Re: [RFC PATCH v9 25/27] x86/cet/shstk: Handle thread Shadow Stack

[Yu-cheng Yu]

On Tue, 2020-02-25 at 13:29 -0800, Kees Cook wrote:
> On Wed, Feb 05, 2020 at 10:19:33AM -0800, Yu-cheng Yu wrote:
> > [...]
> > A 64-bit SHSTK has a fixed size of RLIMIT_STACK. A compat-mode thread SHSTK
> > has a fixed size of 1/4 RLIMIT_STACK.  This allows more threads to share a
> > 32-bit address space.
> 
> I am not understanding this part. :) Entries are sizeof(unsigned long),
> yes? A 1/2 RLIMIT_STACK would cover 32-bit, but 1/4 is less, so why does
> that provide for more threads?

Each thread has a separate shadow stack.  If each shadow stack is smaller, the
address space can accommodate more shadow stack allocations.

> >[...]
> > diff --git a/arch/x86/kernel/cet.c b/arch/x86/kernel/cet.c
> > index cba5c7656aab..5b45abda80a1 100644
> > --- a/arch/x86/kernel/cet.c
> > +++ b/arch/x86/kernel/cet.c
> > @@ -170,6 +170,47 @@ int cet_setup_shstk(void)
> >  	return 0;
> >  }
> >  
> > +int cet_setup_thread_shstk(struct task_struct *tsk)
> > +{
> > +	unsigned long addr, size;
> > +	struct cet_user_state *state;
> > +	struct cet_status *cet = &tsk->thread.cet;
> > +
> > +	if (!cet->shstk_enabled)
> > +		return 0;
> > +
> > +	state = get_xsave_addr(&tsk->thread.fpu.state.xsave,
> > +			       XFEATURE_CET_USER);
> > +
> > +	if (!state)
> > +		return -EINVAL;
> > +
> > +	size = rlimit(RLIMIT_STACK);
> 
> Is SHSTK incompatible with RLIM_INFINITY stack rlimits?

I will change it to:

	size = min(rlimit(RLIMIT_STACK), 4 GB);

> 
> > +
> > +	/*
> > +	 * Compat-mode pthreads share a limited address space.
> > +	 * If each function call takes an average of four slots
> > +	 * stack space, we need 1/4 of stack size for shadow stack.
> > +	 */
> > +	if (in_compat_syscall())
> > +		size /= 4;
> > +
> > +	addr = alloc_shstk(size);
> 
> I assume it'd fail here, but I worry about Stack Clash style attacks.
> I'd like to see test cases that make sure the SHSTK gap is working
> correctly.

I will work on some tests.

Thanks,
Yu-cheng