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KVM: arm64: Trap MTE access and discovery when MTE is disabled

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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>
This commit is contained in:
Fuad Tabba
2026-01-22 11:22:16 +00:00
committed by Marc Zyngier
parent c103c2dfe4
commit f35abcbb8a
3 changed files with 8 additions and 2 deletions
Download Patch File Download Diff File Expand all files Collapse all files

2
arch/arm64/include/asm/kvm_arm.h
View File

@@ -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)

2
arch/arm64/kernel/head.S
View File

@@ -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 */

6
arch/arm64/kvm/arm.c
View File

@@ -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;