[PATCH v3 0/4] arm64, riscv: release shadow stacks on failed fork

[PATCH v3 0/4] arm64, riscv: release shadow stacks on failed fork

[Iván Ezequiel Rodriguez]

clone3(CLONE_VM|CLONE_PIDFD) can fail in put_user() after copy_thread()
has mapped a shadow stack into the shared mm. copy_process() then calls
exit_thread(). arm64 and riscv did not implement it, so the VMA leaked.

x86 already frees via exit_thread() -> shstk_free(). Its separate TLS
leak is when copy_thread() fails after the alloc and skips exit_thread().

Changes since v2:
- Move gcs_reset_thread() into process.c (Mark Brown).
- Spell out the x86 vs arm64/riscv difference above.

v2: https://lore.kernel.org/all/cover.1792519121.git.ivanrwcm25@gmail.com/

Assisted-by: LLM


Iván Ezequiel Rodriguez (4):
  arm64/gcs: release GCS on failed fork
  riscv/shstk: release shadow stack on failed fork
  selftests/arm64: GCS must not leak on a failed clone3
  selftests/riscv: shadow stack must not leak on a failed clone3

 arch/arm64/Kconfig                            |   1 +
 arch/arm64/kernel/process.c                   |  16 ++
 arch/riscv/Kconfig                            |   1 +
 arch/riscv/kernel/process.c                   |   9 +
 tools/testing/selftests/arm64/gcs/Makefile    |   3 +-
 .../selftests/arm64/gcs/gcs-clone-pidfd.c     | 168 +++++++++++++++++
 .../testing/selftests/arm64/gcs/gcs-locking.c |  20 --
 tools/testing/selftests/arm64/gcs/gcs-util.h  |  24 +++
 tools/testing/selftests/arm64/gcs/libc-gcs.c  |  20 --
 tools/testing/selftests/riscv/cfi/Makefile    |   5 +-
 .../selftests/riscv/cfi/shstk-clone-pidfd.c   | 173 ++++++++++++++++++
 11 files changed, 398 insertions(+), 42 deletions(-)
 create mode 100644 tools/testing/selftests/arm64/gcs/gcs-clone-pidfd.c
 create mode 100644 tools/testing/selftests/riscv/cfi/shstk-clone-pidfd.c

-- 
2.43.0

[PATCH v3 1/4] arm64/gcs: release GCS on failed fork

[Iván Ezequiel Rodriguez]

copy_thread() can map a CLONE_VM GCS and still return 0. A later
copy_process() failure calls exit_thread(), but arm64 did not
implement it, so the VMA stayed in the parent's mm. Wire
HAVE_EXIT_THREAD to gcs_free().

x86 already does this via exit_thread() -> shstk_free(). The separate
x86 TLS leak is different: copy_thread() itself fails after the
alloc, so copy_process() skips exit_thread().

args->fn tasks inherit GCS tracking without allocating; clear it so
a failed CLONE_VM copy_process() cannot gcs_free() the caller's
stack. Keep that helper in process.c with the other fork helpers.

Fixes: 506496bcbb42 ("arm64/gcs: Ensure that new threads have a GCS")
Cc: stable@vger.kernel.org
Cc: Mark Brown <broonie@kernel.org>
Assisted-by: LLM
Signed-off-by: Iván Ezequiel Rodriguez <ivanrwcm25@gmail.com>
---
 arch/arm64/Kconfig          |  1 +
 arch/arm64/kernel/process.c | 16 ++++++++++++++++
 2 files changed, 17 insertions(+)

diff --git a/arch/arm64/Kconfig b/arch/arm64/Kconfig
index cfef22aee709..3010ced4b4f2 100644
--- a/arch/arm64/Kconfig
+++ b/arch/arm64/Kconfig
@@ -177,6 +177,7 @@ config ARM64
 	select HAVE_ARM_SMCCC
 	select HAVE_ASM_MODVERSIONS
 	select HAVE_EBPF_JIT
+	select HAVE_EXIT_THREAD
 	select HAVE_C_RECORDMCOUNT
 	select HAVE_CMPXCHG_DOUBLE
 	select HAVE_CMPXCHG_LOCAL
diff --git a/arch/arm64/kernel/process.c b/arch/arm64/kernel/process.c
index 581f80e9b9b7..d17172e60acb 100644
--- a/arch/arm64/kernel/process.c
+++ b/arch/arm64/kernel/process.c
@@ -320,6 +320,13 @@ static int copy_thread_gcs(struct task_struct *p,
 	return 0;
 }
 
