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

[Deepak Gupta <debug@rivosinc.com>]

On Tue, Apr 08, 2025 at 12:51:45PM +0200, Alexandre Ghiti wrote:
>
>On 14/03/2025 22:39, Deepak Gupta wrote:
>>Userspace specifies CLONE_VM to share address space and spawn new thread.
>>`clone` allow userspace to specify a new stack for new thread. However
>>there is no way to specify new shadow stack base address without changing
>>API. This patch allocates a new shadow stack whenever CLONE_VM is given.
>>
>>In case of CLONE_VFORK, parent is suspended until child finishes and thus
>>can child use parent shadow stack. In case of !CLONE_VM, COW kicks in
>>because entire address space is copied from parent to child.
>>
>>`clone3` is extensible and can provide mechanisms using which shadow stack
>>as an input parameter can be provided. This is not settled yet and being
>>extensively discussed on mailing list. Once that's settled, this commit
>>will adapt to that.
>
>
>What's the status of this discussion? Can you provide a Link to it?

It's ongoing right now at v15
https://lore.kernel.org/all/20250408-clone3-shadow-stack-v15-0-3fa245c6e3be@kernel.org/

>
>
>>
>>Reviewed-by: Zong Li <zong.li@sifive.com>
>>Signed-off-by: Deepak Gupta <debug@rivosinc.com>
>>---
>>  arch/riscv/include/asm/mmu_context.h |   7 ++
>>  arch/riscv/include/asm/usercfi.h     |  25 ++++++++
>>  arch/riscv/kernel/process.c          |   9 +++
>>  arch/riscv/kernel/usercfi.c          | 120 +++++++++++++++++++++++++++++++++++
>>  4 files changed, 161 insertions(+)
>>
>>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;
>
>
>The use of the ternary here is weird, can you change that to a if 
>statement?
>
>
>>  		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);
>>+}
>>