[PATCH v3 2/4] KVM: arm64: Trap MTE access and discovery when MTE is disabled

[Fuad Tabba]

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 at google.com>
---
 arch/arm64/include/asm/kvm_arm.h | 2 +-
 arch/arm64/kernel/head.S         | 2 +-
 arch/arm64/kvm/arm.c             | 6 ++++++
 3 files changed, 8 insertions(+), 2 deletions(-)

diff --git a/arch/arm64/include/asm/kvm_arm.h b/arch/arm64/include/asm/kvm_arm.h
index e500600e4b9b..752e3e1604e8 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 4f80da0c0d1d..aeac113e5e74 100644
--- a/arch/arm64/kvm/arm.c
+++ b/arch/arm64/kvm/arm.c
@@ -2044,6 +2044,12 @@ 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;
+	else
+		params->hcr_el2 |= HCR_TID5;
+
 	if (cpus_have_final_cap(ARM64_KVM_HVHE))
 		params->hcr_el2 |= HCR_E2H;
 	params->vttbr = params->vtcr = 0;
-- 
2.52.0.457.g6b5491de43-goog