+static void gcs_reset_thread(struct task_struct *task)
+{
+	task->thread.gcs_base = 0;
+	task->thread.gcs_size = 0;
+	task->thread.gcspr_el0 = 0;
+}
+
 #else
 
 static void flush_gcs(void) { }
@@ -328,6 +335,7 @@ static int copy_thread_gcs(struct task_struct *p,
 {
 	return 0;
 }
+static void gcs_reset_thread(struct task_struct *task) { }
 
 #endif
 
@@ -341,6 +349,12 @@ void flush_thread(void)
 	flush_gcs();
 }
 
+void exit_thread(struct task_struct *tsk)
+{
+	/* Failed fork after copy_thread(); deactivate_mm() does not run. */
+	gcs_free(tsk);
+}
+
 void arch_release_task_struct(struct task_struct *tsk)
 {
 	fpsimd_release_task(tsk);
@@ -503,6 +517,8 @@ int copy_thread(struct task_struct *p, const struct kernel_clone_args *args)
 
 		p->thread.cpu_context.x19 = (unsigned long)args->fn;
 		p->thread.cpu_context.x20 = (unsigned long)args->fn_arg;
+		/* No user GCS alloc here; drop inherited tracking for CLONE_VM. */
+		gcs_reset_thread(p);
 
 		if (system_supports_poe())
 			p->thread.por_el0 = POR_EL0_INIT;
-- 
2.43.0

[PATCH v3 2/4] riscv/shstk: release shadow stack on failed fork

[Iván Ezequiel Rodriguez]

Same hole as arm64: copy_thread() maps a CLONE_VM shadow stack,
copy_process() fails later, exit_thread() ran but was never
implemented. Select HAVE_EXIT_THREAD and call shstk_release().

shstk_release() already documented this path; connect it. Clear
inherited tracking on the args->fn path for the same CLONE_VM
reason as arm64.

Fixes: fd44a4a85516 ("riscv/shstk: If needed allocate a new shadow stack on clone")
Cc: stable@vger.kernel.org
Cc: Deepak Gupta <debug@rivosinc.com>
Assisted-by: LLM
Signed-off-by: Iván Ezequiel Rodriguez <ivanrwcm25@gmail.com>
---
 arch/riscv/Kconfig          | 1 +
 arch/riscv/kernel/process.c | 9 +++++++++
 2 files changed, 10 insertions(+)

diff --git a/arch/riscv/Kconfig b/arch/riscv/Kconfig
index 6c9d61d83d71..ba1ee563aa54 100644
--- a/arch/riscv/Kconfig
+++ b/arch/riscv/Kconfig
@@ -168,6 +168,7 @@ config RISCV
 	select HAVE_FUNCTION_GRAPH_FREGS
 	select HAVE_FUNCTION_TRACER if HAVE_DYNAMIC_FTRACE
 	select HAVE_EBPF_JIT if MMU
+	select HAVE_EXIT_THREAD
 	select HAVE_GENERIC_TIF_BITS
 	select HAVE_GUP_FAST if MMU
 	select HAVE_FUNCTION_ARG_ACCESS_API
diff --git a/arch/riscv/kernel/process.c b/arch/riscv/kernel/process.c
index 7cc5a6a5c020..5bb93377d67f 100644
--- a/arch/riscv/kernel/process.c
+++ b/arch/riscv/kernel/process.c
@@ -12,6 +12,7 @@
 #include <linux/kernel.h>
 #include <linux/sched.h>
 #include <linux/sched/debug.h>
+#include <linux/sched/task.h>
 #include <linux/sched/task_stack.h>
 #include <linux/tick.h>
 #include <linux/ptrace.h>
@@ -213,6 +214,12 @@ void arch_release_task_struct(struct task_struct *tsk)
 		riscv_v_thread_free(tsk);
 }
 
