Re: [PATCH v2 25/39] x86/cet/shstk: Handle thread shadow stack

[Kees Cook]

On Thu, Sep 29, 2022 at 03:29:22PM -0700, Rick Edgecombe wrote:
> [...]
> +#ifdef CONFIG_X86_SHADOW_STACK
> +static int update_fpu_shstk(struct task_struct *dst, unsigned long ssp)
> +{
> +	struct cet_user_state *xstate;
> +
> +	/* If ssp update is not needed. */
> +	if (!ssp)
> +		return 0;

My brain will work to undo the collision of Shadow Stack Pointer with
Stack Smashing Protection. ;)

> [...]
> diff --git a/arch/x86/kernel/shstk.c b/arch/x86/kernel/shstk.c
> index a0b8d4adb2bf..db4e53f9fdaf 100644
> --- a/arch/x86/kernel/shstk.c
> +++ b/arch/x86/kernel/shstk.c
> @@ -118,6 +118,46 @@ void reset_thread_shstk(void)
>  	current->thread.features_locked = 0;
>  }
>  
> +int shstk_alloc_thread_stack(struct task_struct *tsk, unsigned long clone_flags,
> +			     unsigned long stack_size, unsigned long *shstk_addr)

Er, arg 3 is "stack_size". From later:

> +     ret = shstk_alloc_thread_stack(p, clone_flags, args->flags, &shstk_addr);
                                                       ^^^^^^^^^^^

clone_flags and args->flags are identical ... this must be accidentally
working. I was expecting 0 there.

> +{
> +	struct thread_shstk *shstk = &tsk->thread.shstk;
> +	unsigned long addr;
> +
> +	/*
> +	 * If shadow stack is not enabled on the new thread, skip any
> +	 * switch to a new shadow stack.
> +	 */
> +	if (!feature_enabled(CET_SHSTK))
> +		return 0;
> +
> +	/*
> +	 * clone() does not pass stack_size, which was added to clone3().
> +	 * Use RLIMIT_STACK and cap to 4 GB.
> +	 */
> +	if (!stack_size)
> +		stack_size = min_t(unsigned long long, rlimit(RLIMIT_STACK), SZ_4G);

Again, perhaps the clamp should happen in alloc_shstk()?

> +
> +	/*
> +	 * For CLONE_VM, except vfork, the child needs a separate shadow
> +	 * stack.
> +	 */
> +	if ((clone_flags & (CLONE_VFORK | CLONE_VM)) != CLONE_VM)
> +		return 0;
> +
> +
> +	stack_size = PAGE_ALIGN(stack_size);

Uhm, I think a line went missing here. :P

"x86/cet/shstk: Introduce map_shadow_stack syscall" adds the missing:

+	addr = alloc_shstk(0, stack_size, 0, false);

Please add back the original. :)

> +	if (IS_ERR_VALUE(addr))
> +		return PTR_ERR((void *)addr);
> +
> +	shstk->base = addr;
> +	shstk->size = stack_size;
> +
> +	*shstk_addr = addr + stack_size;
> +
> +	return 0;
> +}
> +
>  void shstk_free(struct task_struct *tsk)
>  {
>  	struct thread_shstk *shstk = &tsk->thread.shstk;
> @@ -126,7 +166,13 @@ void shstk_free(struct task_struct *tsk)
>  	    !feature_enabled(CET_SHSTK))
>  		return;
>  
> -	if (!tsk->mm)
> +	/*
> +	 * 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.
> +	 */
> +	if (!tsk->mm || tsk->mm != current->mm)
>  		return;
>  
>  	unmap_shadow_stack(shstk->base, shstk->size);
> -- 
> 2.17.1
> 

-- 
Kees Cook