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SubjectRe: [PATCH v3 6/7] arm64: add sysfs vulnerability show for speculative store bypass
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On 14/01/2019 16:37, Jeremy Linton wrote:
> Hi,
>
> On 01/14/2019 04:15 AM, Marc Zyngier wrote:
>> On 09/01/2019 23:55, Jeremy Linton wrote:
>>> Return status based on ssbd_state and the arm64 SSBS feature. If
>>> the mitigation is disabled, or the firmware isn't responding then
>>> return the expected machine state based on a new blacklist of known
>>> vulnerable cores.
>>>
>>> Signed-off-by: Jeremy Linton <jeremy.linton@arm.com>
>>> ---
>>> arch/arm64/kernel/cpu_errata.c | 48 ++++++++++++++++++++++++++++++++++
>>> 1 file changed, 48 insertions(+)
>>>
>>> diff --git a/arch/arm64/kernel/cpu_errata.c b/arch/arm64/kernel/cpu_errata.c
>>> index ee286d606d9b..c8ff96158b94 100644
>>> --- a/arch/arm64/kernel/cpu_errata.c
>>> +++ b/arch/arm64/kernel/cpu_errata.c
>>> @@ -288,6 +288,7 @@ enable_smccc_arch_workaround_1(const struct arm64_cpu_capabilities *entry)
>>> DEFINE_PER_CPU_READ_MOSTLY(u64, arm64_ssbd_callback_required);
>>>
>>> int ssbd_state __read_mostly = ARM64_SSBD_KERNEL;
>>> +static bool __ssb_safe = true;
>>>
>>> static const struct ssbd_options {
>>> const char *str;
>>> @@ -385,10 +386,18 @@ static bool has_ssbd_mitigation(const struct arm64_cpu_capabilities *entry,
>>> {
>>> struct arm_smccc_res res;
>>> bool required = true;
>>> + bool is_vul;
>>> s32 val;
>>>
>>> WARN_ON(scope != SCOPE_LOCAL_CPU || preemptible());
>>>
>>> + is_vul = is_midr_in_range_list(read_cpuid_id(), entry->midr_range_list);
>>> +
>>> + if (is_vul)
>>> + __ssb_safe = false;
>>> +
>>> + arm64_requested_vuln_attrs |= VULN_SSB;
>>> +
>>> if (this_cpu_has_cap(ARM64_SSBS)) {
>>> required = false;
>>> goto out_printmsg;
>>> @@ -422,6 +431,7 @@ 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;
>>> @@ -476,6 +486,17 @@ static bool has_ssbd_mitigation(const struct arm64_cpu_capabilities *entry,
>>>
>>> return required;
>>> }
>>> +
>>> +/* known vulnerable cores */
>>> +static const struct midr_range arm64_ssb_cpus[] = {
>>> + MIDR_ALL_VERSIONS(MIDR_CORTEX_A57),
>>> + MIDR_ALL_VERSIONS(MIDR_CORTEX_A72),
>>> + MIDR_ALL_VERSIONS(MIDR_CORTEX_A73),
>>> + MIDR_ALL_VERSIONS(MIDR_CORTEX_A75),
>>> + MIDR_ALL_VERSIONS(MIDR_CORTEX_A76),
>>> + {},
>>> +};
>>> +
>>> #endif /* CONFIG_ARM64_SSBD */
>>>
>>> static void __maybe_unused
>>> @@ -762,6 +783,7 @@ const struct arm64_cpu_capabilities arm64_errata[] = {
>>> .capability = ARM64_SSBD,
>>> .type = ARM64_CPUCAP_LOCAL_CPU_ERRATUM,
>>> .matches = has_ssbd_mitigation,
>>> + .midr_range_list = arm64_ssb_cpus,
>>> },
>>> #endif
>>> #ifdef CONFIG_ARM64_ERRATUM_1188873
>>> @@ -809,4 +831,30 @@ ssize_t cpu_show_spectre_v2(struct device *dev, struct device_attribute *attr,
>>> return sprintf(buf, "Vulnerable\n");
>>> }
>>>
>>> +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
>>> + * 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");
>>> + }
>>> +
>>> + if (__ssb_safe)
>>> + return sprintf(buf, "Not affected\n");
>>
>> The kbuild robot reports that this fails if CONFIG_ARM64_SSBD is not
>> selected. What should we print in this case? "Vulnerable"? Or "Unknown"?
>
> The immediate fix is that the __ssb_safe variable should be in its own
> conditional block which is CONFIG_GENERIC_CPU_VULNERABILITIES ||
> CONFIG_ARM64_SSBD. If the mitigation isn't built in then this code won't
> be run anyway because the sysfs entry won't be populated.

But in that case, we should probably assume that the system is
vulnerable, and we get a different default value for __ssb_safe.

> But, these CONFIG_ conditionals are less than ideal (and would be even
> uglier if they were made more efficient). My own opinion at this point
> is that we should really remove the compile time configs and leave the
> mitigation built all the time. The raw code is fairly small, and we
> could add in the nospectre_v2 command line options so that users can
> choose to runtime disable them. That would also remove the need to
> modify the core cpu vulnerabilities sysfs code.

That'd work for me. The whole thing is now an intractable mess, and I'd
welcome some level of simplification.

Thanks,

M.
--
Jazz is not dead. It just smells funny...

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