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Thu, 27 Nov 2025 04:22:13 -0800 (PST) Date: Thu, 27 Nov 2025 12:22:07 +0000 In-Reply-To: <20251127122210.4111702-1-tabba@google.com> Mime-Version: 1.0 References: <20251127122210.4111702-1-tabba@google.com> X-Mailer: git-send-email 2.52.0.487.g5c8c507ade-goog Message-ID: <20251127122210.4111702-3-tabba@google.com> Subject: [PATCH v1 2/5] arm64: Clear HCR_EL2.ATA when MTE is not supported or disabled From: Fuad Tabba To: kvmarm@lists.linux.dev, linux-arm-kernel@lists.infradead.org Cc: maz@kernel.org, oliver.upton@linux.dev, joey.gouly@arm.com, suzuki.poulose@arm.com, yuzenghui@huawei.com, catalin.marinas@arm.com, will@kernel.org, tabba@google.com Content-Type: text/plain; charset="UTF-8" X-CRM114-Version: 20100106-BlameMichelson ( TRE 0.8.0 (BSD) ) MR-646709E3 X-CRM114-CacheID: sfid-20251127_042215_737488_5005CF8E X-CRM114-Status: GOOD ( 15.85 ) X-BeenThere: linux-arm-kernel@lists.infradead.org X-Mailman-Version: 2.1.34 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Sender: "linux-arm-kernel" Errors-To: linux-arm-kernel-bounces+linux-arm-kernel=archiver.kernel.org@lists.infradead.org If MTE is not supported by the hardware, or is disabled in the kernel configuration (CONFIG_ARM64_MTE=n) or command line (arm64.nomte), the kernel stops advertising MTE to userspace and avoids using MTE instructions. However, this is a software-level disable only. When MTE hardware is present and enabled by EL3 firmware, leaving HCR_EL2.ATA set allows the host to execute MTE instructions (STG, LDG, etc.) and access allocation tags in physical memory. This creates a security risk where a malicious or buggy host could lead to system crashes, undefined behavior, or compromise guests. Prevent this by clearing HCR_EL2.ATA when MTE is disabled. Remove it from the HCR_HOST_NVHE_FLAGS default, and conditionally set it in cpu_prepare_hyp_mode() only when system_supports_mte() returns true. This causes MTE instructions to trap to EL2 when HCR_EL2.ATA is cleared. Early boot code in head.S temporarily keeps HCR_ATA set to avoid special-casing initialization paths. This is safe because this code executes before untrusted code runs and will clear HCR_ATA if MTE is disabled. Signed-off-by: Fuad Tabba --- arch/arm64/include/asm/kvm_arm.h | 2 +- arch/arm64/kernel/head.S | 2 +- arch/arm64/kvm/arm.c | 4 ++++ 3 files changed, 6 insertions(+), 2 deletions(-) diff --git a/arch/arm64/include/asm/kvm_arm.h b/arch/arm64/include/asm/kvm_arm.h index 1da290aeedce..a41e3087e00a 100644 --- a/arch/arm64/include/asm/kvm_arm.h +++ b/arch/arm64/include/asm/kvm_arm.h @@ -101,7 +101,7 @@ HCR_BSU_IS | HCR_FB | HCR_TACR | \ HCR_AMO | HCR_SWIO | HCR_TIDCP | HCR_RW | HCR_TLOR | \ HCR_FMO | HCR_IMO | HCR_PTW | HCR_TID3 | HCR_TID1) -#define HCR_HOST_NVHE_FLAGS (HCR_RW | HCR_API | HCR_APK | HCR_ATA) +#define HCR_HOST_NVHE_FLAGS (HCR_RW | HCR_API | HCR_APK) #define HCR_HOST_NVHE_PROTECTED_FLAGS (HCR_HOST_NVHE_FLAGS | HCR_TSC) #define HCR_HOST_VHE_FLAGS (HCR_RW | HCR_TGE | HCR_E2H | HCR_AMO | HCR_IMO | HCR_FMO) diff --git a/arch/arm64/kernel/head.S b/arch/arm64/kernel/head.S index ca04b338cb0d..87a822e5c4ca 100644 --- a/arch/arm64/kernel/head.S +++ b/arch/arm64/kernel/head.S @@ -299,7 +299,7 @@ SYM_INNER_LABEL(init_el2, SYM_L_LOCAL) isb 0: - init_el2_hcr HCR_HOST_NVHE_FLAGS + init_el2_hcr HCR_HOST_NVHE_FLAGS | HCR_ATA init_el2_state /* Hypervisor stub */ diff --git a/arch/arm64/kvm/arm.c b/arch/arm64/kvm/arm.c index 052bf0d4d0b0..c03006b1c5bc 100644 --- a/arch/arm64/kvm/arm.c +++ b/arch/arm64/kvm/arm.c @@ -2030,6 +2030,10 @@ static void __init cpu_prepare_hyp_mode(int cpu, u32 hyp_va_bits) params->hcr_el2 = HCR_HOST_NVHE_PROTECTED_FLAGS; else params->hcr_el2 = HCR_HOST_NVHE_FLAGS; + + if (system_supports_mte()) + params->hcr_el2 |= HCR_ATA; + if (cpus_have_final_cap(ARM64_KVM_HVHE)) params->hcr_el2 |= HCR_E2H; params->vttbr = params->vtcr = 0; -- 2.52.0.487.g5c8c507ade-goog