[PATCH RFT v17 0/8] fork: Support shadow stacks in clone3()

[PATCH RFT v17 0/8] fork: Support shadow stacks in clone3()

[Mark Brown]

The kernel has recently added support for shadow stacks, currently
x86 only using their CET feature but both arm64 and RISC-V have
equivalent features (GCS and Zicfiss respectively), I am actively
working on GCS[1].  With shadow stacks the hardware maintains an
additional stack containing only the return addresses for branch
instructions which is not generally writeable by userspace and ensures
that any returns are to the recorded addresses.  This provides some
protection against ROP attacks and making it easier to collect call
stacks.  These shadow stacks are allocated in the address space of the
userspace process.

Our API for shadow stacks does not currently offer userspace any
flexiblity for managing the allocation of shadow stacks for newly
created threads, instead the kernel allocates a new shadow stack with
the same size as the normal stack whenever a thread is created with the
feature enabled.  The stacks allocated in this way are freed by the
kernel when the thread exits or shadow stacks are disabled for the
thread.  This lack of flexibility and control isn't ideal, in the vast
majority of cases the shadow stack will be over allocated and the
implicit allocation and deallocation is not consistent with other
interfaces.  As far as I can tell the interface is done in this manner
mainly because the shadow stack patches were in development since before
clone3() was implemented.

Since clone3() is readily extensible let's add support for specifying a
shadow stack when creating a new thread or process, keeping the current
implicit allocation behaviour if one is not specified either with
clone3() or through the use of clone().  The user must provide a shadow
stack pointer, this must point to memory mapped for use as a shadow
stackby map_shadow_stack() with an architecture specified shadow stack
token at the top of the stack.

Yuri Khrustalev has raised questions from the libc side regarding
discoverability of extended clone3() structure sizes[2], this seems like
a general issue with clone3().  There was a suggestion to add a hwcap on
arm64 which isn't ideal but is doable there, though architecture
specific mechanisms would also be needed for x86 (and RISC-V if it's
support gets merged before this does).

Please note that the x86 portions of this code are build tested only, I
don't appear to have a system that can run CET available to me.

[1] https://lore.kernel.org/linux-arm-kernel/20241001-arm64-gcs-v13-0-222b78d87eee@kernel.org/T/#mc58f97f27461749ccf400ebabf6f9f937116a86b
[2] https://lore.kernel.org/r/aCs65ccRQtJBnZ_5@arm.com

Signed-off-by: Mark Brown <broonie@kernel.org>
---
Changes in v17:
- Rebase onto v6.16-rc1.
- Link to v16: https://lore.kernel.org/r/20250416-clone3-shadow-stack-v16-0-2ffc9ca3917b@kernel.org

Changes in v16:
- Rebase onto v6.15-rc2.
- Roll in fixes from x86 testing from Rick Edgecombe.
- Rework so that the argument is shadow_stack_token.
- Link to v15: https://lore.kernel.org/r/20250408-clone3-shadow-stack-v15-0-3fa245c6e3be@kernel.org

Changes in v15:
- Rebase onto v6.15-rc1.
- Link to v14: https://lore.kernel.org/r/20250206-clone3-shadow-stack-v14-0-805b53af73b9@kernel.org

Changes in v14:
- Rebase onto v6.14-rc1.
- Link to v13: https://lore.kernel.org/r/20241203-clone3-shadow-stack-v13-0-93b89a81a5ed@kernel.org

Changes in v13:
- Rebase onto v6.13-rc1.
- Link to v12: https://lore.kernel.org/r/20241031-clone3-shadow-stack-v12-0-7183eb8bee17@kernel.org

Changes in v12:
- Add the regular prctl() to the userspace API document since arm64
  support is queued in -next.
- Link to v11: https://lore.kernel.org/r/20241005-clone3-shadow-stack-v11-0-2a6a2bd6d651@kernel.org

Changes in v11:
- Rebase onto arm64 for-next/gcs, which is based on v6.12-rc1, and
  integrate arm64 support.
- Rework the interface to specify a shadow stack pointer rather than a
  base and size like we do for the regular stack.
- Link to v10: https://lore.kernel.org/r/20240821-clone3-shadow-stack-v10-0-06e8797b9445@kernel.org

Changes in v10:
- Integrate fixes & improvements for the x86 implementation from Rick
  Edgecombe.
- Require that the shadow stack be VM_WRITE.
- Require that the shadow stack base and size be sizeof(void *) aligned.
- Clean up trailing newline.
- Link to v9: https://lore.kernel.org/r/20240819-clone3-shadow-stack-v9-0-962d74f99464@kernel.org

Changes in v9:
- Pull token validation earlier and report problems with an error return
  to parent rather than signal delivery to the child.
- Verify that the top of the supplied shadow stack is VM_SHADOW_STACK.
- Rework token validation to only do the page mapping once.
- Drop no longer needed support for testing for signals in selftest.
- Fix typo in comments.
- Link to v8: https://lore.kernel.org/r/20240808-clone3-shadow-stack-v8-0-0acf37caf14c@kernel.org

Changes in v8:
- Fix token verification with user specified shadow stack.
- Don't track user managed shadow stacks for child processes.
- Link to v7: https://lore.kernel.org/r/20240731-clone3-shadow-stack-v7-0-a9532eebfb1d@kernel.org

Changes in v7:
- Rebase onto v6.11-rc1.
- Typo fixes.
- Link to v6: https://lore.kernel.org/r/20240623-clone3-shadow-stack-v6-0-9ee7783b1fb9@kernel.org

Changes in v6:
- Rebase onto v6.10-rc3.
- Ensure we don't try to free the parent shadow stack in error paths of
  x86 arch code.
- Spelling fixes in userspace API document.
- Additional cleanups and improvements to the clone3() tests to support
  the shadow stack tests.
- Link to v5: https://lore.kernel.org/r/20240203-clone3-shadow-stack-v5-0-322c69598e4b@kernel.org

Changes in v5:
- Rebase onto v6.8-rc2.
- Rework ABI to have the user allocate the shadow stack memory with
  map_shadow_stack() and a token.
- Force inlining of the x86 shadow stack enablement.
- Move shadow stack enablement out into a shared header for reuse by
  other tests.
- Link to v4: https://lore.kernel.org/r/20231128-clone3-shadow-stack-v4-0-8b28ffe4f676@kernel.org

Changes in v4:
- Formatting changes.
- Use a define for minimum shadow stack size and move some basic
  validation to fork.c.
- Link to v3: https://lore.kernel.org/r/20231120-clone3-shadow-stack-v3-0-a7b8ed3e2acc@kernel.org

Changes in v3:
- Rebase onto v6.7-rc2.
- Remove stale shadow_stack in internal kargs.
- If a shadow stack is specified unconditionally use it regardless of
  CLONE_ parameters.
- Force enable shadow stacks in the selftest.
- Update changelogs for RISC-V feature rename.
- Link to v2: https://lore.kernel.org/r/20231114-clone3-shadow-stack-v2-0-b613f8681155@kernel.org

Changes in v2:
- Rebase onto v6.7-rc1.
- Remove ability to provide preallocated shadow stack, just specify the
  desired size.
- Link to v1: https://lore.kernel.org/r/20231023-clone3-shadow-stack-v1-0-d867d0b5d4d0@kernel.org

---
Mark Brown (8):
      arm64/gcs: Return a success value from gcs_alloc_thread_stack()
      Documentation: userspace-api: Add shadow stack API documentation
      selftests: Provide helper header for shadow stack testing
      fork: Add shadow stack support to clone3()
      selftests/clone3: Remove redundant flushes of output streams
      selftests/clone3: Factor more of main loop into test_clone3()
      selftests/clone3: Allow tests to flag if -E2BIG is a valid error code
      selftests/clone3: Test shadow stack support

 Documentation/userspace-api/index.rst             |   1 +
 Documentation/userspace-api/shadow_stack.rst      |  44 +++++
 arch/arm64/include/asm/gcs.h                      |   8 +-
 arch/arm64/kernel/process.c                       |   8 +-
 arch/arm64/mm/gcs.c                               |  61 +++++-
 arch/x86/include/asm/shstk.h                      |  11 +-
 arch/x86/kernel/process.c                         |   2 +-
 arch/x86/kernel/shstk.c                           |  57 +++++-
 include/asm-generic/cacheflush.h                  |  11 ++
 include/linux/sched/task.h                        |  17 ++
 include/uapi/linux/sched.h                        |   9 +-
 kernel/fork.c                                     |  96 +++++++--
 tools/testing/selftests/clone3/clone3.c           | 226 ++++++++++++++++++----
 tools/testing/selftests/clone3/clone3_selftests.h |  65 ++++++-
 tools/testing/selftests/ksft_shstk.h              |  98 ++++++++++
 15 files changed, 633 insertions(+), 81 deletions(-)
---
base-commit: 19272b37aa4f83ca52bdf9c16d5d81bdd1354494
change-id: 20231019-clone3-shadow-stack-15d40d2bf536

Best regards,
-- 
Mark Brown <broonie@kernel.org>

[PATCH RFT v17 1/8] arm64/gcs: Return a success value from gcs_alloc_thread_stack()

[Mark Brown]

Currently as a result of templating from x86 code gcs_alloc_thread_stack()
returns a pointer as an unsigned int however on arm64 we don't actually use
this pointer value as anything other than a pass/fail flag. Simplify the
interface to just return an int with 0 on success and a negative error code
on failure.

