Re: [PATCH v26 23/30] x86/cet/shstk: Handle thread shadow stack

[Borislav Petkov]

On Tue, Apr 27, 2021 at 01:43:08PM -0700, Yu-cheng Yu wrote:
> @@ -181,6 +184,12 @@ int copy_thread(unsigned long clone_flags, unsigned long sp, unsigned long arg,
>  	if (clone_flags & CLONE_SETTLS)
>  		ret = set_new_tls(p, tls);
>  
> +#ifdef CONFIG_X86_64

IS_ENABLED

> +	/* Allocate a new shadow stack for pthread */
> +	if (!ret)
> +		ret = shstk_setup_thread(p, clone_flags, stack_size);
> +#endif
> +

And why is this addition here...

>  	if (!ret && unlikely(test_tsk_thread_flag(current, TIF_IO_BITMAP)))
>  		io_bitmap_share(p);

... instead of here?

<---

>  
> diff --git a/arch/x86/kernel/shstk.c b/arch/x86/kernel/shstk.c
> index c815c7507830..d387df84b7f1 100644
> --- a/arch/x86/kernel/shstk.c
> +++ b/arch/x86/kernel/shstk.c
> @@ -70,6 +70,55 @@ int shstk_setup(void)
>  	return 0;
>  }

> +int shstk_setup_thread(struct task_struct *tsk, unsigned long clone_flags,

Judging by what this function does, its name wants to be

shstk_alloc_thread_stack()

or so?

> +		       unsigned long stack_size)
> +{
> +	unsigned long addr, size;
> +	struct cet_user_state *state;
> +	struct cet_status *cet = &tsk->thread.cet;

The tip-tree preferred ordering of variable declarations at the
beginning of a function is reverse fir tree order::

	struct long_struct_name *descriptive_name;
	unsigned long foo, bar;
	unsigned int tmp;
	int ret;

The above is faster to parse than the reverse ordering::

	int ret;
	unsigned int tmp;
	unsigned long foo, bar;
	struct long_struct_name *descriptive_name;

And even more so than random ordering::

	unsigned long foo, bar;
	int ret;
	struct long_struct_name *descriptive_name;
	unsigned int tmp;

> +
> +	if (!cet->shstk_size)
> +		return 0;
> +

This check needs a comment.

> +	if ((clone_flags & (CLONE_VFORK | CLONE_VM)) != CLONE_VM)
> +		return 0;
> +
> +	state = get_xsave_addr(&tsk->thread.fpu.state.xsave,
> +			       XFEATURE_CET_USER);

Let that line stick out.

> +
> +	if (!state)
> +		return -EINVAL;
> +
> +	if (stack_size == 0)

	if (!stack_size)

> +		return -EINVAL;

and that test needs to be done first in the function.

> +
> +	/* Cap shadow stack size to 4 GB */

Why?

> +	size = min_t(unsigned long long, rlimit(RLIMIT_STACK), SZ_4G);
> +	size = min(size, stack_size);
> +
> +	/*
> +	 * Compat-mode pthreads share a limited address space.
> +	 * If each function call takes an average of four slots
> +	 * stack space, allocate 1/4 of stack size for shadow stack.
> +	 */
> +	if (in_compat_syscall())
> +		size /= 4;

<---- newline here.

> +	size = round_up(size, PAGE_SIZE);
> +	addr = alloc_shstk(size);
> +

^ Superfluous newline.

> +	if (IS_ERR_VALUE(addr)) {
> +		cet->shstk_base = 0;
> +		cet->shstk_size = 0;
> +		return PTR_ERR((void *)addr);
> +	}
> +
> +	fpu__prepare_write(&tsk->thread.fpu);
> +	state->user_ssp = (u64)(addr + size);

cet_user_state has u64, cet_status has unsigned longs. Make them all u64.

And since cet_status is per thread, but I had suggested struct
shstk_desc, I think now that that should be called

struct thread_shstk

or so to denote *exactly* what it is.

Thx.

-- 
Regards/Gruss,
    Boris.

https://people.kernel.org/tglx/notes-about-netiquette