[LTP] [PATCH v12 3/3] open16: allow restricted O_CREAT of FIFOs and regular files

Add LTP coverage for kernel commit 30aba6656f61 (Linux 4.19), which
introduced protection against spoofing attacks via O_CREAT of FIFOs
and regular files in world-writable or group-writable sticky
directories.

This commit adds test cases to verify these security restrictions
(Level 1 and Level 2 protections) for opening FIFOs and regular
files in world-writable or group-writable sticky directories when the
file is not owned by the opener.

Signed-off-by: Wei Gao <[email protected]>
---
 runtest/syscalls                          |   1 +
 testcases/kernel/syscalls/open/.gitignore |   1 +
 testcases/kernel/syscalls/open/open16.c   | 135 ++++++++++++++++++++++
 3 files changed, 137 insertions(+)
 create mode 100644 testcases/kernel/syscalls/open/open16.c

diff --git a/runtest/syscalls b/runtest/syscalls
index a021c79da..4fd62efa8 100644
--- a/runtest/syscalls
+++ b/runtest/syscalls
@@ -1008,6 +1008,7 @@ open12 open12
 open13 open13
 open14 open14
 open15 open15
+open16 open16
 
 openat01 openat01
 openat02 openat02
diff --git a/testcases/kernel/syscalls/open/.gitignore b/testcases/kernel/syscalls/open/.gitignore
index af5997572..d2cacc02e 100644
--- a/testcases/kernel/syscalls/open/.gitignore
+++ b/testcases/kernel/syscalls/open/.gitignore
@@ -13,3 +13,4 @@
 /open13
 /open14
 /open15
+/open16
diff --git a/testcases/kernel/syscalls/open/open16.c b/testcases/kernel/syscalls/open/open16.c
new file mode 100644
index 000000000..c74601032
--- /dev/null
+++ b/testcases/kernel/syscalls/open/open16.c
@@ -0,0 +1,135 @@
+// SPDX-License-Identifier: GPL-2.0-or-later
+/*
+ * Copyright (c) 2026 Wei Gao <[email protected]>
+ */
+
+/*\
+ * Verify restricted opening (:manpage:`open(2)` and :manpage:`openat(2)`) of
+ * FIFOs and regular files in sticky directories. This test covers the positive
+ * case where access is allowed when protection is disabled (level 0), and the
+ * negative cases where access is disallowed (EACCES) in world-writable (level
+ * 1) or group-writable (level 2) sticky directories when the file is not owned
+ * by the opener.
+ *
+ * This test requires root to modify /proc/sys/fs/protected_* sysctls and
+ * to manage file ownership and permissions in sticky directories.
+ */
+
+#include <pwd.h>
+#include <stdlib.h>
+#include "tst_test.h"
+#include "tst_safe_file_at.h"
+#include "tst_uid.h"
+
+#define DIR "ltp_tmp_check1"
+#define TEST_FILE "test_file_1"
+#define TEST_FIFO "test_fifo_1"
+#define PROTECTED_REGULAR "/proc/sys/fs/protected_regular"
+#define PROTECTED_FIFOS "/proc/sys/fs/protected_fifos"
+#define TEST_FIFO_PATH DIR "/" TEST_FIFO
+
+static int dir_fd = -1;
+static uid_t uid1, uid2;
+static gid_t gid1;
+
+static struct tcase {
+	char *level;
+	int exp_errno;
+	uid_t owner_uid;
+	int use_nobody_gid;
+	mode_t dir_mode;
+} tcases[] = {
+	{"0", 0,      0,  0, 0777 | S_ISVTX},
+	{"1", EACCES, 0,  0, 0777 | S_ISVTX},
+	{"2", EACCES, -1, 1, 0030 | S_ISVTX},
+};
+
+static void verify_open(unsigned int n)
+{
+	struct tcase *tc = &tcases[n];
+	pid_t pid;
+
+	SAFE_FILE_PRINTF(PROTECTED_REGULAR, "%s", tc->level);
+	SAFE_FILE_PRINTF(PROTECTED_FIFOS, "%s", tc->level);
+
+	if (tc->owner_uid != (uid_t)-1 || tc->use_nobody_gid) {
+		gid_t gid = tc->use_nobody_gid ? gid1 : 0;
+
+		SAFE_CHOWN(DIR, tc->owner_uid, gid);
+	}
+
+	if (tc->dir_mode)
+		SAFE_CHMOD(DIR, tc->dir_mode);
+
+	pid = SAFE_FORK();
+	if (!pid) {
+		SAFE_SETGID(gid1);
+		SAFE_SETUID(uid2);
+
+		if (tc->exp_errno) {
+			TST_EXP_FAIL2(openat(dir_fd, TEST_FILE, O_RDWR | O_CREAT, 0777),
+				      tc->exp_errno, "openat %s (Level %s)", TEST_FILE, tc->level);
+			TST_EXP_FAIL2(open(TEST_FIFO_PATH, O_RDWR | O_CREAT, 0777),
+				      tc->exp_errno, "open %s (Level %s)", TEST_FIFO, tc->level);
+		} else {
+			int fd = TST_EXP_FD(openat(dir_fd, TEST_FILE, O_CREAT | O_RDWR, 0777));
+
+			if (TST_PASS)
+				SAFE_CLOSE(fd);
+
+			fd = TST_EXP_FD(open(TEST_FIFO_PATH, O_RDWR | O_CREAT, 0777));
+			if (TST_PASS)
+				SAFE_CLOSE(fd);
+		}
+
+		exit(0);
+	}
+
+	SAFE_WAITPID(pid, NULL, 0);
+}
+
+static void setup(void)
+{
+	struct passwd *pw;
+
+	pw = SAFE_GETPWNAM("nobody");
+	uid1 = pw->pw_uid;
+	gid1 = pw->pw_gid;
+	uid2 = tst_get_free_uid(uid1);
+
+	umask(0);
+	SAFE_MKDIR(DIR, 0777 | S_ISVTX);
+	dir_fd = SAFE_OPEN(DIR, O_DIRECTORY);
+
+	int fd = SAFE_OPENAT(dir_fd, TEST_FILE, O_CREAT | O_RDWR, 0777);
+
+	SAFE_CLOSE(fd);
+	SAFE_MKFIFO(TEST_FIFO_PATH, 0777);
+	SAFE_CHOWN(TEST_FIFO_PATH, uid1, gid1);
+	SAFE_CHOWN(DIR "/" TEST_FILE, uid1, gid1);
+}
+
+static void cleanup(void)
+{
+	if (dir_fd != -1)
+		SAFE_CLOSE(dir_fd);
+}
+
+static struct tst_test test = {
+	.setup = setup,
+	.cleanup = cleanup,
+	.needs_root = 1,
+	.tcnt = ARRAY_SIZE(tcases),
+	.test = verify_open,
+	.needs_tmpdir = 1,
+	.forks_child = 1,
+	.save_restore = (const struct tst_path_val[]) {
+		{PROTECTED_REGULAR, NULL, TST_SR_TCONF},
+		{PROTECTED_FIFOS, NULL, TST_SR_TCONF},
+		{}
+	},
+	.tags = (const struct tst_tag[]) {
+		{"linux-git", "30aba6656f61"},
+		{}
+	}
+};
-- 
2.54.0


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