Acked-by: Deepak Gupta <debug@rivosinc.com>
Reviewed-by: Catalin Marinas <catalin.marinas@arm.com>
Signed-off-by: Mark Brown <broonie@kernel.org>
---
 arch/arm64/include/asm/gcs.h | 8 ++++----
 arch/arm64/kernel/process.c  | 8 ++++----
 arch/arm64/mm/gcs.c          | 8 ++++----
 3 files changed, 12 insertions(+), 12 deletions(-)

diff --git a/arch/arm64/include/asm/gcs.h b/arch/arm64/include/asm/gcs.h
index f50660603ecf..d8923b5f03b7 100644
--- a/arch/arm64/include/asm/gcs.h
+++ b/arch/arm64/include/asm/gcs.h
@@ -64,8 +64,8 @@ static inline bool task_gcs_el0_enabled(struct task_struct *task)
 void gcs_set_el0_mode(struct task_struct *task);
 void gcs_free(struct task_struct *task);
 void gcs_preserve_current_state(void);
-unsigned long gcs_alloc_thread_stack(struct task_struct *tsk,
-				     const struct kernel_clone_args *args);
+int gcs_alloc_thread_stack(struct task_struct *tsk,
+			   const struct kernel_clone_args *args);
 
 static inline int gcs_check_locked(struct task_struct *task,
 				   unsigned long new_val)
@@ -91,8 +91,8 @@ static inline bool task_gcs_el0_enabled(struct task_struct *task)
 static inline void gcs_set_el0_mode(struct task_struct *task) { }
 static inline void gcs_free(struct task_struct *task) { }
 static inline void gcs_preserve_current_state(void) { }
-static inline unsigned long gcs_alloc_thread_stack(struct task_struct *tsk,
-						   const struct kernel_clone_args *args)
+static inline int gcs_alloc_thread_stack(struct task_struct *tsk,
+					 const struct kernel_clone_args *args)
 {
 	return -ENOTSUPP;
 }
diff --git a/arch/arm64/kernel/process.c b/arch/arm64/kernel/process.c
index a5ca15daeb8a..87e1547d7abf 100644
--- a/arch/arm64/kernel/process.c
+++ b/arch/arm64/kernel/process.c
@@ -297,7 +297,7 @@ static void flush_gcs(void)
 static int copy_thread_gcs(struct task_struct *p,
 			   const struct kernel_clone_args *args)
 {
-	unsigned long gcs;
+	int ret;
 
 	if (!system_supports_gcs())
 		return 0;
@@ -305,9 +305,9 @@ static int copy_thread_gcs(struct task_struct *p,
 	p->thread.gcs_base = 0;
 	p->thread.gcs_size = 0;
 
-	gcs = gcs_alloc_thread_stack(p, args);
-	if (IS_ERR_VALUE(gcs))
-		return PTR_ERR((void *)gcs);
+	ret = gcs_alloc_thread_stack(p, args);
+	if (ret != 0)
+		return ret;
 
 	p->thread.gcs_el0_mode = current->thread.gcs_el0_mode;
 	p->thread.gcs_el0_locked = current->thread.gcs_el0_locked;
diff --git a/arch/arm64/mm/gcs.c b/arch/arm64/mm/gcs.c
index 5c46ec527b1c..1f633a482558 100644
--- a/arch/arm64/mm/gcs.c
+++ b/arch/arm64/mm/gcs.c
@@ -38,8 +38,8 @@ static unsigned long gcs_size(unsigned long size)
 	return max(PAGE_SIZE, size);
 }
 
-unsigned long gcs_alloc_thread_stack(struct task_struct *tsk,
-				     const struct kernel_clone_args *args)
+int gcs_alloc_thread_stack(struct task_struct *tsk,
+			   const struct kernel_clone_args *args)
 {
 	unsigned long addr, size;
 
@@ -59,13 +59,13 @@ unsigned long gcs_alloc_thread_stack(struct task_struct *tsk,
 	size = gcs_size(size);
 	addr = alloc_gcs(0, size);
 	if (IS_ERR_VALUE(addr))
-		return addr;
+		return PTR_ERR((void *)addr);
 
 	tsk->thread.gcs_base = addr;
 	tsk->thread.gcs_size = size;
 	tsk->thread.gcspr_el0 = addr + size - sizeof(u64);
 
-	return addr;
+	return 0;
 }
 
 SYSCALL_DEFINE3(map_shadow_stack, unsigned long, addr, unsigned long, size, unsigned int, flags)

-- 
2.39.5

[PATCH RFT v17 2/8] Documentation: userspace-api: Add shadow stack API documentation

[Mark Brown]

There are a number of architectures with shadow stack features which we are
presenting to userspace with as consistent an API as we can (though there
are some architecture specifics). Especially given that there are some
important considerations for userspace code interacting directly with the
feature let's provide some documentation covering the common aspects.

Reviewed-by: Catalin Marinas <catalin.marinas@arm.com>
Reviewed-by: Kees Cook <kees@kernel.org>
Tested-by: Kees Cook <kees@kernel.org>
Acked-by: Shuah Khan <skhan@linuxfoundation.org>
Acked-by: Yury Khrustalev <yury.khrustalev@arm.com>
Reviewed-by: Deepak Gupta <debug@rivosinc.com>
Signed-off-by: Mark Brown <broonie@kernel.org>
---
 Documentation/userspace-api/index.rst        |  1 +
 Documentation/userspace-api/shadow_stack.rst | 44 ++++++++++++++++++++++++++++
 2 files changed, 45 insertions(+)

diff --git a/Documentation/userspace-api/index.rst b/Documentation/userspace-api/index.rst
index b8c73be4fb11..0167e59b541e 100644
--- a/Documentation/userspace-api/index.rst
+++ b/Documentation/userspace-api/index.rst
@@ -62,6 +62,7 @@ Everything else
 
    ELF
    netlink/index
+   shadow_stack
    sysfs-platform_profile
    vduse
    futex2
diff --git a/Documentation/userspace-api/shadow_stack.rst b/Documentation/userspace-api/shadow_stack.rst
new file mode 100644
index 000000000000..65c665496624
--- /dev/null
+++ b/Documentation/userspace-api/shadow_stack.rst
@@ -0,0 +1,44 @@
+.. SPDX-License-Identifier: GPL-2.0
+
+=============
+Shadow Stacks
+=============
+
+Introduction
+============
+
+Several architectures have features which provide backward edge
+control flow protection through a hardware maintained stack, only
+writeable by userspace through very limited operations.  This feature
+is referred to as shadow stacks on Linux, on x86 it is part of Intel
+Control Enforcement Technology (CET), on arm64 it is Guarded Control
+Stacks feature (FEAT_GCS) and for RISC-V it is the Zicfiss extension.
+It is expected that this feature will normally be managed by the
+system dynamic linker and libc in ways broadly transparent to
+application code, this document covers interfaces and considerations.
+
+
+Enabling
+========
+
+Shadow stacks default to disabled when a userspace process is
+executed, they can be enabled for the current thread with a syscall:
+
+ - For x86 the ARCH_SHSTK_ENABLE arch_prctl()
+ - For other architectures the PR_SET_SHADOW_STACK_ENABLE prctl()
+
+It is expected that this will normally be done by the dynamic linker.
+Any new threads created by a thread with shadow stacks enabled will
+themselves have shadow stacks enabled.
+
+
+Enablement considerations
+=========================
+
+- Returning from the function that enables shadow stacks without first
+  disabling them will cause a shadow stack exception.  This includes
+  any syscall wrapper or other library functions, the syscall will need
+  to be inlined.
+- A lock feature allows userspace to prevent disabling of shadow stacks.
+- Those that change the stack context like longjmp() or use of ucontext
+  changes on signal return will need support from libc.

-- 
2.39.5

[PATCH RFT v17 3/8] selftests: Provide helper header for shadow stack testing

[Mark Brown]

While almost all users of shadow stacks should be relying on the dynamic
linker and libc to enable the feature there are several low level test
programs where it is useful to enable without any libc support, allowing
testing without full system enablement. This low level testing is helpful
during bringup of the support itself, and also in enabling coverage by
automated testing without needing all system components in the target root
filesystems to have enablement.

Provide a header with helpers for this purpose, intended for use only by
test programs directly exercising shadow stack interfaces.

