[LTP] [PATCH v4 2/3] ptrace: add test for /proc/self/mem write rejection

[Jan Polensky <japo@linux.ibm.com>]

Add ptrace12 to verify that /proc/self/mem writes are rejected when
CONFIG_PROC_MEM_FORCE_PTRACE requires ptrace access checks for
/proc/pid/mem writes.

The test maps a page, makes it read-only so the write path needs
FOLL_FORCE, then attempts to write to it through /proc/self/mem. Since
a task cannot ptrace itself, the write is expected to fail with EIO.
If the write succeeds, the test reports TCONF because the required
kernel behavior is not active.

Signed-off-by: Jan Polensky <japo@linux.ibm.com>
---
 runtest/syscalls                            |  1 +
 testcases/kernel/syscalls/ptrace/.gitignore |  1 +
 testcases/kernel/syscalls/ptrace/ptrace12.c | 95 +++++++++++++++++++++
 3 files changed, 97 insertions(+)
 create mode 100644 testcases/kernel/syscalls/ptrace/ptrace12.c

diff --git a/runtest/syscalls b/runtest/syscalls
index b024d4c43a2c..7fc443247361 100644
--- a/runtest/syscalls
+++ b/runtest/syscalls
@@ -1182,6 +1182,7 @@ ptrace09 ptrace09
 ptrace10 ptrace10
 ptrace11 ptrace11
 
+ptrace12 ptrace12
 pwrite01 pwrite01
 pwrite02 pwrite02
 pwrite03 pwrite03
diff --git a/testcases/kernel/syscalls/ptrace/.gitignore b/testcases/kernel/syscalls/ptrace/.gitignore
index 1ee6117e9d5b..8631219312d5 100644
--- a/testcases/kernel/syscalls/ptrace/.gitignore
+++ b/testcases/kernel/syscalls/ptrace/.gitignore
@@ -9,3 +9,4 @@
 /ptrace09
 /ptrace10
 /ptrace11
+/ptrace12
diff --git a/testcases/kernel/syscalls/ptrace/ptrace12.c b/testcases/kernel/syscalls/ptrace/ptrace12.c
new file mode 100644
index 000000000000..d72987a69a92
--- /dev/null
+++ b/testcases/kernel/syscalls/ptrace/ptrace12.c
@@ -0,0 +1,95 @@
+// SPDX-License-Identifier: GPL-2.0-or-later
+/*
+ * Copyright (c) 2026 IBM Corporation
+ */
+
+/*\
+ * Verify that direct writes to /proc/self/mem are correctly rejected
+ * when CONFIG_PROC_MEM_FORCE_PTRACE=y is active.
+ *
+ * When CONFIG_PROC_MEM_FORCE_PTRACE=y is set, the kernel requires
+ * PTRACE_MODE_ATTACH for /proc/pid/mem writes. This means a process
+ * cannot write to its own memory via /proc/self/mem - such writes
+ * should fail with EIO.
+ *
+ * Test behavior:
+ *
+ * - If write fails with EIO: TPASS (correct rejection)
+ * - If write succeeds: TFAIL (policy violation under required config)
+ * - If write fails with other error: TFAIL (unexpected behavior)
+ */
+
+#include <errno.h>
+#include <fcntl.h>
+#include <sys/mman.h>
+#include <unistd.h>
+
+#include "tst_test.h"
+
+static int *test_ptr;
+static int memfd = -1;
+
+static void setup(void)
+{
+	test_ptr = SAFE_MMAP(NULL, sizeof(int), PROT_READ | PROT_WRITE,
+			     MAP_PRIVATE | MAP_ANONYMOUS, -1, 0);
+	*test_ptr = 0;
+
+	/* Force /proc/self/mem to require FOLL_FORCE by targeting a read-only page */
+	SAFE_MPROTECT((void *)test_ptr, sizeof(int), PROT_READ);
+
+	memfd = SAFE_OPEN("/proc/self/mem", O_RDWR);
+}
+
+static void run(void)
+{
+	int test_val = 0xdeadbeef;
+
+	SAFE_LSEEK(memfd, (off_t)test_ptr, SEEK_SET);
+	TEST(write(memfd, &test_val, sizeof(test_val)));
+
+	if (TST_RET == -1 && TST_ERR == EIO) {
+		tst_res(TPASS,
+			"Write to /proc/self/mem correctly rejected with EIO");
+		return;
+	}
+
+	if (TST_RET == -1) {
+		tst_res(TFAIL | TERRNO,
+			"Write to /proc/self/mem failed with unexpected error");
+		return;
+	}
+
+	if (TST_RET == sizeof(test_val)) {
+		tst_res(TFAIL,
+			"Write to /proc/self/mem succeeded under CONFIG_PROC_MEM_FORCE_PTRACE=y");
+		return;
+	}
+
+	tst_res(TFAIL,
+		"Short write to /proc/self/mem: %zd bytes (expected %zu or -1)",
+		TST_RET, sizeof(test_val));
+}
+
+static void cleanup(void)
+{
+	if (memfd >= 0)
+		SAFE_CLOSE(memfd);
+
+	if (test_ptr)
+		SAFE_MUNMAP(test_ptr, sizeof(int));
+}
+
+static struct tst_test test = {
+	.test_all = run,
+	.setup = setup,
+	.cleanup = cleanup,
+	.needs_kconfigs = (const char *[]) {
+		"CONFIG_PROC_MEM_FORCE_PTRACE=y",
+		NULL
+	},
+	.tags = (const struct tst_tag[]) {
+		{"linux-git", "41e8149c8892"},
+		{}
+	}
+};
-- 
2.55.0


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