[PATCH v4 5/6] sample/landlock: Support sample for signal scoping restriction

[Tahera Fahimi]

A sandboxer can receive the character "s" as input from the environment
variable LL_SCOPE to restrict itself from sending a signal to a process
outside its scoped domain.

Example
=======
Create a sandboxed shell and pass the character "s" to LL_SCOPED:
LL_FS_RO=/ LL_FS_RW=. LL_SCOPED="s" ./sandboxer /bin/bash
Try to send a SIGTRAP to a process with process ID <PID> through:
kill -SIGTRAP <PID>
The sandboxed process should not be able to send the signal.

Signed-off-by: Tahera Fahimi <fahimitahera at gmail.com>
---
v4:
- Make it compatible with changes in abstract UNIX socket scoping sample
v3:
- Add a restrict approach on input of LL_SCOPED, so it only allows
  zero or one "s" to be the input.
---
 samples/landlock/sandboxer.c | 17 +++++++++++++----
 1 file changed, 13 insertions(+), 4 deletions(-)

diff --git a/samples/landlock/sandboxer.c b/samples/landlock/sandboxer.c
index 18d072c23a23..618fbf70d38f 100644
--- a/samples/landlock/sandboxer.c
+++ b/samples/landlock/sandboxer.c
@@ -191,11 +191,13 @@ static bool check_ruleset_scope(const char *const env_var,
 				struct landlock_ruleset_attr *ruleset_attr)
 {
 	bool abstract_scoping = false;
+	bool signal_scoping = false;
 	bool ret = true;
 	char *env_type_scope, *env_type_scope_next, *ipc_scoping_name;
 
 	/* scoping is not supported by Landlock ABI */
-	if (!(ruleset_attr->scoped & LANDLOCK_SCOPED_ABSTRACT_UNIX_SOCKET))
+	if (!(ruleset_attr->scoped &
+	      (LANDLOCK_SCOPED_ABSTRACT_UNIX_SOCKET | LANDLOCK_SCOPED_SIGNAL)))
 		return ret;
 
 	env_type_scope = getenv(env_var);
@@ -212,6 +214,9 @@ static bool check_ruleset_scope(const char *const env_var,
 			strsep(&env_type_scope_next, ENV_DELIMITER))) {
 		if (strcmp("a", ipc_scoping_name) == 0 && !abstract_scoping) {
 			abstract_scoping = true;
+		} else if (strcmp("s", ipc_scoping_name) == 0 &&
+			   !signal_scoping) {
+			signal_scoping = true;
 		} else {
 			fprintf(stderr, "Unsupported scoping \"%s\"\n",
 				ipc_scoping_name);
@@ -221,6 +226,8 @@ static bool check_ruleset_scope(const char *const env_var,
 	}
 	if (!abstract_scoping)
 		ruleset_attr->scoped &= ~LANDLOCK_SCOPED_ABSTRACT_UNIX_SOCKET;
+	if (!signal_scoping)
+		ruleset_attr->scoped &= ~LANDLOCK_SCOPED_SIGNAL;
 out_free_name:
 	free(env_type_scope);
 	return ret;
@@ -265,7 +272,8 @@ 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_SCOPED_ABSTRACT_UNIX_SOCKET,
+		.scoped = LANDLOCK_SCOPED_ABSTRACT_UNIX_SOCKET |
+			  LANDLOCK_SCOPED_SIGNAL,
 	};
 
 	if (argc < 2) {
@@ -302,7 +310,7 @@ int main(const int argc, char *const argv[], char *const *const envp)
 			"%s=\"/dev/null:/dev/full:/dev/zero:/dev/pts:/tmp\" "
 			"%s=\"9418\" "
 			"%s=\"80:443\" "
-			"%s=\"a\" "
+			"%s=\"a:s\" "
 			"%s bash -i\n\n",
 			ENV_FS_RO_NAME, ENV_FS_RW_NAME, ENV_TCP_BIND_NAME,
 			ENV_TCP_CONNECT_NAME, ENV_SCOPED_NAME, argv[0]);
@@ -376,7 +384,8 @@ int main(const int argc, char *const argv[], char *const *const envp)
 		__attribute__((fallthrough));
 	case 5:
 		/* Removes LANDLOCK_SCOPED_ABSTRACT_UNIX_SOCKET for ABI < 6 */
-		ruleset_attr.scoped &= ~LANDLOCK_SCOPED_ABSTRACT_UNIX_SOCKET;
+		ruleset_attr.scoped &= ~(LANDLOCK_SCOPED_ABSTRACT_UNIX_SOCKET |
+					 LANDLOCK_SCOPED_SIGNAL);
 		fprintf(stderr,
 			"Hint: You should update the running kernel "
 			"to leverage Landlock features "
-- 
2.34.1