[RFT,v5,4/7] fork: Add shadow stack support to clone3()

diff --git a/arch/x86/include/asm/shstk.h b/arch/x86/include/asm/shstk.h
index 42fee8959df7..8be7b0a909c3 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);
@@ -26,8 +27,10 @@  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 ab49ade31b0d..d2bfcd44de05 100644
--- a/arch/x86/kernel/process.c
+++ b/arch/x86/kernel/process.c
@@ -207,7 +207,7 @@  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 59e15dd8d0f8..24d0e9b825da 100644
--- a/arch/x86/kernel/shstk.c
+++ b/arch/x86/kernel/shstk.c
@@ -191,44 +191,89 @@  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)
+static bool shstk_consume_token(struct task_struct *tsk,
+				unsigned long addr)
+{
+	/*
+	 * SSP is aligned, so reserved bits and mode bit are a zero, just mark
+	 * the token 64-bit.
+	 */
+	u64 expected = (addr - SS_FRAME_SIZE) | BIT(0);
+	u64 val;
+
+	/* This should really be an atomic cpmxchg.  It is not. */
+	__get_user(val, (__user u64 *)addr);
+	if (val != expected)
+		return false;
+
+	if (write_user_shstk_64((u64 __user *)addr, 0))
+		return false;
+
+	return true;
+}
+
+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
+	 * explciitly specify one.
 	 */
-	if (!features_enabled(ARCH_SHSTK_SHSTK))
-		return 0;
+	if (!features_enabled(ARCH_SHSTK_SHSTK)) {
+		if (args->shadow_stack || args->shadow_stack_size)
+			return (unsigned long)ERR_PTR(-EINVAL);
 
-	/*
-	 * For CLONE_VFORK the child will share the parents shadow stack.
-	 * Make sure to clear the internal tracking of the thread shadow
-	 * stack so the freeing logic run for child knows to leave it alone.
-	 */
-	if (clone_flags & CLONE_VFORK) {
-		shstk->base = 0;
-		shstk->size = 0;
 		return 0;
 	}
 
 	/*
-	 * For !CLONE_VM the child will use a copy of the parents shadow
-	 * stack.
+	 * If the user specified a shadow stack then do some basic
+	 * validation and use it, otherwise fall back to a default
+	 * shadow stack size if the clone_flags don't indicate an
+	 * allocation is unneeded.
 	 */
-	if (!(clone_flags & CLONE_VM))
-		return 0;
+	if (args->shadow_stack) {
+		addr = args->shadow_stack;
+		size = args->shadow_stack_size;
 
-	size = adjust_shstk_size(stack_size);
-	addr = alloc_shstk(0, size, 0, false);
-	if (IS_ERR_VALUE(addr))
-		return addr;
+		/* There should be a valid token at the top of the stack. */
+		if (!shstk_consume_token(tsk, addr + size - sizeof(u64)))
+			return (unsigned long)ERR_PTR(-EINVAL);
+	} else {
+		/*
+		 * For CLONE_VFORK the child will share the parents
+		 * shadow stack.  Make sure to clear the internal
+		 * tracking of the thread shadow stack so the freeing
+		 * logic run for child knows to leave it alone.
+		 */
+		if (clone_flags & CLONE_VFORK) {
+			shstk->base = 0;
+			shstk->size = 0;
+			return 0;
+		}
 
-	shstk->base = addr;
-	shstk->size = size;
+		/*
+		 * For !CLONE_VM the child will use a copy of the
+		 * parents shadow stack.
+		 */
+		if (!(clone_flags & CLONE_VM))
+			return 0;
+
+		size = args->stack_size;
+		size = adjust_shstk_size(size);
+		addr = alloc_shstk(0, size, 0, false);
+		if (IS_ERR_VALUE(addr))
+			return addr;
+
+		/* We allocated the shadow stack, we should deallocate it. */
+		shstk->base = addr;
+		shstk->size = size;
+	}
 
 	return addr + size;
 }
diff --git a/include/linux/sched/task.h b/include/linux/sched/task.h
index d362aacf9f89..dd577e8dc881 100644
--- a/include/linux/sched/task.h
+++ b/include/linux/sched/task.h
@@ -43,6 +43,8 @@  struct kernel_clone_args {
 	void *fn_arg;
 	struct cgroup *cgrp;
 	struct css_set *cset;
+	unsigned long shadow_stack;
+	unsigned long shadow_stack_size;
 };
 
 /*
diff --git a/include/uapi/linux/sched.h b/include/uapi/linux/sched.h
index 3bac0a8ceab2..8b7af52548fd 100644
--- a/include/uapi/linux/sched.h
+++ b/include/uapi/linux/sched.h
@@ -84,6 +84,10 @@ 
  *                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: Pointer to the memory allocated for the child
+ *                shadow stack.
+ * @shadow_stack_size: Specify the size of the shadow stack for
+ *                     the child process.
  *
  * The structure is versioned by size and thus extensible.
  * New struct members must go at the end of the struct and
@@ -101,12 +105,15 @@  struct clone_args {
 	__aligned_u64 set_tid;
 	__aligned_u64 set_tid_size;
 	__aligned_u64 cgroup;
+	__aligned_u64 shadow_stack;
+	__aligned_u64 shadow_stack_size;
 };
 #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 104 /* sizeof fourth published struct */
 
 /*
  * Scheduling policies
diff --git a/kernel/fork.c b/kernel/fork.c
index 0d944e92a43f..fca041cc2b8a 100644
--- a/kernel/fork.c
+++ b/kernel/fork.c
@@ -123,6 +123,11 @@ 
  */
 #define MAX_THREADS FUTEX_TID_MASK
 
+/*
+ * Require that shadow stacks can store at least one element
+ */
+#define SHADOW_STACK_SIZE_MIN sizeof(void *)
+
 /*
  * Protected counters by write_lock_irq(&tasklist_lock)
  */
@@ -3062,7 +3067,9 @@  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_size) !=
+		     CLONE_ARGS_SIZE_VER3);
+	BUILD_BUG_ON(sizeof(struct clone_args) != CLONE_ARGS_SIZE_VER3);
 
 	if (unlikely(usize > PAGE_SIZE))
 		return -E2BIG;
@@ -3095,16 +3102,18 @@  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		= args.shadow_stack,
+		.shadow_stack_size	= args.shadow_stack_size,
 	};
 
 	if (args.set_tid &&
@@ -3145,6 +3154,34 @@  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) {
+		if (!kargs->shadow_stack_size)
+			return false;
+
+		if (kargs->shadow_stack_size < SHADOW_STACK_SIZE_MIN)
+			return false;
+
+		if (kargs->shadow_stack_size > rlimit(RLIMIT_STACK))
+			return false;
+
+		/*
+		 * The architecture must check support on the specific
+		 * machine.
+		 */
+		return IS_ENABLED(CONFIG_ARCH_HAS_USER_SHADOW_STACK);
+	} else {
+		return !kargs->shadow_stack_size;
+	}
+}
+
 static bool clone3_args_valid(struct kernel_clone_args *kargs)
 {
 	/* Verify that no unknown flags are passed along. */
@@ -3167,7 +3204,7 @@  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;