Date: Sun, 12 May 2024 19:05:27 +0200
From: Alexandre Ghiti <alex@...ti.fr>
To: Deepak Gupta <debug@...osinc.com>, paul.walmsley@...ive.com,
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Subject: Re: [PATCH v3 15/29] riscv/shstk: If needed allocate a new shadow
 stack on clone

On 04/04/2024 01:35, Deepak Gupta wrote:
> Userspace specifies VM_CLONE to share address space and spawn new thread.


CLONE_VM?


> `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 VM_CLONE is given.
>
> In case of VM_FORK, parent is suspended until child finishes and thus can


You mean CLONE_VFORK here right?


> child use parent shadow stack. In case of !VM_CLONE, 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.
>
> Signed-off-by: Deepak Gupta <debug@...osinc.com>
> ---
>   arch/riscv/include/asm/usercfi.h |  39 ++++++++++
>   arch/riscv/kernel/process.c      |  12 ++-
>   arch/riscv/kernel/usercfi.c      | 121 +++++++++++++++++++++++++++++++
>   3 files changed, 171 insertions(+), 1 deletion(-)
>
> diff --git a/arch/riscv/include/asm/usercfi.h b/arch/riscv/include/asm/usercfi.h
> index 4fa201b4fc4e..b47574a7a8c9 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 +20,42 @@ 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);
> +void set_active_shstk(struct task_struct *task, unsigned long shstk_addr);
> +bool is_shstk_enabled(struct task_struct *task);
> +
> +#else
> +
> +static inline unsigned long shstk_alloc_thread_stack(struct task_struct *tsk,
> +					   const struct kernel_clone_args *args)
> +{
> +	return 0;
> +}
> +
> +static inline void shstk_release(struct task_struct *tsk)
> +{
> +
> +}
> +
> +static inline void set_shstk_base(struct task_struct *task, unsigned long shstk_addr,
> +								unsigned long size)
> +{
> +
> +}
> +
> +static inline void set_active_shstk(struct task_struct *task, unsigned long shstk_addr)
> +{
> +
> +}
> +
> +static inline bool is_shstk_enabled(struct task_struct *task)
> +{
> +	return false;
> +}
> +
>   #endif /* CONFIG_RISCV_USER_CFI */
>   
>   #endif /* __ASSEMBLY__ */
> diff --git a/arch/riscv/kernel/process.c b/arch/riscv/kernel/process.c
> index ce577cdc2af3..ef48a25b0eff 100644
> --- a/arch/riscv/kernel/process.c
> +++ b/arch/riscv/kernel/process.c
> @@ -26,6 +26,7 @@
>   #include <asm/cpuidle.h>
>   #include <asm/vector.h>
>   #include <asm/cpufeature.h>
> +#include <asm/usercfi.h>
>   
>   register unsigned long gp_in_global __asm__("gp");
>   
> @@ -202,7 +203,8 @@ int arch_dup_task_struct(struct task_struct *dst, struct task_struct *src)
>   
>   void exit_thread(struct task_struct *tsk)
>   {
> -
> +	if (IS_ENABLED(CONFIG_RISCV_USER_CFI))
> +		shstk_release(tsk);
>   }
>   
>   int copy_thread(struct task_struct *p, const struct kernel_clone_args *args)
> @@ -210,6 +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);
>   
>   	memset(&p->thread.s, 0, sizeof(p->thread.s));
> @@ -225,11 +228,18 @@ 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 (ssp) /* if needed, set new ssp */
> +			set_active_shstk(p, ssp);
>   		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 c4ed0d4e33d6..11ef7ab925c9 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
> @@ -147,3 +182,89 @@ 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);
> +}

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