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

[Will Deacon]

On Fri, Dec 14, 2018 at 11:28:16AM +0000, Dave Martin wrote:
> 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?

There's at least one tool that parses this stuff to tell you whether you
have/need the mitigations:

https://github.com/speed47/spectre-meltdown-checker

Will