+void exit_thread(struct task_struct *tsk)
+{
+	/* Failed fork after copy_thread(); deactivate_mm() does not run. */
+	shstk_release(tsk);
+}
+
 int arch_dup_task_struct(struct task_struct *dst, struct task_struct *src)
 {
 	fstate_save(src, task_pt_regs(src));
@@ -261,6 +268,8 @@ int copy_thread(struct task_struct *p, const struct kernel_clone_args *args)
 		p->thread.s[0] = (unsigned long)args->fn;
 		p->thread.s[1] = (unsigned long)args->fn_arg;
 		p->thread.ra = (unsigned long)ret_from_fork_kernel_asm;
+		/* No user shstk alloc here; drop inherited tracking for CLONE_VM. */
+		set_shstk_base(p, 0, 0);
 	} else {
 		/* allocate new shadow stack if needed. In case of CLONE_VM we have to */
 		ssp = shstk_alloc_thread_stack(p, args);
-- 
2.43.0

[PATCH v3 3/4] selftests/arm64: GCS must not leak on a failed clone3

[Iván Ezequiel Rodriguez]

KTAP coverage for clone3(CLONE_VM|CLONE_PIDFD) with a bad pidfd
pointer: sixteen failures must not grow the parent's GCS VMA count.

Assisted-by: LLM
Signed-off-by: Iván Ezequiel Rodriguez <ivanrwcm25@gmail.com>
---
 tools/testing/selftests/arm64/gcs/Makefile    |   3 +-
 .../selftests/arm64/gcs/gcs-clone-pidfd.c     | 168 ++++++++++++++++++
 .../testing/selftests/arm64/gcs/gcs-locking.c |  20 ---
 tools/testing/selftests/arm64/gcs/gcs-util.h  |  24 +++
 tools/testing/selftests/arm64/gcs/libc-gcs.c  |  20 ---
 5 files changed, 194 insertions(+), 41 deletions(-)
 create mode 100644 tools/testing/selftests/arm64/gcs/gcs-clone-pidfd.c

diff --git a/tools/testing/selftests/arm64/gcs/Makefile b/tools/testing/selftests/arm64/gcs/Makefile
index 1fbbf0ca1f02..f1365eea31c9 100644
--- a/tools/testing/selftests/arm64/gcs/Makefile
+++ b/tools/testing/selftests/arm64/gcs/Makefile
@@ -6,7 +6,8 @@
 # nolibc.
 #
 
-TEST_GEN_PROGS := basic-gcs libc-gcs gcs-locking gcs-stress gcspushm gcsstr
+TEST_GEN_PROGS := basic-gcs libc-gcs gcs-locking gcs-stress gcspushm gcsstr \
+		  gcs-clone-pidfd
 TEST_GEN_PROGS_EXTENDED := gcs-stress-thread
 
