[PATCH v6 18/26] samples/landlock: Enable users to log sandbox denials

["Mickaël Salaün" <mic@digikod.net>]

By default, denials from within the sandbox are not logged.  Indeed, the
sandboxer's security policy might not be fitted to the set of sandboxed
processes that could be spawned (e.g. from a shell).

For test purpose, parse the LL_FORCE_LOG environment variable to log
every sandbox denials, including after launching the initial sandboxed
program thanks to LANDLOCK_RESTRICT_SELF_LOG_NEW_EXEC_ON.

Cc: Günther Noack <gnoack@google.com>
Signed-off-by: Mickaël Salaün <mic@digikod.net>
Link: https://lore.kernel.org/r/20250308184422.2159360-19-mic@digikod.net
---

Changes since v5:
- Update with new flag.

Changes since v3:
- Extend error message, suggested by Francis Laniel.

Changes since v2:
- New patch.
---
 samples/landlock/sandboxer.c | 37 +++++++++++++++++++++++++++++++++---
 security/landlock/syscalls.c |  8 ++------
 2 files changed, 36 insertions(+), 9 deletions(-)

diff --git a/samples/landlock/sandboxer.c b/samples/landlock/sandboxer.c
index 07fab2ef534e..4e2854c6f9a3 100644
--- a/samples/landlock/sandboxer.c
+++ b/samples/landlock/sandboxer.c
@@ -58,6 +58,7 @@ static inline int landlock_restrict_self(const int ruleset_fd,
 #define ENV_TCP_BIND_NAME "LL_TCP_BIND"
 #define ENV_TCP_CONNECT_NAME "LL_TCP_CONNECT"
 #define ENV_SCOPED_NAME "LL_SCOPED"
+#define ENV_FORCE_LOG_NAME "LL_FORCE_LOG"
 #define ENV_DELIMITER ":"
 
 static int str2num(const char *numstr, __u64 *num_dst)
@@ -295,7 +296,7 @@ static bool check_ruleset_scope(const char *const env_var,
 
 /* clang-format on */
 
-#define LANDLOCK_ABI_LAST 6
+#define LANDLOCK_ABI_LAST 7
 
 #define XSTR(s) #s
 #define STR(s) XSTR(s)
@@ -322,6 +323,9 @@ static const char help[] =
 	"  - \"a\" to restrict opening abstract unix sockets\n"
 	"  - \"s\" to restrict sending signals\n"
 	"\n"
+	"A sandboxer should not log denied access requests to avoid spamming logs, "
+	"but to test audit we can set " ENV_FORCE_LOG_NAME "=1\n"
+	"\n"
 	"Example:\n"
 	ENV_FS_RO_NAME "=\"${PATH}:/lib:/usr:/proc:/etc:/dev/urandom\" "
 	ENV_FS_RW_NAME "=\"/dev/null:/dev/full:/dev/zero:/dev/pts:/tmp\" "
@@ -340,7 +344,7 @@ int main(const int argc, char *const argv[], char *const *const envp)
 	const char *cmd_path;
 	char *const *cmd_argv;
 	int ruleset_fd, abi;
-	char *env_port_name;
+	char *env_port_name, *env_force_log;
 	__u64 access_fs_ro = ACCESS_FS_ROUGHLY_READ,
 	      access_fs_rw = ACCESS_FS_ROUGHLY_READ | ACCESS_FS_ROUGHLY_WRITE;
 
