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

Kees Cook
2022-10-03T20:29:20Z

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