 LDLIBS+=-lpthread
diff --git a/tools/testing/selftests/arm64/gcs/gcs-clone-pidfd.c b/tools/testing/selftests/arm64/gcs/gcs-clone-pidfd.c
new file mode 100644
index 000000000000..96400ea376c0
--- /dev/null
+++ b/tools/testing/selftests/arm64/gcs/gcs-clone-pidfd.c
@@ -0,0 +1,168 @@
+// SPDX-License-Identifier: GPL-2.0
+/*
+ * clone3(CLONE_VM|CLONE_PIDFD) with an unwritable pidfd pointer must
+ * not leak a GCS 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 GCS, do not return from main(): libc frames above
+ * that point have no GCS entries.
+ */
+#define _GNU_SOURCE
+
+#include <errno.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/auxv.h>
+#include <sys/mman.h>
+#include <sys/syscall.h>
+#include <sys/wait.h>
+#include <unistd.h>
+
+#include <asm/hwcap.h>
+
+#include "kselftest.h"
+#include "gcs-util.h"
+
+#ifndef HWCAP_GCS
+#define HWCAP_GCS		(1UL << 32)
+#endif
+
+#ifndef __NR_clone3
+#define __NR_clone3 435
+#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;
+}
+
+/*
+ * CLONE_VM maps a new GCS. CLONE_PIDFD lets put_user() fail after that.
+ * Use a raw svc: the child resumes on an empty GCS, so a libc return
+ * after clone3 would fault; exit the same way.
+ */
+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__("x8") = __NR_clone3;
+	register long ret __asm__("x0") = (long)&args;
+	register long sz __asm__("x1") = sizeof(args);
+	int st;
+
+	asm volatile("svc #0"
+		     : "+r"(ret)
+		     : "r"(num), "r"(sz)
+		     : "memory", "cc");
+	if (ret == 0) {
+		register long n __asm__("x8") = __NR_exit;
+		register long e __asm__("x0") = 0;
+
+		asm volatile("svc #0" : : "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 gcs_mode;
+	long rc;
+	int pidfd = -1;
+	int vmas0, vmas1;
+	int i, n_efault = 0;
+
+	ksft_print_header();
+
+	if (!(getauxval(AT_HWCAP) & HWCAP_GCS))
+		ksft_exit_skip("GCS not supported\n");
+
+	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_syscall2(__NR_prctl, PR_GET_SHADOW_STACK_STATUS, &gcs_mode);
+	if (rc)
+		ksft_exit_fail_msg("PR_GET_SHADOW_STACK_STATUS: %ld\n", rc);
+
+	if (!(gcs_mode & PR_SHADOW_STACK_ENABLE)) {
+		rc = my_syscall2(__NR_prctl, PR_SET_SHADOW_STACK_STATUS,
+				 PR_SHADOW_STACK_ENABLE);
+		if (rc == -EOPNOTSUPP || rc == -EINVAL || rc == -EBUSY)
+			ksft_exit_skip("cannot enable GCS: %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 GCS VMA leak (%d -> %d)\n", vmas0, vmas1);
+
+	ksft_finished();
+}
diff --git a/tools/testing/selftests/arm64/gcs/gcs-locking.c b/tools/testing/selftests/arm64/gcs/gcs-locking.c
index 80a1317f6097..8877fd2d2eab 100644
--- a/tools/testing/selftests/arm64/gcs/gcs-locking.c
+++ b/tools/testing/selftests/arm64/gcs/gcs-locking.c
@@ -18,26 +18,6 @@
 
 #include "gcs-util.h"
 
-#define my_syscall2(num, arg1, arg2)                                          \
-({                                                                            \
-	register long _num  __asm__ ("x8") = (num);                           \
-	register long _arg1 __asm__ ("x0") = (long)(arg1);                    \
-	register long _arg2 __asm__ ("x1") = (long)(arg2);                    \
-	register long _arg3 __asm__ ("x2") = 0;                               \
-	register long _arg4 __asm__ ("x3") = 0;                               \
-	register long _arg5 __asm__ ("x4") = 0;                               \
-	                                                                      \
-	__asm__  volatile (                                                   \
-		"svc #0\n"                                                    \
-		: "=r"(_arg1)                                                 \
-		: "r"(_arg1), "r"(_arg2),                                     \
-		  "r"(_arg3), "r"(_arg4),                                     \
-		  "r"(_arg5), "r"(_num)					      \
-		: "memory", "cc"                                              \
-	);                                                                    \
-	_arg1;                                                                \
-})
-
 /* No mode bits are rejected for locking */
 TEST(lock_all_modes)
 {
diff --git a/tools/testing/selftests/arm64/gcs/gcs-util.h b/tools/testing/selftests/arm64/gcs/gcs-util.h
index 7a81bb07ed4b..46f6ed9cf54d 100644
--- a/tools/testing/selftests/arm64/gcs/gcs-util.h
+++ b/tools/testing/selftests/arm64/gcs/gcs-util.h
@@ -43,6 +43,30 @@
 #define GCS_CAP(x) (((unsigned long)(x) & GCS_CAP_ADDR_MASK) | \
 		    GCS_CAP_VALID_TOKEN)
 
+/*
+ * prctl(..., arg1, arg2) with unused registers forced to zero.  The
+ * kernel rejects non-zero unused args; libc may leave garbage there.
+ */
+#define my_syscall2(num, arg1, arg2)					\
+({									\
+	register long _num  __asm__ ("x8") = (num);			\
+	register long _arg1 __asm__ ("x0") = (long)(arg1);		\
+	register long _arg2 __asm__ ("x1") = (long)(arg2);		\
+	register long _arg3 __asm__ ("x2") = 0;				\
+	register long _arg4 __asm__ ("x3") = 0;				\
+	register long _arg5 __asm__ ("x4") = 0;				\
+									\
+	__asm__ volatile (						\
+		"svc #0\n"						\
+		: "=r"(_arg1)						\
+		: "r"(_arg1), "r"(_arg2),				\
+		  "r"(_arg3), "r"(_arg4),				\
+		  "r"(_arg5), "r"(_num)					\
+		: "memory", "cc"					\
+	);								\
+	_arg1;								\
+})
+
 static inline unsigned long *get_gcspr(void)
 {
 	unsigned long *gcspr;
diff --git a/tools/testing/selftests/arm64/gcs/libc-gcs.c b/tools/testing/selftests/arm64/gcs/libc-gcs.c
index ddb0b2b20155..4e9fa9b82f0f 100644
--- a/tools/testing/selftests/arm64/gcs/libc-gcs.c
+++ b/tools/testing/selftests/arm64/gcs/libc-gcs.c
@@ -24,26 +24,6 @@
 