Reviewed-by: Rick Edgecombe <rick.p.edgecombe@intel.com>
Reviewed-by: Kees Cook <kees@kernel.org>
Tested-by: Kees Cook <kees@kernel.org>
Acked-by: Shuah Khan <skhan@linuxfoundation.org>
Signed-off-by: Mark Brown <broonie@kernel.org>
---
 tools/testing/selftests/ksft_shstk.h | 98 ++++++++++++++++++++++++++++++++++++
 1 file changed, 98 insertions(+)

diff --git a/tools/testing/selftests/ksft_shstk.h b/tools/testing/selftests/ksft_shstk.h
new file mode 100644
index 000000000000..fecf91218ea5
--- /dev/null
+++ b/tools/testing/selftests/ksft_shstk.h
@@ -0,0 +1,98 @@
+/* SPDX-License-Identifier: GPL-2.0-only */
+/*
+ * Helpers for shadow stack enablement, this is intended to only be
+ * used by low level test programs directly exercising interfaces for
+ * working with shadow stacks.
+ *
+ * Copyright (C) 2024 ARM Ltd.
+ */
+
+#ifndef __KSFT_SHSTK_H
+#define __KSFT_SHSTK_H
+
+#include <asm/mman.h>
+
+/* This is currently only defined for x86 */
+#ifndef SHADOW_STACK_SET_TOKEN
+#define SHADOW_STACK_SET_TOKEN (1ULL << 0)
+#endif
+
+static bool shadow_stack_enabled;
+
+#ifdef __x86_64__
+#define ARCH_SHSTK_ENABLE	0x5001
+#define ARCH_SHSTK_SHSTK	(1ULL <<  0)
+
+#define ARCH_PRCTL(arg1, arg2)					\
+({								\
+	long _ret;						\
+	register long _num  asm("eax") = __NR_arch_prctl;	\
+	register long _arg1 asm("rdi") = (long)(arg1);		\
+	register long _arg2 asm("rsi") = (long)(arg2);		\
+								\
+	asm volatile (						\
+		"syscall\n"					\
+		: "=a"(_ret)					\
+		: "r"(_arg1), "r"(_arg2),			\
+		  "0"(_num)					\
+		: "rcx", "r11", "memory", "cc"			\
+	);							\
+	_ret;							\
+})
+
+#define ENABLE_SHADOW_STACK
+static __always_inline void enable_shadow_stack(void)
+{
+	int ret = ARCH_PRCTL(ARCH_SHSTK_ENABLE, ARCH_SHSTK_SHSTK);
+	if (ret == 0)
+		shadow_stack_enabled = true;
+}
+
+#endif
+
+#ifdef __aarch64__
+#define PR_SET_SHADOW_STACK_STATUS      75
+# define PR_SHADOW_STACK_ENABLE         (1UL << 0)
+
+#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;                                                                \
+})
+
+#define ENABLE_SHADOW_STACK
+static __always_inline void enable_shadow_stack(void)
+{
+	int ret;
+
+	ret = my_syscall2(__NR_prctl, PR_SET_SHADOW_STACK_STATUS,
+			  PR_SHADOW_STACK_ENABLE);
+	if (ret == 0)
+		shadow_stack_enabled = true;
+}
+
+#endif
+
+#ifndef __NR_map_shadow_stack
+#define __NR_map_shadow_stack 453
+#endif
+
+#ifndef ENABLE_SHADOW_STACK
+static inline void enable_shadow_stack(void) { }
+#endif
+
+#endif

-- 
2.39.5

[PATCH RFT v17 4/8] fork: Add shadow stack support to clone3()

[Mark Brown]

Unlike with the normal stack there is no API for configuring the shadow
stack for a new thread, instead the kernel will dynamically allocate a
new shadow stack with the same size as the normal stack. This appears to
be due to the shadow stack series having been in development since
before the more extensible clone3() was added rather than anything more
deliberate.

Add a parameter to clone3() specifying the shadow stack pointer to use
for the new thread, this is inconsistent with the way we specify the
normal stack but during review concerns were expressed about having to
identify where the shadow stack pointer should be placed especially in
cases where the shadow stack has been previously active.  If no shadow
stack is specified then the existing implicit allocation behaviour is
maintained.

If a shadow stack pointer is specified then it is required to have an
architecture defined token placed on the stack, this will be consumed by
the new task.  If no valid token is present then this will be reported
with -EINVAL.  This token prevents new threads being created pointing at
the shadow stack of an existing running thread.

If the architecture does not support shadow stacks the shadow stack
pointer must be not be specified, architectures that do support the
feature are expected to enforce the same requirement on individual
systems that lack shadow stack support.

Update the existing arm64 and x86 implementations to pay attention to
the newly added arguments, in order to maintain compatibility we use the
existing behaviour if no shadow stack is specified. Since we are now
using more fields from the kernel_clone_args we pass that into the
shadow stack code rather than individual fields.

Portions of the x86 architecture code were written by Rick Edgecombe.

Acked-by: Yury Khrustalev <yury.khrustalev@arm.com>
Signed-off-by: Mark Brown <broonie@kernel.org>
---
 arch/arm64/mm/gcs.c              | 53 +++++++++++++++++++++-
 arch/x86/include/asm/shstk.h     | 11 +++--
 arch/x86/kernel/process.c        |  2 +-
 arch/x86/kernel/shstk.c          | 57 +++++++++++++++++++++---
 include/asm-generic/cacheflush.h | 11 +++++
 include/linux/sched/task.h       | 17 +++++++
 include/uapi/linux/sched.h       |  9 ++--
 kernel/fork.c                    | 96 +++++++++++++++++++++++++++++++++++-----
 8 files changed, 230 insertions(+), 26 deletions(-)

diff --git a/arch/arm64/mm/gcs.c b/arch/arm64/mm/gcs.c
index 1f633a482558..586494f7669b 100644
--- a/arch/arm64/mm/gcs.c
+++ b/arch/arm64/mm/gcs.c
@@ -43,8 +43,23 @@ int gcs_alloc_thread_stack(struct task_struct *tsk,
 {
 	unsigned long addr, size;
 
-	if (!system_supports_gcs())
+	if (!system_supports_gcs()) {
+		if (args->shadow_stack_token)
+			return -EINVAL;
+
+		return 0;
+	}
+
+	/*
+	 * If the user specified a GCS then use it, otherwise fall
+	 * back to a default allocation strategy. Validation is done
+	 * in arch_shstk_validate_clone().
+	 */
+	if (args->shadow_stack_token) {
+		tsk->thread.gcs_base = 0;
+		tsk->thread.gcs_size = 0;
 		return 0;
+	}
 
 	if (!task_gcs_el0_enabled(tsk))
 		return 0;
@@ -68,6 +83,42 @@ int gcs_alloc_thread_stack(struct task_struct *tsk,
 	return 0;
 }
 
+static bool gcs_consume_token(struct vm_area_struct *vma, struct page *page,
+			      unsigned long user_addr)
+{
+	u64 expected = GCS_CAP(user_addr);
+	u64 *token = page_address(page) + offset_in_page(user_addr);
+
+	if (!cmpxchg_to_user_page(vma, page, user_addr, token, expected, 0))
+		return false;
+	set_page_dirty_lock(page);
+
+	return true;
+}
+
+int arch_shstk_validate_clone(struct task_struct *tsk,
+			      struct vm_area_struct *vma,
+			      struct page *page,
+			      struct kernel_clone_args *args)
+{
+	unsigned long gcspr_el0;
+	int ret = 0;
+
+	/* Ensure that a token written as a result of a pivot is visible */
+	gcsb_dsync();
+
+	gcspr_el0 = args->shadow_stack_token;
+	if (!gcs_consume_token(vma, page, gcspr_el0))
+		return -EINVAL;
+
+	tsk->thread.gcspr_el0 = gcspr_el0 + sizeof(u64);
+
+	/* Ensure that our token consumption visible */
+	gcsb_dsync();
+
+	return ret;
+}
+
 SYSCALL_DEFINE3(map_shadow_stack, unsigned long, addr, unsigned long, size, unsigned int, flags)
 {
 	unsigned long alloc_size;
diff --git a/arch/x86/include/asm/shstk.h b/arch/x86/include/asm/shstk.h
index ba6f2fe43848..827e983430aa 100644
--- a/arch/x86/include/asm/shstk.h
+++ b/arch/x86/include/asm/shstk.h
@@ -6,6 +6,7 @@
 #include <linux/types.h>
 
 struct task_struct;
+struct kernel_clone_args;
 struct ksignal;
 
