linux-kernel - [PATCH v3 2/2] arm64: Turn on KPTI only on CPUs that need it

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Date:   Fri, 19 Jan 2018 04:22:48 -0800
From:   Jayachandran C <jnair@...iumnetworks.com>
To:     Will Deacon <will.deacon@....com>, Jon Masters <jcm@...masters.org>
Cc:     marc.zyngier@....com, linux-arm-kernel@...ts.infradead.org,
        lorenzo.pieralisi@....com, ard.biesheuvel@...aro.org,
        catalin.marinas@....com, linux-kernel@...r.kernel.org,
        labbott@...hat.com, christoffer.dall@...aro.org,
        Jayachandran C <jnair@...iumnetworks.com>
Subject: [PATCH v3 2/2] arm64: Turn on KPTI only on CPUs that need it

Whitelist Broadcom Vulcan/Cavium ThunderX2 processors in
unmap_kernel_at_el0(). These CPUs are not vulnerable to
CVE-2017-5754 and do not need KPTI when KASLR is off.

Signed-off-by: Jayachandran C <jnair@...iumnetworks.com>
---
 arch/arm64/kernel/cpufeature.c | 7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/arch/arm64/kernel/cpufeature.c b/arch/arm64/kernel/cpufeature.c
index 647d44b..fb698ca 100644
--- a/arch/arm64/kernel/cpufeature.c
+++ b/arch/arm64/kernel/cpufeature.c
@@ -866,6 +866,13 @@ static bool unmap_kernel_at_el0(const struct arm64_cpu_capabilities *entry,
 	if (IS_ENABLED(CONFIG_RANDOMIZE_BASE))
 		return true;
 
+	/* 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;
+	}
+
 	/* Defer to CPU feature registers */
 	return !cpuid_feature_extract_unsigned_field(pfr0,
 						     ID_AA64PFR0_CSV3_SHIFT);
-- 
2.7.4