Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1756171AbeAHJUS (ORCPT + 1 other); Mon, 8 Jan 2018 04:20:18 -0500 Received: from usa-sjc-mx-foss1.foss.arm.com ([217.140.101.70]:35786 "EHLO foss.arm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1755900AbeAHJUM (ORCPT ); Mon, 8 Jan 2018 04:20:12 -0500 Subject: Re: [v2,03/11] arm64: Take into account ID_AA64PFR0_EL1.CSV3 To: Jayachandran C , Will Deacon Cc: linux-arm-kernel@lists.infradead.org, lorenzo.pieralisi@arm.com, ard.biesheuvel@linaro.org, catalin.marinas@arm.com, linux-kernel@vger.kernel.org, labbott@redhat.com, christoffer.dall@linaro.org References: <1515157961-20963-4-git-send-email-will.deacon@arm.com> <20180108072253.GA178830@jc-sabre> From: Marc Zyngier Organization: ARM Ltd Message-ID: <9bc1f137-d78c-e46e-e1bc-f49160d5f289@arm.com> Date: Mon, 8 Jan 2018 09:20:09 +0000 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:52.0) Gecko/20100101 Thunderbird/52.5.2 MIME-Version: 1.0 In-Reply-To: <20180108072253.GA178830@jc-sabre> Content-Type: text/plain; charset=utf-8 Content-Language: en-GB Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Return-Path: On 08/01/18 07:24, Jayachandran C wrote: > On Fri, Jan 05, 2018 at 01:12:33PM +0000, Will Deacon wrote: >> For non-KASLR kernels where the KPTI behaviour has not been overridden >> on the command line we can use ID_AA64PFR0_EL1.CSV3 to determine whether >> or not we should unmap the kernel whilst running at EL0. >> >> Reviewed-by: Suzuki K Poulose >> Signed-off-by: Will Deacon >> --- >> arch/arm64/include/asm/sysreg.h | 1 + >> arch/arm64/kernel/cpufeature.c | 8 +++++++- >> 2 files changed, 8 insertions(+), 1 deletion(-) >> >> diff --git a/arch/arm64/include/asm/sysreg.h b/arch/arm64/include/asm/sysreg.h >> index 08cc88574659..ae519bbd3f9e 100644 >> --- a/arch/arm64/include/asm/sysreg.h >> +++ b/arch/arm64/include/asm/sysreg.h >> @@ -437,6 +437,7 @@ >> #define ID_AA64ISAR1_DPB_SHIFT 0 >> >> /* id_aa64pfr0 */ >> +#define ID_AA64PFR0_CSV3_SHIFT 60 >> #define ID_AA64PFR0_SVE_SHIFT 32 >> #define ID_AA64PFR0_GIC_SHIFT 24 >> #define ID_AA64PFR0_ASIMD_SHIFT 20 >> diff --git a/arch/arm64/kernel/cpufeature.c b/arch/arm64/kernel/cpufeature.c >> index 9f0545dfe497..d723fc071f39 100644 >> --- a/arch/arm64/kernel/cpufeature.c >> +++ b/arch/arm64/kernel/cpufeature.c >> @@ -145,6 +145,7 @@ static const struct arm64_ftr_bits ftr_id_aa64isar1[] = { >> }; >> >> static const struct arm64_ftr_bits ftr_id_aa64pfr0[] = { >> + ARM64_FTR_BITS(FTR_HIDDEN, FTR_NONSTRICT, FTR_LOWER_SAFE, ID_AA64PFR0_CSV3_SHIFT, 4, 0), >> ARM64_FTR_BITS(FTR_VISIBLE, FTR_STRICT, FTR_LOWER_SAFE, ID_AA64PFR0_SVE_SHIFT, 4, 0), >> ARM64_FTR_BITS(FTR_HIDDEN, FTR_STRICT, FTR_LOWER_SAFE, ID_AA64PFR0_GIC_SHIFT, 4, 0), >> S_ARM64_FTR_BITS(FTR_VISIBLE, FTR_STRICT, FTR_LOWER_SAFE, ID_AA64PFR0_ASIMD_SHIFT, 4, ID_AA64PFR0_ASIMD_NI), >> @@ -851,6 +852,8 @@ static int __kpti_forced; /* 0: not forced, >0: forced on, <0: forced off */ >> static bool unmap_kernel_at_el0(const struct arm64_cpu_capabilities *entry, >> int __unused) >> { >> + u64 pfr0 = read_sanitised_ftr_reg(SYS_ID_AA64PFR0_EL1); >> + >> /* Forced on command line? */ >> if (__kpti_forced) { >> pr_info_once("kernel page table isolation forced %s by command line option\n", >> @@ -862,7 +865,9 @@ static bool unmap_kernel_at_el0(const struct arm64_cpu_capabilities *entry, >> if (IS_ENABLED(CONFIG_RANDOMIZE_BASE)) >> return true; >> >> - return false; >> + /* Defer to CPU feature registers */ >> + return !cpuid_feature_extract_unsigned_field(pfr0, >> + ID_AA64PFR0_CSV3_SHIFT); > > If I read this correctly, this enables KPTI on all processors without the CSV3 > set (which seems to be a future capability). > > Turning on KPTI has a small but significant overhead, so I think we should turn > it off on processors that are not vulnerable to CVE-2017-5754. Can we add something > like this: > > --->8 > diff --git a/arch/arm64/kernel/cpufeature.c b/arch/arm64/kernel/cpufeature.c > index 19ed09b..202b037 100644 > --- a/arch/arm64/kernel/cpufeature.c > +++ b/arch/arm64/kernel/cpufeature.c > @@ -862,6 +862,13 @@ static bool unmap_kernel_at_el0(const struct arm64_cpu_capabilities *entry, > return __kpti_forced > 0; > } > > + /* Don't force KPTI for CPUs that are not vulnerable */ > + switch (read_cpuid_id() & MIDR_CPU_MODEL_MASK) { > + case MIDR_CAVIUM_THUNDERX2: > + case MIDR_BRCM_VULCAN: > + return false; > + } > + > /* Useful for KASLR robustness */ > if (IS_ENABLED(CONFIG_RANDOMIZE_BASE)) > return true; > KPTI is also an improvement for KASLR. Why would you deprive a user of the choice to further secure their system? Thanks, M. -- Jazz is not dead. It just smells funny...