 #ifdef CONFIG_X86_USER_SHADOW_STACK
@@ -16,8 +17,8 @@ struct thread_shstk {
 
 long shstk_prctl(struct task_struct *task, int option, unsigned long arg2);
 void reset_thread_features(void);
-unsigned long shstk_alloc_thread_stack(struct task_struct *p, unsigned long clone_flags,
-				       unsigned long stack_size);
+unsigned long shstk_alloc_thread_stack(struct task_struct *p,
+				       const struct kernel_clone_args *args);
 void shstk_free(struct task_struct *p);
 int setup_signal_shadow_stack(struct ksignal *ksig);
 int restore_signal_shadow_stack(void);
@@ -28,8 +29,10 @@ static inline long shstk_prctl(struct task_struct *task, int option,
 			       unsigned long arg2) { return -EINVAL; }
 static inline void reset_thread_features(void) {}
 static inline unsigned long shstk_alloc_thread_stack(struct task_struct *p,
-						     unsigned long clone_flags,
-						     unsigned long stack_size) { return 0; }
+						     const struct kernel_clone_args *args)
+{
+	return 0;
+}
 static inline void shstk_free(struct task_struct *p) {}
 static inline int setup_signal_shadow_stack(struct ksignal *ksig) { return 0; }
 static inline int restore_signal_shadow_stack(void) { return 0; }
diff --git a/arch/x86/kernel/process.c b/arch/x86/kernel/process.c
index 704883c21f3a..56bf3394360a 100644
--- a/arch/x86/kernel/process.c
+++ b/arch/x86/kernel/process.c
@@ -209,7 +209,7 @@ int copy_thread(struct task_struct *p, const struct kernel_clone_args *args)
 	 * is disabled, new_ssp will remain 0, and fpu_clone() will know not to
 	 * update it.
 	 */
-	new_ssp = shstk_alloc_thread_stack(p, clone_flags, args->stack_size);
+	new_ssp = shstk_alloc_thread_stack(p, args);
 	if (IS_ERR_VALUE(new_ssp))
 		return PTR_ERR((void *)new_ssp);
 
diff --git a/arch/x86/kernel/shstk.c b/arch/x86/kernel/shstk.c
index 2ddf23387c7e..9d82dd0bed1f 100644
--- a/arch/x86/kernel/shstk.c
+++ b/arch/x86/kernel/shstk.c
@@ -191,18 +191,65 @@ void reset_thread_features(void)
 	current->thread.features_locked = 0;
 }
 
-unsigned long shstk_alloc_thread_stack(struct task_struct *tsk, unsigned long clone_flags,
-				       unsigned long stack_size)
+int arch_shstk_validate_clone(struct task_struct *t,
+			      struct vm_area_struct *vma,
+			      struct page *page,
+			      struct kernel_clone_args *args)
+{
+	/*
+	 * SSP is aligned, so reserved bits and mode bit are a zero, just mark
+	 * the token 64-bit.
+	 */
+	void *maddr = page_address(page);
+	int offset, token;
+	unsigned long ssp;
+	u64 expected;
+
+	if (!features_enabled(ARCH_SHSTK_SHSTK))
+		return 0;
+
+	token = args->shadow_stack_token;
+	ssp = token + 8;
+	expected = (ssp + FRAME_SIZE) | BIT(0);
+	offset = offset_in_page(token);
+
+	if (!cmpxchg_to_user_page(vma, page, token, (unsigned long *)(maddr + offset),
+				  expected, 0))
+		return -EINVAL;
+	set_page_dirty_lock(page);
+
+	return 0;
+}
+
+unsigned long shstk_alloc_thread_stack(struct task_struct *tsk,
+				       const struct kernel_clone_args *args)
 {
 	struct thread_shstk *shstk = &tsk->thread.shstk;
+	unsigned long clone_flags = args->flags;
 	unsigned long addr, size;
 
 	/*
 	 * If shadow stack is not enabled on the new thread, skip any
-	 * switch to a new shadow stack.
+	 * implicit switch to a new shadow stack and reject attempts to
+	 * explicitly specify one.
 	 */
-	if (!features_enabled(ARCH_SHSTK_SHSTK))
+	if (!features_enabled(ARCH_SHSTK_SHSTK)) {
+		if (args->shadow_stack_token)
+			return (unsigned long)ERR_PTR(-EINVAL);
+
 		return 0;
+	}
+
+	/*
+	 * If the user specified a shadow stack then use it, otherwise
+	 * fall back to a default allocation strategy. Validation is
+	 * done in arch_shstk_validate_clone().
+	 */
+	if (args->shadow_stack_token) {
+		shstk->base = 0;
+		shstk->size = 0;
+		return args->shadow_stack_token + 8;
+	}
 
 	/*
 	 * For CLONE_VFORK the child will share the parents shadow stack.
@@ -222,7 +269,7 @@ unsigned long shstk_alloc_thread_stack(struct task_struct *tsk, unsigned long cl
 	if (!(clone_flags & CLONE_VM))
 		return 0;
 
-	size = adjust_shstk_size(stack_size);
+	size = adjust_shstk_size(args->stack_size);
 	addr = alloc_shstk(0, size, 0, false);
 	if (IS_ERR_VALUE(addr))
 		return addr;
diff --git a/include/asm-generic/cacheflush.h b/include/asm-generic/cacheflush.h
index 7ee8a179d103..96cc0c7a5c90 100644
--- a/include/asm-generic/cacheflush.h
+++ b/include/asm-generic/cacheflush.h
@@ -124,4 +124,15 @@ static inline void flush_cache_vunmap(unsigned long start, unsigned long end)
 	} while (0)
 #endif
 
+#ifndef cmpxchg_to_user_page
+#define cmpxchg_to_user_page(vma, page, vaddr, ptr, old, new)  \
+({							  \
+	bool ret;						  \
+								  \
+	ret = try_cmpxchg(ptr, &old, new);			  \
+	flush_icache_user_page(vma, page, vaddr, sizeof(*ptr));	  \
+	ret;							  \
+})
+#endif
+
 #endif /* _ASM_GENERIC_CACHEFLUSH_H */
diff --git a/include/linux/sched/task.h b/include/linux/sched/task.h
index ca1db4b92c32..c34f3cb68822 100644
--- a/include/linux/sched/task.h
+++ b/include/linux/sched/task.h
@@ -16,6 +16,7 @@ struct task_struct;
 struct rusage;
 union thread_union;
 struct css_set;
+struct vm_area_struct;
 
 /* All the bits taken by the old clone syscall. */
 #define CLONE_LEGACY_FLAGS 0xffffffffULL
@@ -44,6 +45,7 @@ struct kernel_clone_args {
 	struct cgroup *cgrp;
 	struct css_set *cset;
 	unsigned int kill_seq;
+	unsigned long shadow_stack_token;
 };
 
 /*
@@ -237,4 +239,19 @@ static inline void task_unlock(struct task_struct *p)
 
 DEFINE_GUARD(task_lock, struct task_struct *, task_lock(_T), task_unlock(_T))
 
+#ifdef CONFIG_ARCH_HAS_USER_SHADOW_STACK
+int arch_shstk_validate_clone(struct task_struct *p,
+			      struct vm_area_struct *vma,
+			      struct page *page,
+			      struct kernel_clone_args *args);
+#else
+static inline int arch_shstk_validate_clone(struct task_struct *p,
+					    struct vm_area_struct *vma,
+					    struct page *page,
+					    struct kernel_clone_args *args)
+{
+	return 0;
+}
+#endif
+
 #endif /* _LINUX_SCHED_TASK_H */
diff --git a/include/uapi/linux/sched.h b/include/uapi/linux/sched.h
index 359a14cc76a4..9cf5c419e109 100644
--- a/include/uapi/linux/sched.h
+++ b/include/uapi/linux/sched.h
@@ -84,6 +84,7 @@
  *                kernel's limit of nested PID namespaces.
  * @cgroup:       If CLONE_INTO_CGROUP is specified set this to
  *                a file descriptor for the cgroup.
+ * @shadow_stack_token: Pointer to shadow stack token at top of stack.
  *
  * The structure is versioned by size and thus extensible.
  * New struct members must go at the end of the struct and
@@ -101,12 +102,14 @@ struct clone_args {
 	__aligned_u64 set_tid;
 	__aligned_u64 set_tid_size;
 	__aligned_u64 cgroup;
+	__aligned_u64 shadow_stack_token;
 };
 #endif
 
-#define CLONE_ARGS_SIZE_VER0 64 /* sizeof first published struct */
-#define CLONE_ARGS_SIZE_VER1 80 /* sizeof second published struct */
-#define CLONE_ARGS_SIZE_VER2 88 /* sizeof third published struct */
+#define CLONE_ARGS_SIZE_VER0  64 /* sizeof first published struct */
+#define CLONE_ARGS_SIZE_VER1  80 /* sizeof second published struct */
+#define CLONE_ARGS_SIZE_VER2  88 /* sizeof third published struct */
+#define CLONE_ARGS_SIZE_VER3  96 /* sizeof fourth published struct */
 
 /*
  * Scheduling policies
diff --git a/kernel/fork.c b/kernel/fork.c
index 1ee8eb11f38b..89c19996235d 100644
--- a/kernel/fork.c
+++ b/kernel/fork.c
@@ -1902,6 +1902,51 @@ static bool need_futex_hash_allocate_default(u64 clone_flags)
 	return true;
 }
 
