANDLOCK_ACCESS_NET_CONNECT_TCP;
 	}
+
+	if (check_ruleset_scope(ENV_SCOPED_NAME, &ruleset_attr))
+		return 1;
 
 	ruleset_fd =
 		landlock_create_ruleset(&ruleset_attr, sizeof(ruleset_attr), 0);