Re: [PATCH 5/6] arm64: add sysfs vulnerability show for speculative store bypass

[Will Deacon <will.deacon@arm.com>]

On Fri, Dec 14, 2018 at 10:34:31AM +0000, Steven Price wrote:
> On 06/12/2018 23:44, Jeremy Linton wrote:
> > From: Mian Yousaf Kaukab <ykaukab@suse.de>
> > 
> > Return status based no ssbd_state and the arm64 SSBS feature.
>                       ^^ on
> 
> > Return string "Unknown" in case CONFIG_ARM64_SSBD is
> > disabled or arch workaround2 is not available
> > in the firmware.
> > 
> > Signed-off-by: Mian Yousaf Kaukab <ykaukab@suse.de>
> > [Added SSBS logic]
> > Signed-off-by: Jeremy Linton <jeremy.linton@arm.com>
> > ---
> >  arch/arm64/kernel/cpu_errata.c | 28 ++++++++++++++++++++++++++++
> >  1 file changed, 28 insertions(+)
> > 
> > diff --git a/arch/arm64/kernel/cpu_errata.c b/arch/arm64/kernel/cpu_errata.c
> > index 6505c93d507e..8aeb5ca38db8 100644
> > --- a/arch/arm64/kernel/cpu_errata.c
> > +++ b/arch/arm64/kernel/cpu_errata.c
> > @@ -423,6 +423,7 @@ static bool has_ssbd_mitigation(const struct arm64_cpu_capabilities *entry,
> >  		ssbd_state = ARM64_SSBD_UNKNOWN;
> >  		return false;
> >  
> > +	/* machines with mixed mitigation requirements must not return this */
> >  	case SMCCC_RET_NOT_REQUIRED:
> >  		pr_info_once("%s mitigation not required\n", entry->desc);
> >  		ssbd_state = ARM64_SSBD_MITIGATED;
> > @@ -828,4 +829,31 @@ ssize_t cpu_show_spectre_v2(struct device *dev, struct device_attribute *attr,
> >  	}
> >  }
> >  
> > +ssize_t cpu_show_spec_store_bypass(struct device *dev,
> > +		struct device_attribute *attr, char *buf)
> > +{
> > +	/*
> > +	 *  Two assumptions: First, get_ssbd_state() reflects the worse case
> > +	 *  for hetrogenous machines, and that if SSBS is supported its
>                                                   ^^^^ SSBD
> > +	 *  supported by all cores.
> > +	 */
> > +	switch (arm64_get_ssbd_state()) {
> > +	case ARM64_SSBD_MITIGATED:
> > +		return sprintf(buf, "Not affected\n");
> > +
> > +	case ARM64_SSBD_KERNEL:
> > +	case ARM64_SSBD_FORCE_ENABLE:
> > +		if (cpus_have_cap(ARM64_SSBS))
> > +			return sprintf(buf, "Not affected\n");
> > +		return sprintf(buf,
> > +			"Mitigation: Speculative Store Bypass disabled\n");
> 
> NIT: To me this reads as the mitigation is disabled. Can we call it
> "Speculative Store Bypass Disable" (with a capital 'D' and without the
> 'd at the end)?

Whilst I agree that the strings are reasonably confusing (especially when
you pile on the double-negatives all the way up the stack!), we really
have no choice but to follow x86's lead with these strings.

I don't think it's worth forking the ABI in an attempt to make this clearer.

Will