[v12,11/28] riscv/shstk: If needed allocate a new shadow stack on clone

diff --git a/arch/riscv/include/asm/mmu_context.h b/arch/riscv/include/asm/mmu_context.h
index 8c4bc49a3a0f..dbf27a78df6c 100644
--- a/arch/riscv/include/asm/mmu_context.h
+++ b/arch/riscv/include/asm/mmu_context.h
@@ -48,6 +48,13 @@  static inline unsigned long mm_untag_mask(struct mm_struct *mm)
 }
 #endif
 
+#define deactivate_mm deactivate_mm
+static inline void deactivate_mm(struct task_struct *tsk,
+				 struct mm_struct *mm)
+{
+	shstk_release(tsk);
+}
+
 #include <asm-generic/mmu_context.h>
 
 #endif /* _ASM_RISCV_MMU_CONTEXT_H */
diff --git a/arch/riscv/include/asm/usercfi.h b/arch/riscv/include/asm/usercfi.h
index 5f2027c51917..82d28ac98d76 100644
--- a/arch/riscv/include/asm/usercfi.h
+++ b/arch/riscv/include/asm/usercfi.h
@@ -8,6 +8,9 @@ 
 #ifndef __ASSEMBLY__
 #include <linux/types.h>
 
+struct task_struct;
+struct kernel_clone_args;
+
 #ifdef CONFIG_RISCV_USER_CFI
 struct cfi_status {
 	unsigned long ubcfi_en : 1; /* Enable for backward cfi. */
@@ -17,6 +20,28 @@  struct cfi_status {
 	unsigned long shdw_stk_size; /* size of shadow stack */
 };
 
+unsigned long shstk_alloc_thread_stack(struct task_struct *tsk,
+				       const struct kernel_clone_args *args);
+void shstk_release(struct task_struct *tsk);
+void set_shstk_base(struct task_struct *task, unsigned long shstk_addr, unsigned long size);
+unsigned long get_shstk_base(struct task_struct *task, unsigned long *size);
+void set_active_shstk(struct task_struct *task, unsigned long shstk_addr);
+bool is_shstk_enabled(struct task_struct *task);
+
+#else
+
+#define shstk_alloc_thread_stack(tsk, args) 0
+
+#define shstk_release(tsk)
+
+#define get_shstk_base(task, size) 0UL
+
+#define set_shstk_base(task, shstk_addr, size)
+
+#define set_active_shstk(task, shstk_addr)
+
+#define is_shstk_enabled(task) false
+
 #endif /* CONFIG_RISCV_USER_CFI */
 
 #endif /* __ASSEMBLY__ */
diff --git a/arch/riscv/kernel/process.c b/arch/riscv/kernel/process.c
index 7c244de77180..99acb6342a37 100644
--- a/arch/riscv/kernel/process.c
+++ b/arch/riscv/kernel/process.c
@@ -29,6 +29,7 @@ 
 #include <asm/vector.h>
 #include <asm/cpufeature.h>
 #include <asm/exec.h>
+#include <asm/usercfi.h>
 
 #if defined(CONFIG_STACKPROTECTOR) && !defined(CONFIG_STACKPROTECTOR_PER_TASK)
 #include <linux/stackprotector.h>
@@ -211,6 +212,7 @@  int copy_thread(struct task_struct *p, const struct kernel_clone_args *args)
 	unsigned long clone_flags = args->flags;
 	unsigned long usp = args->stack;
 	unsigned long tls = args->tls;
+	unsigned long ssp = 0;
 	struct pt_regs *childregs = task_pt_regs(p);
 
 	/* Ensure all threads in this mm have the same pointer masking mode. */
@@ -229,11 +231,18 @@  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;
 	} else {
+		/* allocate new shadow stack if needed. In case of CLONE_VM we have to */
+		ssp = shstk_alloc_thread_stack(p, args);
+		if (IS_ERR_VALUE(ssp))
+			return PTR_ERR((void *)ssp);
+
 		*childregs = *(current_pt_regs());
 		/* Turn off status.VS */
 		riscv_v_vstate_off(childregs);
 		if (usp) /* User fork */
 			childregs->sp = usp;
+		/* if needed, set new ssp */
+		ssp ? set_active_shstk(p, ssp) : 0;
 		if (clone_flags & CLONE_SETTLS)
 			childregs->tp = tls;
 		childregs->a0 = 0; /* Return value of fork() */
diff --git a/arch/riscv/kernel/usercfi.c b/arch/riscv/kernel/usercfi.c
index 24022809a7b5..73cf87dab186 100644
--- a/arch/riscv/kernel/usercfi.c
+++ b/arch/riscv/kernel/usercfi.c
@@ -19,6 +19,41 @@ 
 
 #define SHSTK_ENTRY_SIZE sizeof(void *)
 
