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

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Date:   Mon, 10 Dec 2018 14:59:31 +0000
From:   Catalin Marinas <catalin.marinas@....com>
To:     Julien Thierry <julien.thierry@....com>
Cc:     peterz@...radead.org, will.deacon@....com,
        linux-kernel@...r.kernel.org, mingo@...hat.com,
        james.morse@....com, hpa@...or.com,
        linux-arm-kernel@...ts.infradead.org,
        Suzuki K Poulose <Suzuki.Poulose@....com>
Subject: Re: [PATCH v2 2/2] arm64: uaccess: Implement unsafe accessors

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).

-- 
Catalin