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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. 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. Additionally, set `HCR_EL2.TID5` when MTE is disabled. This traps reads of `GMID_EL1` (Multiple tag transfer ID register) to EL2, preventing the discovery of MTE parameters (such as tag block size) when the feature is suppressed. 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 <tabba@google.com> Link: https://patch.msgid.link/20260122112218.531948-3-tabba@google.com Signed-off-by: Marc Zyngier <maz@kernel.org>