Diagnosing ARMv8.5 Memory Tagging Extension Tag-Check Faults on AArch64

Recognizable Condition

A Tag Check Fault (TCF) occurs when the processor detects a mismatch between the tag stored in the pointer's upper byte and the tag associated with the physical memory location being accessed. This manifests as a synchronous abort. To identify this condition, inspect the Exception Syndrome Register (ESR_ELx); a TCF is confirmed when bit 0 (TCF) is set. The Fault Status Code (FSC) will be 0x20 for an instruction abort or 0x24 for a data abort. The Fault Address Register (FAR_ELx) contains the virtual address that triggered the mismatch.

Cause and Diagnostic Table

Observed Symptom	Likely Cause	Key Evidence
ESR_ELx.TCF=1, FSC=0x20/0x24, FAR_ELx in user-space	Software-generated pointer lost its Upper Byte Tag	Pointer casts to uintptr_t, arithmetic overflows, missing -mbranch-protection
Abort in region explicitly allocated as untagged	MTE enabled for process but region is untagged	prctl(PR_GET_TAGGED_ADDR_CTRL) result, mmap flags
Abort on specific core revision despite valid software	Silicon errata (e.g., ARM errata 1655431)	CPU part/revision from /proc/cpuinfo, vendor errata sheet
MTE faults on hardware lacking MTE support	Kernel configured with CONFIG_ARM64_MTE=y on unsupported CPU	/proc/cpuinfo missing 'mte' flag

Ordered Diagnostic Checks

1. Confirm Exception Type: Read ESR_ELx from the fault handler. Verify bit 0 (TCF) is 1 and check if the FSC is 0x20 (instruction) or 0x24 (data).
2. Verify Hardware Support: Run

   grep -i mte /proc/cpuinfo

   as a user with read permissions. The output must contain the mte flag. If absent, the hardware does not support MTE.
3. Check Kernel Configuration: Verify the kernel was built with MTE support by checking

   zcat /proc/config.gz | grep CONFIG_ARM64_MTE

   or inspecting /boot/config-$(uname -r). It must show CONFIG_ARM64_MTE=y.
4. Inspect Process Tag Control: In the target process, call

   prctl(PR_GET_TAGGED_ADDR_CTRL, 0, 0, 0, 0)

   . A non-zero return confirms the kernel is managing tags for this process.
5. Review Pointer Logic: Audit source code for casts between pointers and integer types narrower than uintptr_t. Check for pointer arithmetic that may overflow into the tag space (bits 56-63).
6. Cross-Reference Silicon Errata: Extract the CPU part and revision from /proc/cpuinfo. Compare these against the SoC vendor's errata list for known spurious MTE faults.

Fixes Tied to Findings

Software Tag Loss

• Replace narrowing casts (e.g., to unsigned long on 32-bit compat) with intptr_t or uintptr_t.
• Recompile with -mbranch-protection=standard to enable automatic tag injection and -moutline-atomics if using atomic outlines.
• If interoperability requires stripping tags, wrap the access by disabling MTE for the process:

  prctl(PR_SET_TAGGED_ADDR_CTRL, PR_TAGGED_ADDR_ENABLE, 0)

  .

Untagged Region Access

• Ensure memory is mapped with MAP_TAGGED if tagging is required.
• For regions that must remain untagged, disable MTE globally via the boot parameter mte=off (note: this removes protection for all processes).

Silicon Errata

• Apply vendor workarounds, such as inserting a DSB SY followed by an ISB instruction before the memory access.
• If the erratum is core-specific, disable MTE on the affected core via cpuhotplug.

Kernel Misconfiguration

• If the CPU lacks MTE support, rebuild the kernel with CONFIG_ARM64_MTE=n.

Escalation Criteria

If the fault persists after applying software fixes and verifying errata, escalate to the silicon vendor or ARM. Provide the following data:

• Register Dump: ESR_ELx, ELR_ELx, FAR_ELx, and the tag bits (56-63) of the fault address.
• System Context: Output of uname -a, /proc/cpuinfo (part and revision), and the kernel .config.
• Workload: A minimal reproducible example or the specific workload triggering the fault.

Practical Verification

To verify the MTE diagnostic path, you can create a minimal test program. Allocate memory using malloc, apply a tag using __builtin_arm_sttag, and then intentionally clear the upper byte of the pointer before dereferencing it. Run this under perf record -e arm64_mte_tag_check_fault ./test. A successful test will record a sample with ESR_ELx.TCF set.

Limitations: Disabling MTE via prctl or boot parameters masks genuine memory-safety bugs. Always limit disablement to the smallest possible scope (e.g., a specific process) to maintain system security.