[PATCH v2 4/4] selftests/riscv: shadow stack must not leak on a failed clone3

["Iván Ezequiel Rodriguez" <ivanrwcm25@gmail.com>]

clone3(CLONE_VM|CLONE_PIDFD) with an unwritable pidfd pointer fails
in put_user() after copy_thread() has mapped a shadow stack.
copy_process() then calls exit_thread(). Without HAVE_EXIT_THREAD
that free is a no-op and the VMA stays in the parent's mm.

Use kselftest.h / KTAP. Skip paths that cannot run the regression
run before ksft_set_plan(). Then confirm a valid clone3 writes a
pidfd, reject 16 bad-pidfd clones with -EFAULT, and require the
shadow-stack VMA count (VmFlags " ss") to stay unchanged.

ENOSYS, EINVAL, and EPERM from the valid clone skip. The rejected
child exits with a raw ecall. The test expects /proc already
mounted. The Makefile skips this binary when the compiler cannot
target zicfiss/zicfilp.

Assisted-by: LLM
Signed-off-by: Iván Ezequiel Rodriguez <ivanrwcm25@gmail.com>
---
 tools/testing/selftests/riscv/cfi/Makefile    |   5 +-
 .../selftests/riscv/cfi/shstk-clone-pidfd.c   | 173 ++++++++++++++++++
 2 files changed, 177 insertions(+), 1 deletion(-)
 create mode 100644 tools/testing/selftests/riscv/cfi/shstk-clone-pidfd.c

diff --git a/tools/testing/selftests/riscv/cfi/Makefile b/tools/testing/selftests/riscv/cfi/Makefile
index 418b4b5325a5..dbecb1fd3b82 100644
--- a/tools/testing/selftests/riscv/cfi/Makefile
+++ b/tools/testing/selftests/riscv/cfi/Makefile
@@ -13,10 +13,13 @@ endif
 
 
 ifeq ($(shell $(CC) $(CFLAGS) -nostdlib -xc /dev/null -o /dev/null > /dev/null 2>&1; echo $$?),0)
-TEST_GEN_PROGS := cfitests
+TEST_GEN_PROGS := cfitests shstk-clone-pidfd
 
 $(OUTPUT)/cfitests: cfitests.c shadowstack.c
 	$(CC) -o $@ $(CFLAGS) $(LDFLAGS) $^
+
+$(OUTPUT)/shstk-clone-pidfd: shstk-clone-pidfd.c
+	$(CC) -o $@ $(CFLAGS) $(LDFLAGS) $^
 else
 
