Messages in this thread | | | Date | Thu, 13 Aug 2020 18:59:31 +0100 | From | Marc Zyngier <> | Subject | Re: [PATCH] arm64: Add KRYO4XX gold CPU core to spectre-v2 safe list |
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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.
M. -- Jazz is not dead. It just smells funny...
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