+bool is_shstk_enabled(struct task_struct *task)
+{
+	return task->thread_info.user_cfi_state.ubcfi_en ? true : false;
+}
+
+void set_shstk_base(struct task_struct *task, unsigned long shstk_addr, unsigned long size)
+{
+	task->thread_info.user_cfi_state.shdw_stk_base = shstk_addr;
+	task->thread_info.user_cfi_state.shdw_stk_size = size;
+}
+
+unsigned long get_shstk_base(struct task_struct *task, unsigned long *size)
+{
+	if (size)
+		*size = task->thread_info.user_cfi_state.shdw_stk_size;
+	return task->thread_info.user_cfi_state.shdw_stk_base;
+}
+
+void set_active_shstk(struct task_struct *task, unsigned long shstk_addr)
+{
+	task->thread_info.user_cfi_state.user_shdw_stk = shstk_addr;
+}
+
+/*
+ * If size is 0, then to be compatible with regular stack we want it to be as big as
+ * regular stack. Else PAGE_ALIGN it and return back
+ */
+static unsigned long calc_shstk_size(unsigned long size)
+{
+	if (size)
+		return PAGE_ALIGN(size);
+
+	return PAGE_ALIGN(min_t(unsigned long long, rlimit(RLIMIT_STACK), SZ_4G));
+}
+
 /*
  * Writes on shadow stack can either be `sspush` or `ssamoswap`. `sspush` can happen
  * implicitly on current shadow stack pointed to by CSR_SSP. `ssamoswap` takes pointer to
@@ -142,3 +177,88 @@  SYSCALL_DEFINE3(map_shadow_stack, unsigned long, addr, unsigned long, size, unsi
 
 	return allocate_shadow_stack(addr, aligned_size, size, set_tok);
 }
+
+/*
+ * This gets called during clone/clone3/fork. And is needed to allocate a shadow stack for
+ * cases where CLONE_VM is specified and thus a different stack is specified by user. We
+ * thus need a separate shadow stack too. How does separate shadow stack is specified by
+ * user is still being debated. Once that's settled, remove this part of the comment.
+ * This function simply returns 0 if shadow stack are not supported or if separate shadow
+ * stack allocation is not needed (like in case of !CLONE_VM)
+ */
+unsigned long shstk_alloc_thread_stack(struct task_struct *tsk,
+				       const struct kernel_clone_args *args)
+{
+	unsigned long addr, size;
+
+	/* If shadow stack is not supported, return 0 */
+	if (!cpu_supports_shadow_stack())
+		return 0;
+
+	/*
+	 * If shadow stack is not enabled on the new thread, skip any
+	 * switch to a new shadow stack.
+	 */
+	if (!is_shstk_enabled(tsk))
+		return 0;
+
+	/*
+	 * For CLONE_VFORK the child will share the parents shadow stack.
+	 * Set base = 0 and size = 0, this is special means to track this state
+	 * so the freeing logic run for child knows to leave it alone.
+	 */
+	if (args->flags & CLONE_VFORK) {
+		set_shstk_base(tsk, 0, 0);
+		return 0;
+	}
+
+	/*
+	 * For !CLONE_VM the child will use a copy of the parents shadow
+	 * stack.
+	 */
+	if (!(args->flags & CLONE_VM))
+		return 0;
+
+	/*
+	 * reaching here means, CLONE_VM was specified and thus a separate shadow
+	 * stack is needed for new cloned thread. Note: below allocation is happening
+	 * using current mm.
+	 */
+	size = calc_shstk_size(args->stack_size);
+	addr = allocate_shadow_stack(0, size, 0, false);
+	if (IS_ERR_VALUE(addr))
+		return addr;
+
+	set_shstk_base(tsk, addr, size);
+
+	return addr + size;
+}
+
+void shstk_release(struct task_struct *tsk)
+{
+	unsigned long base = 0, size = 0;
+	/* If shadow stack is not supported or not enabled, nothing to release */
+	if (!cpu_supports_shadow_stack() || !is_shstk_enabled(tsk))
+		return;
+
+	/*
+	 * When fork() with CLONE_VM fails, the child (tsk) already has a
+	 * shadow stack allocated, and exit_thread() calls this function to
+	 * free it.  In this case the parent (current) and the child share
+	 * the same mm struct. Move forward only when they're same.
+	 */
+	if (!tsk->mm || tsk->mm != current->mm)
+		return;
+
+	/*
+	 * We know shadow stack is enabled but if base is NULL, then
+	 * this task is not managing its own shadow stack (CLONE_VFORK). So
+	 * skip freeing it.
+	 */
+	base = get_shstk_base(tsk, &size);
+	if (!base)
+		return;
+
+	vm_munmap(base, size);
+	set_shstk_base(tsk, 0, 0);
+}