@@ -351,6 +355,8 @@ int main(const int argc, char *const argv[], char *const *const envp)
 		.scoped = LANDLOCK_SCOPE_ABSTRACT_UNIX_SOCKET |
 			  LANDLOCK_SCOPE_SIGNAL,
 	};
+	int supported_restrict_flags = LANDLOCK_RESTRICT_SELF_LOG_NEW_EXEC_ON;
+	int set_restrict_flags = 0;
 
 	if (argc < 2) {
 		fprintf(stderr, help, argv[0]);
@@ -422,6 +428,13 @@ int main(const int argc, char *const argv[], char *const *const envp)
 		/* Removes LANDLOCK_SCOPE_* for ABI < 6 */
 		ruleset_attr.scoped &= ~(LANDLOCK_SCOPE_ABSTRACT_UNIX_SOCKET |
 					 LANDLOCK_SCOPE_SIGNAL);
+		__attribute__((fallthrough));
+	case 6:
+		/* Removes LANDLOCK_RESTRICT_SELF_LOG_NEW_EXEC_ON for ABI < 7 */
+		supported_restrict_flags &=
+			~LANDLOCK_RESTRICT_SELF_LOG_NEW_EXEC_ON;
+
+		/* Must be printed for any ABI < LANDLOCK_ABI_LAST. */
 		fprintf(stderr,
 			"Hint: You should update the running kernel "
 			"to leverage Landlock features "
@@ -456,6 +469,24 @@ int main(const int argc, char *const argv[], char *const *const envp)
 	if (check_ruleset_scope(ENV_SCOPED_NAME, &ruleset_attr))
 		return 1;
 
+	/* Enables optional logs. */
+	env_force_log = getenv(ENV_FORCE_LOG_NAME);
+	if (env_force_log) {
+		if (strcmp(env_force_log, "1") != 0) {
+			fprintf(stderr, "Unknown value for " ENV_FORCE_LOG_NAME
+					" (only \"1\" is handled)\n");
+			return 1;
+		}
+		if (!(supported_restrict_flags &
+		      LANDLOCK_RESTRICT_SELF_LOG_NEW_EXEC_ON)) {
+			fprintf(stderr,
+				"Audit logs not supported by current kernel\n");
+			return 1;
+		}
+		set_restrict_flags |= LANDLOCK_RESTRICT_SELF_LOG_NEW_EXEC_ON;
+		unsetenv(ENV_FORCE_LOG_NAME);
+	}
+
 	ruleset_fd =
 		landlock_create_ruleset(&ruleset_attr, sizeof(ruleset_attr), 0);
 	if (ruleset_fd < 0) {
@@ -483,7 +514,7 @@ int main(const int argc, char *const argv[], char *const *const envp)
 		perror("Failed to restrict privileges");
 		goto err_close_ruleset;
 	}
-	if (landlock_restrict_self(ruleset_fd, 0)) {
+	if (landlock_restrict_self(ruleset_fd, set_restrict_flags)) {
 		perror("Failed to enforce ruleset");
 		goto err_close_ruleset;
 	}
diff --git a/security/landlock/syscalls.c b/security/landlock/syscalls.c
index e38b22075ca1..cae0a0fd08d8 100644
--- a/security/landlock/syscalls.c
+++ b/security/landlock/syscalls.c
@@ -495,12 +495,8 @@ SYSCALL_DEFINE2(landlock_restrict_self, const int, ruleset_fd, const __u32,
 	/* Translates "off" flag to boolean. */
 	log_subdomains = !(flags & LANDLOCK_RESTRICT_SELF_LOG_SUBDOMAINS_OFF);
 
-	/*
-	 * It is allowed to set %LANDLOCK_RESTRICT_SELF_LOG_SUBDOMAINS_OFF with -1
-	 * as @ruleset_fd, but no other flag must be set.
-	 */
-	if (!(ruleset_fd == -1 &&
-	      flags == LANDLOCK_RESTRICT_SELF_LOG_SUBDOMAINS_OFF)) {
+	if (!(flags == LANDLOCK_RESTRICT_SELF_LOG_SUBDOMAINS_OFF &&
+	      ruleset_fd == -1)) {
 		/* Gets and checks the ruleset. */
 		ruleset = get_ruleset_from_fd(ruleset_fd, FMODE_CAN_READ);
 		if (IS_ERR(ruleset))
-- 
2.48.1