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

[Suzuki K Poulose]

On 12/10/2018 02:59 PM, Catalin Marinas wrote:
> On Fri, Dec 07, 2018 at 08:38:11AM +0000, Julien Thierry wrote:
> > 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.
>
> OK, so the PSTATE.PAN bit only makes sense when SCTLR_EL1.SPAN is 0, IOW
> the PAN hardware feature has been enabled. Maybe you could write it
> (together with some comment):
>
> 	} else if (cpus_have_const_cap(ARM64_ALT_PAN_NOT_UAO) &&
> 		 !(read_sysreg(sctlr_el1) & SCTLR_EL1_SPAN)) {
> 		 /* only if PAN is present and enabled */
> 		return !read_sysreg_s(SYS_PSTATE_PAN)
> 	}
>
> On the cpufeature.c side of things, it seems that we enable the
> static_branch before calling the cpu_enable. I wonder whether changing
> the order here would help with avoid the SCTLR_EL1 read (not sure what
> else it would break; cc'ing Suzuki).


I doubt if we would gain anything by moving it around. 
cpus_have_const_cap() would fall back to test_bit() until we mark that
the static_branches have been updated explicitly, which happens after
we have issued the stop_machine(). So, even if we move the static branch
per capability, we don't gain much.

Is that what you were looking for ?

Cheers
Suzuki