+static int shstk_validate_clone(struct task_struct *p,
+				struct kernel_clone_args *args)
+{
+	struct mm_struct *mm;
+	struct vm_area_struct *vma;
+	struct page *page;
+	unsigned long addr;
+	int ret;
+
+	if (!IS_ENABLED(CONFIG_ARCH_HAS_USER_SHADOW_STACK))
+		return 0;
+
+	if (!args->shadow_stack_token)
+		return 0;
+
+	mm = get_task_mm(p);
+	if (!mm)
+		return -EFAULT;
+
+	mmap_read_lock(mm);
+
+	addr = untagged_addr_remote(mm, args->shadow_stack_token);
+	page = get_user_page_vma_remote(mm, addr, FOLL_FORCE | FOLL_WRITE,
+					&vma);
+	if (IS_ERR(page)) {
+		ret = -EFAULT;
+		goto out;
+	}
+
+	if (!(vma->vm_flags & VM_SHADOW_STACK) ||
+	    !(vma->vm_flags & VM_WRITE)) {
+		ret = -EFAULT;
+		goto out_page;
+	}
+
+	ret = arch_shstk_validate_clone(p, vma, page, args);
+
+out_page:
+	put_page(page);
+out:
+	mmap_read_unlock(mm);
+	mmput(mm);
+	return ret;
+}
+
 /*
  * This creates a new process as a copy of the old one,
  * but does not actually start it yet.
@@ -2176,6 +2221,9 @@ __latent_entropy struct task_struct *copy_process(
 	if (retval)
 		goto bad_fork_cleanup_namespaces;
 	retval = copy_thread(p, args);
+	if (retval)
+		goto bad_fork_cleanup_io;
+	retval = shstk_validate_clone(p, args);
 	if (retval)
 		goto bad_fork_cleanup_io;
 
@@ -2757,7 +2805,9 @@ noinline static int copy_clone_args_from_user(struct kernel_clone_args *kargs,
 		     CLONE_ARGS_SIZE_VER1);
 	BUILD_BUG_ON(offsetofend(struct clone_args, cgroup) !=
 		     CLONE_ARGS_SIZE_VER2);
-	BUILD_BUG_ON(sizeof(struct clone_args) != CLONE_ARGS_SIZE_VER2);
+	BUILD_BUG_ON(offsetofend(struct clone_args, shadow_stack_token) !=
+		     CLONE_ARGS_SIZE_VER3);
+	BUILD_BUG_ON(sizeof(struct clone_args) != CLONE_ARGS_SIZE_VER3);
 
 	if (unlikely(usize > PAGE_SIZE))
 		return -E2BIG;
@@ -2790,16 +2840,17 @@ noinline static int copy_clone_args_from_user(struct kernel_clone_args *kargs,
 		return -EINVAL;
 
 	*kargs = (struct kernel_clone_args){
-		.flags		= args.flags,
-		.pidfd		= u64_to_user_ptr(args.pidfd),
-		.child_tid	= u64_to_user_ptr(args.child_tid),
-		.parent_tid	= u64_to_user_ptr(args.parent_tid),
-		.exit_signal	= args.exit_signal,
-		.stack		= args.stack,
-		.stack_size	= args.stack_size,
-		.tls		= args.tls,
-		.set_tid_size	= args.set_tid_size,
-		.cgroup		= args.cgroup,
+		.flags			= args.flags,
+		.pidfd			= u64_to_user_ptr(args.pidfd),
+		.child_tid		= u64_to_user_ptr(args.child_tid),
+		.parent_tid		= u64_to_user_ptr(args.parent_tid),
+		.exit_signal		= args.exit_signal,
+		.stack			= args.stack,
+		.stack_size		= args.stack_size,
+		.tls			= args.tls,
+		.set_tid_size		= args.set_tid_size,
+		.cgroup			= args.cgroup,
+		.shadow_stack_token	= args.shadow_stack_token,
 	};
 
 	if (args.set_tid &&
@@ -2840,6 +2891,27 @@ static inline bool clone3_stack_valid(struct kernel_clone_args *kargs)
 	return true;
 }
 
+/**
+ * clone3_shadow_stack_valid - check and prepare shadow stack
+ * @kargs: kernel clone args
+ *
+ * Verify that shadow stacks are only enabled if supported.
+ */
+static inline bool clone3_shadow_stack_valid(struct kernel_clone_args *kargs)
+{
+	if (!kargs->shadow_stack_token)
+		return true;
+
+	if (!IS_ALIGNED(kargs->shadow_stack_token, sizeof(void *)))
+		return false;
+
+	/*
+	 * The architecture must check support on the specific
+	 * machine.
+	 */
+	return IS_ENABLED(CONFIG_ARCH_HAS_USER_SHADOW_STACK);
+}
+
 static bool clone3_args_valid(struct kernel_clone_args *kargs)
 {
 	/* Verify that no unknown flags are passed along. */
@@ -2862,7 +2934,7 @@ static bool clone3_args_valid(struct kernel_clone_args *kargs)
 	    kargs->exit_signal)
 		return false;
 
-	if (!clone3_stack_valid(kargs))
+	if (!clone3_stack_valid(kargs) || !clone3_shadow_stack_valid(kargs))
 		return false;
 
 	return true;

-- 
2.39.5

[PATCH RFT v17 5/8] selftests/clone3: Remove redundant flushes of output streams

[Mark Brown]

Since there were widespread issues with output not being flushed the
kselftest framework was modified to explicitly set the output streams
unbuffered in commit 58e2847ad2e6 ("selftests: line buffer test
program's stdout") so there is no need to explicitly flush in the clone3
tests.

Reviewed-by: Kees Cook <kees@kernel.org>
Tested-by: Kees Cook <kees@kernel.org>
Acked-by: Shuah Khan <skhan@linuxfoundation.org>
Reviewed-by: Catalin Marinas <catalin.marinas@arm.com>
Signed-off-by: Mark Brown <broonie@kernel.org>
---
 tools/testing/selftests/clone3/clone3_selftests.h | 2 --
 1 file changed, 2 deletions(-)

diff --git a/tools/testing/selftests/clone3/clone3_selftests.h b/tools/testing/selftests/clone3/clone3_selftests.h
index eeca8005723f..939b26c86d42 100644
--- a/tools/testing/selftests/clone3/clone3_selftests.h
+++ b/tools/testing/selftests/clone3/clone3_selftests.h
@@ -35,8 +35,6 @@ struct __clone_args {
 
 static pid_t sys_clone3(struct __clone_args *args, size_t size)
 {
-	fflush(stdout);
-	fflush(stderr);
 	return syscall(__NR_clone3, args, size);
 }
 

-- 
2.39.5

[PATCH RFT v17 6/8] selftests/clone3: Factor more of main loop into test_clone3()

[Mark Brown]

In order to make it easier to add more configuration for the tests and
more support for runtime detection of when tests can be run pass the
structure describing the tests into test_clone3() rather than picking
the arguments out of it and have that function do all the per-test work.

No functional change.

Reviewed-by: Kees Cook <kees@kernel.org>
Tested-by: Kees Cook <kees@kernel.org>
Acked-by: Shuah Khan <skhan@linuxfoundation.org>
Reviewed-by: Catalin Marinas <catalin.marinas@arm.com>
Signed-off-by: Mark Brown <broonie@kernel.org>
---
 tools/testing/selftests/clone3/clone3.c | 77 ++++++++++++++++-----------------
 1 file changed, 37 insertions(+), 40 deletions(-)

diff --git a/tools/testing/selftests/clone3/clone3.c b/tools/testing/selftests/clone3/clone3.c
index e61f07973ce5..e066b201fa64 100644
--- a/tools/testing/selftests/clone3/clone3.c
+++ b/tools/testing/selftests/clone3/clone3.c
@@ -30,6 +30,19 @@ enum test_mode {
 	CLONE3_ARGS_INVAL_EXIT_SIGNAL_NSIG,
 };
 
+typedef bool (*filter_function)(void);
+typedef size_t (*size_function)(void);
+
+struct test {
+	const char *name;
+	uint64_t flags;
+	size_t size;
+	size_function size_function;
+	int expected;
+	enum test_mode test_mode;
+	filter_function filter;
+};
+
 static int call_clone3(uint64_t flags, size_t size, enum test_mode test_mode)
 {
 	struct __clone_args args = {
@@ -109,30 +122,40 @@ static int call_clone3(uint64_t flags, size_t size, enum test_mode test_mode)
 	return 0;
 }
 
-static bool test_clone3(uint64_t flags, size_t size, int expected,
-			enum test_mode test_mode)
+static void test_clone3(const struct test *test)
 {
+	size_t size;
 	int ret;
 
+	if (test->filter && test->filter()) {
+		ksft_test_result_skip("%s\n", test->name);
+		return;
+	}
+
+	if (test->size_function)
+		size = test->size_function();
+	else
+		size = test->size;
+
+	ksft_print_msg("Running test '%s'\n", test->name);
+
 	ksft_print_msg(
 		"[%d] Trying clone3() with flags %#" PRIx64 " (size %zu)\n",
-		getpid(), flags, size);
-	ret = call_clone3(flags, size, test_mode);
+		getpid(), test->flags, size);
+	ret = call_clone3(test->flags, size, test->test_mode);
 	ksft_print_msg("[%d] clone3() with flags says: %d expected %d\n",
-			getpid(), ret, expected);
-	if (ret != expected) {
+			getpid(), ret, test->expected);
+	if (ret != test->expected) {
 		ksft_print_msg(
 			"[%d] Result (%d) is different than expected (%d)\n",
-			getpid(), ret, expected);
-		return false;
+			getpid(), ret, test->expected);
+		ksft_test_result_fail("%s\n", test->name);
+		return;
 	}
 
