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

[Dave Martin]

On Fri, Dec 14, 2018 at 10:41:42AM +0000, Steven Price wrote:
> On 14/12/2018 10:36, Will Deacon wrote:
> > 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@xxxxxxx>
> >>>
> >>> 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@xxxxxxx>
> >>> [Added SSBS logic]
> >>> Signed-off-by: Jeremy Linton <jeremy.linton@xxxxxxx>
> >>> ---
> >>>  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.
> 
> Ah, sorry I hadn't checked the x86 string - yes we should match that.

This is rather why I feel these strings are either a) useless or
b) should be documented somewhere.

Putting at least a skeleton document somewhere could be a good start,
and would require little effort.


What decisions do we expect userspace to make based on this information?

Cheers
---Dave