linux-kernel - [PATCH 24/29] arm64: Turn on KPTI only on CPUs that need it

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Date:   Wed, 28 Feb 2018 11:56:46 +0800
From:   Alex Shi <alex.shi@...aro.org>
To:     Marc Zyngier <marc.zyngier@....com>,
        Will Deacon <will.deacon@....com>,
        Ard Biesheuvel <ard.biesheuvel@...aro.org>,
        Catalin Marinas <catalin.marinas@....com>,
        stable@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
        linux-kernel@...r.kernel.org
Cc:     Jayachandran C <jnair@...iumnetworks.com>,
        Alex Shi <alex.shi@...aro.org>
Subject: [PATCH 24/29] arm64: Turn on KPTI only on CPUs that need it

From: Jayachandran C <jnair@...iumnetworks.com>

commit 0ba2e29c7fc1 upstream.

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.

Acked-by: Will Deacon <will.deacon@....com>
Signed-off-by: Jayachandran C <jnair@...iumnetworks.com>
Signed-off-by: Catalin Marinas <catalin.marinas@....com>
Signed-off-by: Will Deacon <will.deacon@....com>
Signed-off-by: Alex Shi <alex.shi@...aro.org>
---
 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 6200b81..e62583d 100644
--- a/arch/arm64/kernel/cpufeature.c
+++ b/arch/arm64/kernel/cpufeature.c
@@ -766,6 +766,13 @@ 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