samples/landlock: Add support for file reparenting

Add LANDLOCK_ACCESS_FS_REFER to the "roughly write" access rights and
leverage the Landlock ABI version to only try to enforce it if it is
supported by the running kernel.
Reviewed-by: Paul Moore <paul@paul-moore.com>
Signed-off-by: Mickaël Salaün <mic@digikod.net>
Link: https://lore.kernel.org/r/20220506161102.525323-10-mic@digikod.net

samples/landlock/sandboxer.c

@@ -159,7 +159,11 @@ static int populate_ruleset(const char *const env_var, const int ruleset_fd,
 	LANDLOCK_ACCESS_FS_MAKE_SOCK | \
 	LANDLOCK_ACCESS_FS_MAKE_FIFO | \
 	LANDLOCK_ACCESS_FS_MAKE_BLOCK | \
-	LANDLOCK_ACCESS_FS_MAKE_SYM)
+	LANDLOCK_ACCESS_FS_MAKE_SYM | \
+	LANDLOCK_ACCESS_FS_REFER)
+
+#define ACCESS_ABI_2 ( \
+	LANDLOCK_ACCESS_FS_REFER)
 
 /* clang-format on */
 
@@ -167,10 +171,11 @@ int main(const int argc, char *const argv[], char *const *const envp)
 {
 	const char *cmd_path;
 	char *const *cmd_argv;
-	int ruleset_fd;
+	int ruleset_fd, abi;
+	__u64 access_fs_ro = ACCESS_FS_ROUGHLY_READ,
+	      access_fs_rw = ACCESS_FS_ROUGHLY_READ | ACCESS_FS_ROUGHLY_WRITE;
 	struct landlock_ruleset_attr ruleset_attr = {
-		.handled_access_fs = ACCESS_FS_ROUGHLY_READ |
-				     ACCESS_FS_ROUGHLY_WRITE,
+		.handled_access_fs = access_fs_rw,
 	};
 
 	if (argc < 2) {
@@ -196,12 +201,11 @@ int main(const int argc, char *const argv[], char *const *const envp)
 		return 1;
 	}
 
-	ruleset_fd =
-		landlock_create_ruleset(&ruleset_attr, sizeof(ruleset_attr), 0);
-	if (ruleset_fd < 0) {
+	abi = landlock_create_ruleset(NULL, 0, LANDLOCK_CREATE_RULESET_VERSION);
+	if (abi < 0) {
 		const int err = errno;
 
-		perror("Failed to create a ruleset");
+		perror("Failed to check Landlock compatibility");
 		switch (err) {
 		case ENOSYS:
 			fprintf(stderr,
@@ -221,13 +225,23 @@ int main(const int argc, char *const argv[], char *const *const envp)
 		}
 		return 1;
 	}
-	if (populate_ruleset(ENV_FS_RO_NAME, ruleset_fd,
-			     ACCESS_FS_ROUGHLY_READ)) {
+	/* Best-effort security. */
+	if (abi < 2) {
+		ruleset_attr.handled_access_fs &= ~ACCESS_ABI_2;
+		access_fs_ro &= ~ACCESS_ABI_2;
+		access_fs_rw &= ~ACCESS_ABI_2;
+	}
+
+	ruleset_fd =
+		landlock_create_ruleset(&ruleset_attr, sizeof(ruleset_attr), 0);
+	if (ruleset_fd < 0) {
+		perror("Failed to create a ruleset");
+		return 1;
+	}
+	if (populate_ruleset(ENV_FS_RO_NAME, ruleset_fd, access_fs_ro)) {
 		goto err_close_ruleset;
 	}
-	if (populate_ruleset(ENV_FS_RW_NAME, ruleset_fd,
-			     ACCESS_FS_ROUGHLY_READ |
-				     ACCESS_FS_ROUGHLY_WRITE)) {
+	if (populate_ruleset(ENV_FS_RW_NAME, ruleset_fd, access_fs_rw)) {
 		goto err_close_ruleset;
 	}
 	if (prctl(PR_SET_NO_NEW_PRIVS, 1, 0, 0, 0)) {