-	return true;
+	ksft_test_result_pass("%s\n", test->name);
 }
 
-typedef bool (*filter_function)(void);
-typedef size_t (*size_function)(void);
-
 static bool not_root(void)
 {
 	if (getuid() != 0) {
@@ -160,16 +183,6 @@ static size_t page_size_plus_8(void)
 	return getpagesize() + 8;
 }
 
-struct test {
-	const char *name;
-	uint64_t flags;
-	size_t size;
-	size_function size_function;
-	int expected;
-	enum test_mode test_mode;
-	filter_function filter;
-};
-
 static const struct test tests[] = {
 	{
 		.name = "simple clone3()",
@@ -319,24 +332,8 @@ int main(int argc, char *argv[])
 	ksft_set_plan(ARRAY_SIZE(tests));
 	test_clone3_supported();
 
-	for (i = 0; i < ARRAY_SIZE(tests); i++) {
-		if (tests[i].filter && tests[i].filter()) {
-			ksft_test_result_skip("%s\n", tests[i].name);
-			continue;
-		}
-
-		if (tests[i].size_function)
-			size = tests[i].size_function();
-		else
-			size = tests[i].size;
-
-		ksft_print_msg("Running test '%s'\n", tests[i].name);
-
-		ksft_test_result(test_clone3(tests[i].flags, size,
-					     tests[i].expected,
-					     tests[i].test_mode),
-				 "%s\n", tests[i].name);
-	}
+	for (i = 0; i < ARRAY_SIZE(tests); i++)
+		test_clone3(&tests[i]);
 
 	ksft_finished();
 }

-- 
2.39.5

[PATCH RFT v17 7/8] selftests/clone3: Allow tests to flag if -E2BIG is a valid error code

[Mark Brown]

The clone_args structure is extensible, with the syscall passing in the
length of the structure. Inside the kernel we use copy_struct_from_user()
to read the struct but this has the unfortunate side effect of silently
accepting some overrun in the structure size providing the extra data is
all zeros. This means that we can't discover the clone3() features that
the running kernel supports by simply probing with various struct sizes.
We need to check this for the benefit of test systems which run newer
kselftests on old kernels.

Add a flag which can be set on a test to indicate that clone3() may return
-E2BIG due to the use of newer struct versions. Currently no tests need
this but it will become an issue for testing clone3() support for shadow
stacks, the support for shadow stacks is already present on x86.

Reviewed-by: Kees Cook <kees@kernel.org>
Tested-by: Kees Cook <kees@kernel.org>
Acked-by: Shuah Khan <skhan@linuxfoundation.org>
Reviewed-by: Catalin Marinas <catalin.marinas@arm.com>
Signed-off-by: Mark Brown <broonie@kernel.org>
---
 tools/testing/selftests/clone3/clone3.c | 6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/tools/testing/selftests/clone3/clone3.c b/tools/testing/selftests/clone3/clone3.c
index e066b201fa64..5b8b7d640e70 100644
--- a/tools/testing/selftests/clone3/clone3.c
+++ b/tools/testing/selftests/clone3/clone3.c
@@ -39,6 +39,7 @@ struct test {
 	size_t size;
 	size_function size_function;
 	int expected;
+	bool e2big_valid;
 	enum test_mode test_mode;
 	filter_function filter;
 };
@@ -146,6 +147,11 @@ static void test_clone3(const struct test *test)
 	ksft_print_msg("[%d] clone3() with flags says: %d expected %d\n",
 			getpid(), ret, test->expected);
 	if (ret != test->expected) {
+		if (test->e2big_valid && ret == -E2BIG) {
+			ksft_print_msg("Test reported -E2BIG\n");
+			ksft_test_result_skip("%s\n", test->name);
+			return;
+		}
 		ksft_print_msg(
 			"[%d] Result (%d) is different than expected (%d)\n",
 			getpid(), ret, test->expected);

-- 
2.39.5

[PATCH RFT v17 8/8] selftests/clone3: Test shadow stack support

[Mark Brown]

Add basic test coverage for specifying the shadow stack for a newly
created thread via clone3(), including coverage of the newly extended
argument structure.  We check that a user specified shadow stack can be
provided, and that invalid combinations of parameters are rejected.

In order to facilitate testing on systems without userspace shadow stack
support we manually enable shadow stacks on startup, this is architecture
specific due to the use of an arch_prctl() on x86. Due to interactions with
potential userspace locking of features we actually detect support for
shadow stacks on the running system by attempting to allocate a shadow
stack page during initialisation using map_shadow_stack(), warning if this
succeeds when the enable failed.

In order to allow testing of user configured shadow stacks on
architectures with that feature we need to ensure that we do not return
from the function where the clone3() syscall is called in the child
process, doing so would trigger a shadow stack underflow.  To do this we
use inline assembly rather than the standard syscall wrapper to call
clone3().  In order to avoid surprises we also use a syscall rather than
the libc exit() function., this should be overly cautious.

Acked-by: Shuah Khan <skhan@linuxfoundation.org>
Signed-off-by: Mark Brown <broonie@kernel.org>
---
 tools/testing/selftests/clone3/clone3.c           | 143 +++++++++++++++++++++-
 tools/testing/selftests/clone3/clone3_selftests.h |  63 ++++++++++
 2 files changed, 205 insertions(+), 1 deletion(-)

diff --git a/tools/testing/selftests/clone3/clone3.c b/tools/testing/selftests/clone3/clone3.c
index 5b8b7d640e70..6fd2b3238e2c 100644
--- a/tools/testing/selftests/clone3/clone3.c
+++ b/tools/testing/selftests/clone3/clone3.c
@@ -3,6 +3,7 @@
 /* Based on Christian Brauner's clone3() example */
 
 #define _GNU_SOURCE
+#include <asm/mman.h>
 #include <errno.h>
 #include <inttypes.h>
 #include <linux/types.h>
@@ -11,6 +12,7 @@
 #include <stdint.h>
 #include <stdio.h>
 #include <stdlib.h>
+#include <sys/mman.h>
 #include <sys/syscall.h>
 #include <sys/types.h>
 #include <sys/un.h>
@@ -19,8 +21,12 @@
 #include <sched.h>
 
 #include "../kselftest.h"
+#include "../ksft_shstk.h"
 #include "clone3_selftests.h"
 
+static bool shadow_stack_supported;
+static size_t max_supported_args_size;
+
 enum test_mode {
 	CLONE3_ARGS_NO_TEST,
 	CLONE3_ARGS_ALL_0,
@@ -28,6 +34,10 @@ enum test_mode {
 	CLONE3_ARGS_INVAL_EXIT_SIGNAL_NEG,
 	CLONE3_ARGS_INVAL_EXIT_SIGNAL_CSIG,
 	CLONE3_ARGS_INVAL_EXIT_SIGNAL_NSIG,
+	CLONE3_ARGS_SHADOW_STACK,
+	CLONE3_ARGS_SHADOW_STACK_MISALIGNED,
+	CLONE3_ARGS_SHADOW_STACK_NO_TOKEN,
+	CLONE3_ARGS_SHADOW_STACK_NORMAL_MEMORY,
 };
 
 typedef bool (*filter_function)(void);
@@ -44,6 +54,44 @@ struct test {
 	filter_function filter;
 };
 
+
+/*
+ * We check for shadow stack support by attempting to use
+ * map_shadow_stack() since features may have been locked by the
+ * dynamic linker resulting in spurious errors when we attempt to
+ * enable on startup.  We warn if the enable failed.
+ */
+static void test_shadow_stack_supported(void)
+{
+	long ret;
+
+	ret = syscall(__NR_map_shadow_stack, 0, getpagesize(), 0);
+	if (ret == -1) {
+		ksft_print_msg("map_shadow_stack() not supported\n");
+	} else if ((void *)ret == MAP_FAILED) {
+		ksft_print_msg("Failed to map shadow stack\n");
+	} else {
+		ksft_print_msg("Shadow stack supportd\n");
+		shadow_stack_supported = true;
+
+		if (!shadow_stack_enabled)
+			ksft_print_msg("Mapped but did not enable shadow stack\n");
+	}
+}
+
+static void *get_shadow_stack_page(unsigned long flags)
+{
+	unsigned long long page;
+
+	page = syscall(__NR_map_shadow_stack, 0, getpagesize(), flags);
+	if ((void *)page == MAP_FAILED) {
+		ksft_print_msg("map_shadow_stack() failed: %d\n", errno);
+		return 0;
+	}
+
+	return (void *)page;
+}
+
 static int call_clone3(uint64_t flags, size_t size, enum test_mode test_mode)
 {
 	struct __clone_args args = {
@@ -57,6 +105,7 @@ static int call_clone3(uint64_t flags, size_t size, enum test_mode test_mode)
 	} args_ext;
 
 	pid_t pid = -1;
+	void *p;
 	int status;
 
