Re: [PATCH v29 23/32] x86/cet/shstk: Add user-mode shadow stack support

[Yu, Yu-cheng]

On 8/26/2021 9:25 AM, Borislav Petkov wrote:
> On Fri, Aug 20, 2021 at 11:11:52AM -0700, Yu-cheng Yu wrote:
[...]
> > +
> > +int shstk_setup(void)
> > +{
> > +	struct thread_shstk *shstk = &current->thread.shstk;
> > +	unsigned long addr, size;
> > +	int err;
> > +
> > +	if (!cpu_feature_enabled(X86_FEATURE_SHSTK))
> > +		return 0;
> > +
> > +	size = round_up(min_t(unsigned long long, rlimit(RLIMIT_STACK), SZ_4G), PAGE_SIZE);
> > +	addr = alloc_shstk(size);
> > +	if (IS_ERR_VALUE(addr))
> > +		return PTR_ERR((void *)addr);
> > +
> > +	start_update_msrs();
>
> You're setting CET_U with the MSR writes below. Why do you need to do
> XRSTOR here? To zero out PL[012]_SSP?
>
> If so, you can WRMSR those too - no need for a full XRSTOR...

Because on context switches the whole xstates are switched together, we 
need to make sure all are in registers.

> > +	err = wrmsrl_safe(MSR_IA32_PL3_SSP, addr + size);
> > +	if (!err)
> > +		wrmsrl_safe(MSR_IA32_U_CET, CET_SHSTK_EN);
> > +	end_update_msrs();
> > +
> > +	if (!err) {
> > +		shstk->base = addr;
> > +		shstk->size = size;
> > +	}
> > +
> > +	return err;
> > +}