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

[Yu, Yu-cheng]

On 5/10/2021 7:15 AM, Borislav Petkov wrote:
> On Tue, Apr 27, 2021 at 01:43:08PM -0700, Yu-cheng Yu wrote:

[...]

> > 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?

This is not necessary.  I will make it just stack_size, which is passed 
in from copy_thread().

> > +	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.

So this struct will be:

struct thread_shstk {
	u64 shstk_base;
	u64 shstk_size;
	u64 locked:1;
	u64 ibt:1;
};

Ok?

Thanks,
Yu-cheng