 	memset(&args_ext, 0, sizeof(args_ext));
@@ -89,6 +138,26 @@ static int call_clone3(uint64_t flags, size_t size, enum test_mode test_mode)
 	case CLONE3_ARGS_INVAL_EXIT_SIGNAL_NSIG:
 		args.exit_signal = 0x00000000000000f0ULL;
 		break;
+	case CLONE3_ARGS_SHADOW_STACK:
+		p = get_shadow_stack_page(SHADOW_STACK_SET_TOKEN);
+		p += getpagesize() - sizeof(void *);
+		args.shadow_stack_token = (unsigned long long)p;
+		break;
+	case CLONE3_ARGS_SHADOW_STACK_MISALIGNED:
+		p = get_shadow_stack_page(SHADOW_STACK_SET_TOKEN);
+		p += getpagesize() - sizeof(void *) - 1;
+		args.shadow_stack_token = (unsigned long long)p;
+		break;
+	case CLONE3_ARGS_SHADOW_STACK_NORMAL_MEMORY:
+		p = malloc(getpagesize());
+		p += getpagesize() - sizeof(void *);
+		args.shadow_stack_token = (unsigned long long)p;
+		break;
+	case CLONE3_ARGS_SHADOW_STACK_NO_TOKEN:
+		p = get_shadow_stack_page(0);
+		p += getpagesize() - sizeof(void *);
+		args.shadow_stack_token = (unsigned long long)p;
+		break;
 	}
 
 	memcpy(&args_ext.args, &args, sizeof(struct __clone_args));
@@ -102,7 +171,12 @@ static int call_clone3(uint64_t flags, size_t size, enum test_mode test_mode)
 
 	if (pid == 0) {
 		ksft_print_msg("I am the child, my PID is %d\n", getpid());
-		_exit(EXIT_SUCCESS);
+		/*
+		 * Use a raw syscall to ensure we don't get issues
+		 * with manually specified shadow stack and exit handlers.
+		 */
+		syscall(__NR_exit, EXIT_SUCCESS);
+		ksft_print_msg("CHILD FAILED TO EXIT PID is %d\n", getpid());
 	}
 
 	ksft_print_msg("I am the parent (%d). My child's pid is %d\n",
@@ -184,6 +258,26 @@ static bool no_timenamespace(void)
 	return true;
 }
 
+static bool have_shadow_stack(void)
+{
+	if (shadow_stack_supported) {
+		ksft_print_msg("Shadow stack supported\n");
+		return true;
+	}
+
+	return false;
+}
+
+static bool no_shadow_stack(void)
+{
+	if (!shadow_stack_supported) {
+		ksft_print_msg("Shadow stack not supported\n");
+		return true;
+	}
+
+	return false;
+}
+
 static size_t page_size_plus_8(void)
 {
 	return getpagesize() + 8;
@@ -327,6 +421,50 @@ static const struct test tests[] = {
 		.expected = -EINVAL,
 		.test_mode = CLONE3_ARGS_NO_TEST,
 	},
+	{
+		.name = "Shadow stack on system with shadow stack",
+		.size = 0,
+		.expected = 0,
+		.e2big_valid = true,
+		.test_mode = CLONE3_ARGS_SHADOW_STACK,
+		.filter = no_shadow_stack,
+	},
+	{
+		.name = "Shadow stack with misaligned address",
+		.flags = CLONE_VM,
+		.size = 0,
+		.expected = -EINVAL,
+		.e2big_valid = true,
+		.test_mode = CLONE3_ARGS_SHADOW_STACK_MISALIGNED,
+		.filter = no_shadow_stack,
+	},
+	{
+		.name = "Shadow stack with normal memory",
+		.flags = CLONE_VM,
+		.size = 0,
+		.expected = -EFAULT,
+		.e2big_valid = true,
+		.test_mode = CLONE3_ARGS_SHADOW_STACK_NORMAL_MEMORY,
+		.filter = no_shadow_stack,
+	},
+	{
+		.name = "Shadow stack with no token",
+		.flags = CLONE_VM,
+		.size = 0,
+		.expected = -EINVAL,
+		.e2big_valid = true,
+		.test_mode = CLONE3_ARGS_SHADOW_STACK_NO_TOKEN,
+		.filter = no_shadow_stack,
+	},
+	{
+		.name = "Shadow stack on system without shadow stack",
+		.flags = CLONE_VM,
+		.size = 0,
+		.expected = -EFAULT,
+		.e2big_valid = true,
+		.test_mode = CLONE3_ARGS_SHADOW_STACK_NORMAL_MEMORY,
+		.filter = have_shadow_stack,
+	},
 };
 
 int main(int argc, char *argv[])
@@ -334,9 +472,12 @@ int main(int argc, char *argv[])
 	size_t size;
 	int i;
 
+	enable_shadow_stack();
+
 	ksft_print_header();
 	ksft_set_plan(ARRAY_SIZE(tests));
 	test_clone3_supported();
+	test_shadow_stack_supported();
 
 	for (i = 0; i < ARRAY_SIZE(tests); i++)
 		test_clone3(&tests[i]);
diff --git a/tools/testing/selftests/clone3/clone3_selftests.h b/tools/testing/selftests/clone3/clone3_selftests.h
index 939b26c86d42..8151c4fc971a 100644
--- a/tools/testing/selftests/clone3/clone3_selftests.h
+++ b/tools/testing/selftests/clone3/clone3_selftests.h
@@ -31,12 +31,75 @@ struct __clone_args {
 	__aligned_u64 set_tid;
 	__aligned_u64 set_tid_size;
 	__aligned_u64 cgroup;
+#ifndef CLONE_ARGS_SIZE_VER2
+#define CLONE_ARGS_SIZE_VER2 88	/* sizeof third published struct */
+#endif
+	__aligned_u64 shadow_stack_token;
+#ifndef CLONE_ARGS_SIZE_VER3
+#define CLONE_ARGS_SIZE_VER3 96 /* sizeof fourth published struct */
+#endif
 };
 
+/*
+ * For architectures with shadow stack support we need to be
+ * absolutely sure that the clone3() syscall will be inline and not a
+ * function call so we open code.
+ */
+#ifdef __x86_64__
+static __always_inline pid_t sys_clone3(struct __clone_args *args, size_t size)
+{
+	register long _num  __asm__ ("rax") = __NR_clone3;
+	register long _args __asm__ ("rdi") = (long)(args);
+	register long _size __asm__ ("rsi") = (long)(size);
+	long ret;
+
+	__asm__ volatile (
+		"syscall\n"
+		: "=a"(ret)
+		: "r"(_args), "r"(_size),
+		  "0"(_num)
+		: "rcx", "r11", "memory", "cc"
+	);
+
+	if (ret < 0) {
+		errno = -ret;
+		return -1;
+	}
+
+	return ret;
+}
+#elif defined(__aarch64__)
+static __always_inline pid_t sys_clone3(struct __clone_args *args, size_t size)
+{
+	register long _num  __asm__ ("x8") = __NR_clone3;
+	register long _args __asm__ ("x0") = (long)(args);
+	register long _size __asm__ ("x1") = (long)(size);
+	register long arg2 __asm__ ("x2") = 0;
+	register long arg3 __asm__ ("x3") = 0;
+	register long arg4 __asm__ ("x4") = 0;
+
+	__asm__ volatile (
+		"svc #0\n"
+		: "=r"(_args)
+		: "r"(_args), "r"(_size),
+		  "r"(_num), "r"(arg2),
+		  "r"(arg3), "r"(arg4)
+		: "memory", "cc"
+	);
+
+	if ((int)_args < 0) {
+		errno = -((int)_args);
+		return -1;
+	}
+
+	return _args;
+}
+#else
 static pid_t sys_clone3(struct __clone_args *args, size_t size)
 {
 	return syscall(__NR_clone3, args, size);
 }
+#endif
 
 static inline void test_clone3_supported(void)
 {

-- 
2.39.5

Re: [PATCH RFT v17 4/8] fork: Add shadow stack support to clone3()

[Yury Khrustalev]

On Mon, Jun 09, 2025 at 01:54:05PM +0100, Mark Brown wrote:
> Unlike with the normal stack there is no API for configuring the shadow
> stack for a new thread, instead the kernel will dynamically allocate a
> new shadow stack with the same size as the normal stack. This appears to
> be due to the shadow stack series having been in development since
> before the more extensible clone3() was added rather than anything more
> deliberate.
> 
> Add a parameter to clone3() specifying the shadow stack pointer to use
> for the new thread, this is inconsistent with the way we specify the
> normal stack but during review concerns were expressed about having to
> identify where the shadow stack pointer should be placed especially in
> cases where the shadow stack has been previously active.  If no shadow
> stack is specified then the existing implicit allocation behaviour is
> maintained.
> 
> If a shadow stack pointer is specified then it is required to have an
> architecture defined token placed on the stack, this will be consumed by
> the new task.  If no valid token is present then this will be reported
> with -EINVAL.  This token prevents new threads being created pointing at
> the shadow stack of an existing running thread.
> 
> If the architecture does not support shadow stacks the shadow stack
> pointer must be not be specified, architectures that do support the
> feature are expected to enforce the same requirement on individual
> systems that lack shadow stack support.
> 
> Update the existing arm64 and x86 implementations to pay attention to
> the newly added arguments, in order to maintain compatibility we use the
> existing behaviour if no shadow stack is specified. Since we are now
> using more fields from the kernel_clone_args we pass that into the
> shadow stack code rather than individual fields.
> 
> Portions of the x86 architecture code were written by Rick Edgecombe.
> 
> Acked-by: Yury Khrustalev <yury.khrustalev@arm.com>

LGTM. I've tested this change on the FVP model along with my Glibc patch
series [1] and confirm that it works fine. The Glibc patch is at RFC stage
and will require more work, but it's enough to test this series.