 #include "gcs-util.h"
 
-#define my_syscall2(num, arg1, arg2)                                          \
-({                                                                            \
-	register long _num  __asm__ ("x8") = (num);                           \
-	register long _arg1 __asm__ ("x0") = (long)(arg1);                    \
-	register long _arg2 __asm__ ("x1") = (long)(arg2);                    \
-	register long _arg3 __asm__ ("x2") = 0;                               \
-	register long _arg4 __asm__ ("x3") = 0;                               \
-	register long _arg5 __asm__ ("x4") = 0;                               \
-	                                                                      \
-	__asm__  volatile (                                                   \
-		"svc #0\n"                                                    \
-		: "=r"(_arg1)                                                 \
-		: "r"(_arg1), "r"(_arg2),                                     \
-		  "r"(_arg3), "r"(_arg4),                                     \
-		  "r"(_arg5), "r"(_num)					      \
-		: "memory", "cc"                                              \
-	);                                                                    \
-	_arg1;                                                                \
-})
-
 static noinline void gcs_recurse(int depth)
 {
 	if (depth)
-- 
2.43.0

[PATCH v3 4/4] selftests/riscv: shadow stack must not leak on a failed clone3

[Iván Ezequiel Rodriguez]

Same clone3 bad-pidfd check as the arm64 GCS selftest.

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

Re: [PATCH v3 3/4] selftests/arm64: GCS must not leak on a failed clone3

[Mark Brown]

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On Wed, Oct 28, 2026 at 12:43:14PM -0300, Iván Ezequiel Rodriguez wrote:
> KTAP coverage for clone3(CLONE_VM|CLONE_PIDFD) with a bad pidfd
> pointer: sixteen failures must not grow the parent's GCS VMA count.

Thanks, this is a lot cleaner and easier to understand than the original
version.  It'd probably be nicer to do the refactoring of the existing
code in a separate commit but that's more nice to have than essential.

> +TEST_GEN_PROGS := basic-gcs libc-gcs gcs-locking gcs-stress gcspushm gcsstr \
> +                 gcs-clone-pidfd

We should update .gitignore as well (or merge the test into libc-gcs but
that's a bunch more work and definitely into bikeshed territory).

I verified that this fails with the current kernel and that the kernel
change causes it to pass so with the above:

Reviewed-by: Mark Brown <broonie@kernel.org>
Tested-by: Mark Brown <broonie@kernel.org>

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Re: [PATCH v3 1/4] arm64/gcs: release GCS on failed fork

[Mark Brown]

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On Wed, Oct 28, 2026 at 12:43:12PM -0300, Iván Ezequiel Rodriguez wrote:
> copy_thread() can map a CLONE_VM GCS and still return 0. A later
> copy_process() failure calls exit_thread(), but arm64 did not
> implement it, so the VMA stayed in the parent's mm. Wire
> HAVE_EXIT_THREAD to gcs_free().

One extremely minor nit below, I verified that this fails with the
current kernel and that the kernel change causes it to pass so with or
without dealing with that:

Reviewed-by: Mark Brown <broonie@kernel.org>
Tested-by: Mark Brown <broonie@kernel.org>

> @@ -328,6 +335,7 @@ static int copy_thread_gcs(struct task_struct *p,
>  {
>  	return 0;
>  }
> +static void gcs_reset_thread(struct task_struct *task) { }
>  

This looks like there's a missing blank line but it fits in with the
missing blank before the stub for copy_thread_gcs().  Possibly slightly
neater to put this before there next to the similar flush_gcs() stub but
meh.

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