Re: [PATCH v2 2/2] arm64: uaccess: Implement unsafe accessors

[Julien Thierry]

On 12/06/2018 06:25 PM, Catalin Marinas wrote:
> On Mon, Dec 03, 2018 at 01:55:18PM +0000, Julien Thierry wrote:
> > diff --git a/arch/arm64/include/asm/uaccess.h b/arch/arm64/include/asm/uaccess.h
> > index 07c3408..cabfcae 100644
> > --- a/arch/arm64/include/asm/uaccess.h
> > +++ b/arch/arm64/include/asm/uaccess.h
> > @@ -233,6 +233,23 @@ static inline void uaccess_enable_not_uao(void)
> >   	__uaccess_enable(ARM64_ALT_PAN_NOT_UAO);
> >   }
> >   
> > +#define unsafe_user_region_active	uaccess_region_active
> > +static inline bool uaccess_region_active(void)
> > +{
> > +	if (system_uses_ttbr0_pan()) {
> > +		u64 ttbr;
> > +
> > +		ttbr = read_sysreg(ttbr1_el1);
> > +		return ttbr & TTBR_ASID_MASK;
>
> Nitpick: could write this in 1-2 lines.

True, I can do that in 1 line.

> > +	} else if (cpus_have_const_cap(ARM64_ALT_PAN_NOT_UAO)) {
> > +		return (read_sysreg(sctlr_el1) & SCTLR_EL1_SPAN) ?
> > +				false :
> > +				!read_sysreg_s(SYS_PSTATE_PAN);
> > +	}
>
> ARM64_ALT_PAN_NOT_UAO implies ARM64_HAS_PAN which implies SCTLR_EL1.SPAN
> is 0 at run-time. Is this to cope with the case of being called prior to
> cpu_enable_pan()?

Yes, the issue I can into is that for cpufeatures, .cpu_enable() 
callbacks are called inside stop_machine() which obviously might_sleep 
and so attempts to check whether user_access is on. But for features 
that get enabled before PAN, the PAN bit will be set.

Thanks,

Julien