Tested-by: Yury Khrustalev <yury.khrustalev@arm.com>

[1]: https://inbox.sourceware.org/libc-alpha/20250610151320.885131-1-yury.khrustalev@arm.com/

Thanks,
Yury

Re: [PATCH RFT v17 3/8] selftests: Provide helper header for shadow stack testing

[Catalin Marinas]

On Mon, Jun 09, 2025 at 01:54:04PM +0100, Mark Brown wrote:
> While almost all users of shadow stacks should be relying on the dynamic
> linker and libc to enable the feature there are several low level test
> programs where it is useful to enable without any libc support, allowing
> testing without full system enablement. This low level testing is helpful
> during bringup of the support itself, and also in enabling coverage by
> automated testing without needing all system components in the target root
> filesystems to have enablement.
> 
> Provide a header with helpers for this purpose, intended for use only by
> test programs directly exercising shadow stack interfaces.
> 
> Reviewed-by: Rick Edgecombe <rick.p.edgecombe@intel.com>
> Reviewed-by: Kees Cook <kees@kernel.org>
> Tested-by: Kees Cook <kees@kernel.org>
> Acked-by: Shuah Khan <skhan@linuxfoundation.org>
> Signed-off-by: Mark Brown <broonie@kernel.org>

Acked-by: Catalin Marinas <catalin.marinas@arm.com>

Re: [PATCH RFT v17 4/8] fork: Add shadow stack support to clone3()

[Catalin Marinas]

On Mon, Jun 09, 2025 at 01:54:05PM +0100, Mark Brown wrote:
> +int arch_shstk_validate_clone(struct task_struct *tsk,
> +			      struct vm_area_struct *vma,
> +			      struct page *page,
> +			      struct kernel_clone_args *args)
> +{
> +	unsigned long gcspr_el0;
> +	int ret = 0;
> +
> +	/* Ensure that a token written as a result of a pivot is visible */
> +	gcsb_dsync();
> +	gcspr_el0 = args->shadow_stack_token;
> +	if (!gcs_consume_token(vma, page, gcspr_el0))
> +		return -EINVAL;
> +
> +	tsk->thread.gcspr_el0 = gcspr_el0 + sizeof(u64);
> +
> +	/* Ensure that our token consumption visible */
> +	gcsb_dsync();
> +
> +	return ret;
> +}

What are the scenarios where we need the barriers? We have one via
map_shadow_stack() that would cover the first one. IIUC, GCSSS2 also
generates a GCSB event (or maybe I got it all wrong; I need to read the
spec).

> diff --git a/kernel/fork.c b/kernel/fork.c
> index 1ee8eb11f38b..89c19996235d 100644
> --- a/kernel/fork.c
> +++ b/kernel/fork.c
> @@ -1902,6 +1902,51 @@ static bool need_futex_hash_allocate_default(u64 clone_flags)
>  	return true;
>  }
>  
> +static int shstk_validate_clone(struct task_struct *p,
> +				struct kernel_clone_args *args)
> +{
> +	struct mm_struct *mm;
> +	struct vm_area_struct *vma;
> +	struct page *page;
> +	unsigned long addr;
> +	int ret;
> +
> +	if (!IS_ENABLED(CONFIG_ARCH_HAS_USER_SHADOW_STACK))
> +		return 0;
> +
> +	if (!args->shadow_stack_token)
> +		return 0;
> +
> +	mm = get_task_mm(p);
> +	if (!mm)
> +		return -EFAULT;
> +
> +	mmap_read_lock(mm);
> +
> +	addr = untagged_addr_remote(mm, args->shadow_stack_token);

I think down the line, get_user_page_vma_remote() already does an
untagged_addr_remote(). But it does it after the vma look-up, so we
still need the untagging early.

That said, would we ever allowed a tagged pointer for the shadow stack?

> @@ -2840,6 +2891,27 @@ static inline bool clone3_stack_valid(struct kernel_clone_args *kargs)
>  	return true;
>  }
>  
> +/**
> + * clone3_shadow_stack_valid - check and prepare shadow stack
> + * @kargs: kernel clone args
> + *
> + * Verify that shadow stacks are only enabled if supported.
> + */
> +static inline bool clone3_shadow_stack_valid(struct kernel_clone_args *kargs)
> +{
> +	if (!kargs->shadow_stack_token)
> +		return true;
> +
> +	if (!IS_ALIGNED(kargs->shadow_stack_token, sizeof(void *)))
> +		return false;
> +
> +	/*
> +	 * The architecture must check support on the specific
> +	 * machine.
> +	 */
> +	return IS_ENABLED(CONFIG_ARCH_HAS_USER_SHADOW_STACK);

I don't understand the comment here. It implies some kind of fallback
for further arch checks but it's just a return.

BTW, clone3_stack_valid() has an access_ok() check as well. Shall we add
it here? That's where the size would have come in handy but IIUC the
decision was to drop it (fine by me, just validate that the token is
accessible).

-- 
Catalin

Re: [PATCH RFT v17 4/8] fork: Add shadow stack support to clone3()

[Mark Brown]

[-- Attachment #1: Type: text/plain, Size: 3836 bytes --]

On Fri, Jun 27, 2025 at 06:19:33PM +0100, Catalin Marinas wrote:
> On Mon, Jun 09, 2025 at 01:54:05PM +0100, Mark Brown wrote:

> > +	/* Ensure that a token written as a result of a pivot is visible */
> > +	gcsb_dsync();
> > +	gcspr_el0 = args->shadow_stack_token;
> > +	if (!gcs_consume_token(vma, page, gcspr_el0))
> > +		return -EINVAL;
> > +
> > +	tsk->thread.gcspr_el0 = gcspr_el0 + sizeof(u64);
> > +
> > +	/* Ensure that our token consumption visible */
> > +	gcsb_dsync();

> What are the scenarios where we need the barriers? We have one via
> map_shadow_stack() that would cover the first one. IIUC, GCSSS2 also
> generates a GCSB event (or maybe I got it all wrong; I need to read the
> spec).

I think now that gcs_consume_token() does a cmpxchg they're redundant,
your analysis covers the first one (anything that puts a valid token
in memory should have a barrier) and now gcs_consume_token() does a
cmpxchg the second one should also be redundant thanks to R_FZRGP.  It
would be good if someone double checked though.

Originally gcs_consume_token() was using regular accesses as for the
example in DDI0487 L.a K3.3 and was tried on two addresses, I missed
dropping the barriers when changing to a cmpxchg.

> > +static int shstk_validate_clone(struct task_struct *p,
> > +				struct kernel_clone_args *args)
> > +{

> > +	mmap_read_lock(mm);

> > +	addr = untagged_addr_remote(mm, args->shadow_stack_token);

> I think down the line, get_user_page_vma_remote() already does an
> untagged_addr_remote(). But it does it after the vma look-up, so we
> still need the untagging early.

> That said, would we ever allowed a tagged pointer for the shadow stack?

For arm64 you can architecturally use tags as per G_HMJHM.  I_WBHHX says
that GCS accesses are tag unchecked, but tags are used on GCSSS1 as per
I_MGLTC and I_MBHFS.  We'll need new ABI to allow userspace to get a
PROT_MTE GCS though, I'd planned on extending map_shadow_stack() for
that, and adding handling in the token validation here.

There's also the fact that the untagging should be very cheap in the
context of what we're doing so it seems sensible to just have it,
especially generic code which applies to all arches.

> > +static inline bool clone3_shadow_stack_valid(struct kernel_clone_args *kargs)
> > +{
> > +	if (!kargs->shadow_stack_token)
> > +		return true;
> > +
> > +	if (!IS_ALIGNED(kargs->shadow_stack_token, sizeof(void *)))
> > +		return false;
> > +
> > +	/*
> > +	 * The architecture must check support on the specific
> > +	 * machine.
> > +	 */
> > +	return IS_ENABLED(CONFIG_ARCH_HAS_USER_SHADOW_STACK);

> I don't understand the comment here. It implies some kind of fallback
> for further arch checks but it's just a return.

Here we're just doing initial triage that a shadow stack could possibly
be valid, the check here is there to fail if there's one specified but
there is no support in the kernel (eg, for architectures that don't have
the feature at all like arm32).  The comment is trying to say that we're
not attempting to validate that we can actually use shadow stacks on the
current system, just that the support exists in the kernel.  I'll reword
the comment, it's not clear.

> BTW, clone3_stack_valid() has an access_ok() check as well. Shall we add
> it here? That's where the size would have come in handy but IIUC the
> decision was to drop it (fine by me, just validate that the token is
> accessible).

AIUI the main reason for doing that for the normal stack is to report an
error before we actually start the thread and have it fault trying to
access an invalid stack since we don't otherwise look at the memory,
like you say with shadow stacks we'll consume the token before we start
the new thread so we get the equivalent error reporting as part of that.
I don't think the extra check would buy us much.

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