Messages in this thread | ![/](/images/icornerl.gif) | | Date | Fri, 14 Aug 2020 10:04:59 +0530 | From | Sai Prakash Ranjan <> | Subject | Re: [PATCH] arm64: Add KRYO4XX gold CPU core to spectre-v2 safe list |
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On 2020-08-13 23:29, Marc Zyngier wrote: > On 2020-08-13 13:33, Sai Prakash Ranjan wrote: >> On 2020-08-13 16:09, Marc Zyngier wrote: >>> On 2020-08-13 10:40, Will Deacon wrote: >>>> On Thu, Aug 13, 2020 at 02:49:37PM +0530, Sai Prakash Ranjan wrote: >>>>> On 2020-08-13 14:33, Will Deacon wrote: >>>>> > On Thu, Aug 13, 2020 at 01:48:34PM +0530, Sai Prakash Ranjan wrote: >>>>> > > KRYO4XX gold/big CPU cores are based on Cortex-A76 which has CSV2 >>>>> > > bits set and are spectre-v2 safe. But on big.LITTLE systems where >>>>> > > they are coupled with other CPU cores such as the KRYO4XX silver >>>>> > > based on Cortex-A55 which are spectre-v2 safe but do not have CSV2 >>>>> > > bits set, the system wide safe value will be set to the lowest value >>>>> > > of CSV2 bits as per FTR_LOWER_SAFE defined for CSV2 bits of register >>>>> > > ID_AA64PFR0_EL1. >>>>> > > >>>>> > > This is a problem when booting a guest kernel on gold CPU cores >>>>> > > where it will incorrectly report ARM_SMCCC_ARCH_WORKAROUND_1 warning >>>>> > > and consider them as vulnerable for Spectre variant 2 due to system >>>>> > > wide safe value which is used in kvm emulation code when reading id >>>>> > > registers. One wrong way of fixing this is to set the FTR_HIGHER_SAFE >>>>> > > for CSV2 bits, so instead add the KRYO4XX gold CPU core to the safe >>>>> > > list which will be consulted even when the sanitised read reports >>>>> > > that CSV2 bits are not set for KRYO4XX gold cores. >>>>> > > >>>>> > > Reported-by: Stephen Boyd <swboyd@chromium.org> >>>>> > > Signed-off-by: Sai Prakash Ranjan <saiprakash.ranjan@codeaurora.org> >>>>> > > --- >>>>> > > arch/arm64/kernel/cpu_errata.c | 1 + >>>>> > > 1 file changed, 1 insertion(+) >>>>> > > >>>>> > > diff --git a/arch/arm64/kernel/cpu_errata.c >>>>> > > b/arch/arm64/kernel/cpu_errata.c >>>>> > > index 6bd1d3ad037a..6cbdd2d98a2a 100644 >>>>> > > --- a/arch/arm64/kernel/cpu_errata.c >>>>> > > +++ b/arch/arm64/kernel/cpu_errata.c >>>>> > > @@ -545,6 +545,7 @@ static const struct midr_range >>>>> > > spectre_v2_safe_list[] = { >>>>> > > MIDR_ALL_VERSIONS(MIDR_HISI_TSV110), >>>>> > > MIDR_ALL_VERSIONS(MIDR_QCOM_KRYO_3XX_SILVER), >>>>> > > MIDR_ALL_VERSIONS(MIDR_QCOM_KRYO_4XX_SILVER), >>>>> > > + MIDR_ALL_VERSIONS(MIDR_QCOM_KRYO_4XX_GOLD), >>>>> > >>>>> > We shouldn't be putting CPUs in the safe list when they have CSV2 >>>>> > reporting >>>>> > that they are mitigated in hardware, so I don't think this is the right >>>>> > approach. >>>>> > >>>>> >>>>> Ok but the only thing I find wrong in this approach is that it is a >>>>> redundant >>>>> information because CSV2 is already advertising the mitigation, but >>>>> again >>>>> CSV2 check is done first so it doesn't really hurt to add it to the >>>>> safe >>>>> list because we already know that it is safe. >>>> >>>> It simply doesn't scale. That's why CSV2 exists in the first place, >>>> so we >>>> don't have to modify the kernel everytime a new CPU is invented. >>>> >>>>> > Sounds more like KVM should advertise CSV2 for the vCPUs if all of the >>>>> > physical CPUs without CSV2 set are on the safe list. But then again, KVM >>>>> > has always been slightly in denial about big.LITTLE because you can't >>>>> > sensibly expose it to a guest if there are detectable differences... >>>>> > >>>>> >>>>> Sorry but I don't see how the guest kernel will see the CSV2 bits >>>>> set for >>>>> gold CPU cores without actually adding them to the safe list or >>>>> reading the >>>>> not sanitised value of ID_AA64PFR0_EL1 ? >>>> >>>> Well that's for somebody to figure out in the patch. I'm just saying >>>> that >>>> adding cores to the safe list when they already have a CSV2 field >>>> conveying >>>> the same information is the wrong approach. The right appproach is >>>> for KVM >>>> to expose CSV2 as set when the system is not affected by the >>>> erratum. >>> >>> A sensible way to fix this would be with something like that: >>> >>> diff --git a/arch/arm64/kvm/sys_regs.c b/arch/arm64/kvm/sys_regs.c >>> index 077293b5115f..2735db21ff0d 100644 >>> --- a/arch/arm64/kvm/sys_regs.c >>> +++ b/arch/arm64/kvm/sys_regs.c >>> @@ -1131,6 +1131,9 @@ static u64 read_id_reg(const struct kvm_vcpu >>> *vcpu, >>> if (!vcpu_has_sve(vcpu)) >>> val &= ~(0xfUL << ID_AA64PFR0_SVE_SHIFT); >>> val &= ~(0xfUL << ID_AA64PFR0_AMU_SHIFT); >>> + if (!(val & (0xfUL << ID_AA64PFR0_CSV2_SHIFT)) && >>> + get_spectre_v2_workaround_state() == >>> ARM64_BP_HARDEN_NOT_REQUIRED) >>> + val |= (1UL << ID_AA64PFR0_CSV2_SHIFT); >>> } else if (id == SYS_ID_AA64ISAR1_EL1 && !vcpu_has_ptrauth(vcpu)) { >>> val &= ~((0xfUL << ID_AA64ISAR1_APA_SHIFT) | >>> (0xfUL << ID_AA64ISAR1_API_SHIFT) | >>> >> >> Thanks Marc, I gave this a go on SC7180 where the issue was seen and >> this fix is good. >> >> Tested-by: Sai Prakash Ranjan <saiprakash.ranjan@codeaurora.org> > > There is still a problem with this approach. A late CPU could > come up after a guest has been started. If that CPU identified > as vulnerable by the host kernel, get_spectre_v2_workaround_state() > will return a different value, breaking the guest (or more > likely, leaving it exposed to Spectre-v2 attacks). > > We'd need to disable the late onlining of CPUs that would change > the mitigation state, and this is... ugly. >
Ugh, yes indeed and here I was thinking that these things are straightforward :(
Thanks, Sai
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