[PATCH v2 1/7] arm64: Factor out PAN enabling/disabling into separate uaccess_* macros

[catalin.marinas@arm.com (Catalin Marinas)]

On Mon, Sep 05, 2016 at 04:38:28PM +0100, Mark Rutland wrote:
> On Fri, Sep 02, 2016 at 04:02:07PM +0100, Catalin Marinas wrote:
> >  /*
> > + * User access enabling/disabling.
> > + */
> > +#define uaccess_disable(alt)						\
> > +do {									\
> > +	asm(ALTERNATIVE("nop", SET_PSTATE_PAN(1), alt,			\
> > +			CONFIG_ARM64_PAN));				\
> > +} while (0)
> > +
> > +#define uaccess_enable(alt)						\
> > +do {									\
> > +	asm(ALTERNATIVE("nop", SET_PSTATE_PAN(0), alt,			\
> > +			CONFIG_ARM64_PAN));				\
> > +} while (0)
> 
> Passing the alternative down is somewhat confusing. e.g. in the futex
> case it looks like we're only doing something when PAN is present,
> whereas we'll manipulate TTBR0 in the absence of PAN.

I agree it's confusing (I got it wrong first time as well and used the
wrong alternative for futex).

> If I've understood correctly, we need this to distinguish regular
> load/store uaccess sequences (eg. the futex code) from potentially
> patched unprivileged load/store sequences (e.g. {get,put}_user) when
> poking PSTATE.PAN.
> 
> So perhaps we could ahve something like:
> 
> * privileged_uaccess_{enable,disable}()
>   Which toggle TTBR0, or PAN (always).
>   These would handle cases like the futex/swp code.
>  
> * (unprivileged_)uaccess_{enable,disable}()
>   Which toggle TTBR0, or PAN (in the absence of UAO).
>   These would handle cases like the {get,put}_user sequences.
> 
> Though perhaps that is just as confusing. ;)

I find it more confusing. In the non-UAO case, get_user etc. would
normally have to use privileged_uaccess_enable() since ldr is not
replaced with ldtr. Maybe uaccess_enable_for_exclusives() but it doesn't
look any better. I think adding some comments to the code
(uaccess_enable macro) would work better, clarifying what the
alternative is for.

-- 
Catalin