# MemTagSanitizer

## Introduction

Note: this page describes a tool under development. Part of this functionality is planned but not implemented. Hardware capable of running MemTagSanitizer does not exist as of Oct 2019.

MemTagSanitizer is a fast memory error detector and a code hardening tool based on the Armv8.5-A Memory Tagging Extension. It detects a similar class of errors as AddressSanitizer or HardwareAssistedAddressSanitizer, but with much lower overhead.

MemTagSanitizer overhead is expected to be in low single digits, both CPU and memory. There are plans for a debug mode with slightly higher memory overhead and better diagnostics. The primary use case of MemTagSanitizer is code hardening in production binaries, where it is expected to be a strong mitigation for both stack and heap-based memory bugs.

## Usage

Compile and link your program with the `-fsanitize=memtag` flag. This will only work when targeting AArch64 Android with the memory tagging extension. One possible way to achieve that is to add `--target=aarch64-linux-android -march=armv8+memtag` to your compilation flags.

Note that doing this will override existing flags of the same type. Assuming that you are already targeting AArch64 Android, an alternative is to add `-Xclang -target-feature -Xclang +mte` to your compilation flags. This adds the memory tagging feature, without changing anything else.

## Implementation

See HardwareAssistedAddressSanitizer for a general overview of a tag-based approach to memory safety. MemTagSanitizer follows a similar implementation strategy, but with the tag storage (shadow) provided by the hardware.

A quick overview of MTE hardware capabilities:

- Every 16 aligned bytes of memory can be assigned a 4-bit Allocation Tag.

- Every pointer can have a 4-bit Address Tag that is in its most significant byte.

- Most memory access instructions generate an exception if Address Tag != Allocation Tag.

- Special instructions are provided for fast tag manipulation.

## Stack instrumentation

Stack-based memory errors are detected by updating Allocation Tag for each local variable to a random value at the start of its lifetime, and resetting it to the stack pointer Address Tag at the end of it. Unallocated stack space is expected to match the Address Tag of SP; this allows to skip tagging of any variable when memory safety can be statically proven.

Allocating a truly random tag for each stack variable in a large function may incur significant code size overhead, because it means that each variable’s address is an independent, non-rematerializable value; thus a function with N local variables will have extra N live values to keep through most of its life time.

For this reason MemTagSanitizer generates at most one random tag per function, called a “base tag”. Other stack variables, if there are any, are assigned tags at a fixed offset from the base.

Please refer to this document for more details about stack instrumentation.

## Heap tagging

Note: this part is not implemented as of Oct 2019.

MemTagSanitizer will use Scudo Hardened Allocator with additional code to update memory tags when

- New memory is obtained from the system.

- An allocation is freed.

There is no need to change Allocation Tags for the bulk of the allocated memory in malloc(), as long as a pointer with the matching Address Tag is returned.

## More information

- LLVM Developer Meeting 2018 talk on Memory Tagging

- Memory Tagging Whitepaper