 $(shell echo "Toolchain doesn't support CFI, skipping CFI kselftest." >&2)
diff --git a/tools/testing/selftests/riscv/cfi/shstk-clone-pidfd.c b/tools/testing/selftests/riscv/cfi/shstk-clone-pidfd.c
new file mode 100644
index 000000000000..1f16d9c513e6
--- /dev/null
+++ b/tools/testing/selftests/riscv/cfi/shstk-clone-pidfd.c
@@ -0,0 +1,173 @@
+// SPDX-License-Identifier: GPL-2.0
+/*
+ * clone3(CLONE_VM|CLONE_PIDFD) with an unwritable pidfd pointer must
+ * not leak a shadow-stack VMA. copy_thread() maps one, then put_user()
+ * fails and copy_process() calls exit_thread(). Without HAVE_EXIT_THREAD
+ * that free is a no-op and the VMA stays in the parent's mm.
+ *
+ * After enabling the shadow stack, do not return from main(): libc
+ * frames above that point have no shadow-stack entries.
+ */
+#define _GNU_SOURCE
+
+#include <errno.h>
+#include <linux/prctl.h>
+#include <linux/sched.h>
+#include <sched.h>
+#include <signal.h>
+#include <stdint.h>
+#include <stdio.h>
+#include <stdlib.h>
+#include <string.h>
+#include <sys/mman.h>
+#include <sys/syscall.h>
+#include <sys/wait.h>
+#include <unistd.h>
+
+#include "../../kselftest.h"
+#include "cfi_rv_test.h"
+
+#ifndef __NR_clone3
+#define __NR_clone3 435
+#endif
+
+#ifndef PR_GET_SHADOW_STACK_STATUS
+#define PR_GET_SHADOW_STACK_STATUS	74
+#endif
+#ifndef PR_SET_SHADOW_STACK_STATUS
+#define PR_SET_SHADOW_STACK_STATUS	75
+#endif
+#ifndef PR_SHADOW_STACK_ENABLE
+#define PR_SHADOW_STACK_ENABLE		(1UL << 0)
+#endif
+
+#define NCLONE			16
+#define STACK_SIZE		(64 * 1024)
+
+static int count_ss_vmas(void)
+{
+	FILE *f;
+	char line[256];
+	int vmas = 0;
+
+	f = fopen("/proc/self/smaps", "r");
+	if (!f)
+		ksft_exit_fail_msg("fopen /proc/self/smaps: %s\n",
+				   strerror(errno));
+
+	while (fgets(line, sizeof(line), f)) {
+		if (!strncmp(line, "VmFlags:", 8) && strstr(line, " ss"))
+			vmas++;
+	}
+	fclose(f);
+	return vmas;
+}
+
+/*
+ * Raw ecall: the child resumes on an empty shadow stack, so a libc
+ * return after clone3 would fault.
+ */
+static long clone_pidfd(void *stack, size_t stack_size, int *pidfd_ptr)
+{
+	struct clone_args args = {
+		.flags = CLONE_VM | CLONE_PIDFD,
+		.pidfd = (uint64_t)(uintptr_t)pidfd_ptr,
+		.exit_signal = SIGCHLD,
+		.stack = (uint64_t)(uintptr_t)stack,
+		.stack_size = stack_size,
+	};
+	register long num __asm__("a7") = __NR_clone3;
+	register long ret __asm__("a0") = (long)&args;
+	register long sz __asm__("a1") = sizeof(args);
+	int st;
+
+	asm volatile("ecall"
+		     : "+r"(ret)
+		     : "r"(num), "r"(sz)
+		     : "memory", "cc");
+	if (ret == 0) {
+		register long n __asm__("a7") = __NR_exit;
+		register long e __asm__("a0") = 0;
+
+		asm volatile("ecall" : : "r"(n), "r"(e) : "memory");
+		__builtin_unreachable();
+	}
+	if (ret < 0)
+		return ret;
+	if (waitpid((pid_t)ret, &st, 0) < 0)
+		return -errno;
+	if (!WIFEXITED(st) || WEXITSTATUS(st) != 0)
+		return 1;
+	return 0;
+}
+
+int main(void)
+{
+	void *stack;
+	void *bad_pidfd;
+	unsigned long ss_mode = 0;
+	long rc;
+	int pidfd = -1;
+	int vmas0, vmas1;
+	int i, n_efault = 0;
+
+	ksft_print_header();
+
+	stack = mmap(NULL, STACK_SIZE, PROT_READ | PROT_WRITE,
+		     MAP_PRIVATE | MAP_ANONYMOUS | MAP_STACK, -1, 0);
+	if (stack == MAP_FAILED)
+		ksft_exit_fail_msg("mmap stack: %s\n", strerror(errno));
+
+	bad_pidfd = mmap(NULL, 4096, PROT_NONE,
+			 MAP_PRIVATE | MAP_ANONYMOUS, -1, 0);
+	if (bad_pidfd == MAP_FAILED)
+		ksft_exit_fail_msg("mmap pidfd page: %s\n", strerror(errno));
+
+	rc = my_syscall5(__NR_prctl, PR_GET_SHADOW_STACK_STATUS, &ss_mode,
+			 0, 0, 0);
+	if (rc == -ENOSYS || rc == -EINVAL || rc == -EOPNOTSUPP)
+		ksft_exit_skip("shadow stack not available: %ld\n", rc);
+	if (rc)
+		ksft_exit_fail_msg("PR_GET_SHADOW_STACK_STATUS: %ld\n", rc);
+
+	if (!(ss_mode & PR_SHADOW_STACK_ENABLE)) {
+		rc = my_syscall5(__NR_prctl, PR_SET_SHADOW_STACK_STATUS,
+				 PR_SHADOW_STACK_ENABLE, 0, 0, 0);
+		if (rc == -EOPNOTSUPP || rc == -EINVAL || rc == -EBUSY)
+			ksft_exit_skip("cannot enable shadow stack: %ld\n",
+				       rc);
+		if (rc)
+			ksft_exit_fail_msg("PR_SET_SHADOW_STACK_STATUS: %ld\n",
+					   rc);
+	}
+
+	rc = clone_pidfd(stack, STACK_SIZE, &pidfd);
+	if (rc == -ENOSYS || rc == -EINVAL || rc == -EPERM)
+		ksft_exit_skip("clone3 valid pidfd unavailable: %s\n",
+			       strerror(-rc));
+
+	ksft_set_plan(3);
+
+	ksft_test_result(rc == 0 && pidfd >= 0,
+			 "clone3 valid pidfd (rc=%ld pidfd=%d)\n",
+			 rc, pidfd);
+	if (pidfd >= 0)
+		close(pidfd);
+
+	vmas0 = count_ss_vmas();
+
+	for (i = 0; i < NCLONE; i++) {
+		rc = clone_pidfd(stack, STACK_SIZE, bad_pidfd);
+		if (rc == -EFAULT)
+			n_efault++;
+	}
+	ksft_test_result(n_efault == NCLONE,
+			 "clone3 bad pidfd returns -EFAULT x%d\n", NCLONE);
+
+	vmas1 = count_ss_vmas();
+	ksft_test_result(vmas1 == vmas0,
+			 "no shadow-stack VMA leak (%d -> %d)\n",
+			 vmas0, vmas1);
+
+	ksft_finished();
+}
-- 
2.43.0