Exploit Mitigations - The rustc book

# Exploit Mitigations

This chapter documents the exploit mitigations supported by the Rust compiler, and is by no means an extensive survey of the Rust programming language’s security features.

This chapter is for software engineers working with the Rust programming language, and assumes prior knowledge of the Rust programming language and its toolchain.

## Introduction

The Rust programming language provides memory[1] and thread[2] safety guarantees via its ownership[3], references and borrowing[4], and slice types[5] features. However, Unsafe Rust[6] introduces unsafe blocks, unsafe functions and methods, unsafe traits, and new types that are not subject to the borrowing rules.

Parts of the Rust standard library are implemented as safe abstractions over unsafe code (and historically have been vulnerable to memory corruption[7]). Furthermore, the Rust code and documentation encourage creating safe abstractions over unsafe code. This can cause a false sense of security if unsafe code is not properly reviewed and tested.

Unsafe Rust introduces features that do not provide the same memory and thread safety guarantees. This causes programs or libraries to be susceptible to memory corruption (CWE-119)[8] and concurrency issues (CWE-557)[9]. Modern C and C++ compilers provide exploit mitigations to increase the difficulty to exploit vulnerabilities resulting from these issues. Therefore, the Rust compiler must also support these exploit mitigations in order to mitigate vulnerabilities resulting from the use of Unsafe Rust. This chapter documents these exploit mitigations and how they apply to Rust.

This chapter does not discuss the effectiveness of these exploit mitigations as they vary greatly depending on several factors besides their design and implementation, but rather describe what they do, so their effectiveness can be understood within a given context.

## Exploit mitigations

This section documents the exploit mitigations applicable to the Rust compiler when building programs for the Linux operating system on the AMD64 architecture and equivalent.1

The Rust Programming Language currently has no specification. The Rust compiler (i.e., rustc) is the language reference implementation. All references to “the Rust compiler” in this chapter refer to the language reference implementation.

Table I
Summary of exploit mitigations supported by the Rust compiler when building programs for the Linux operating system on the AMD64 architecture and equivalent.

Exploit mitigation | Supported and enabled by default | Since
Position-independent executable | Yes | 0.12.0 (2014-10-09)
Integer overflow checks | Yes (enabled when debug assertions are enabled, and disabled when debug assertions are disabled) | 1.1.0 (2015-06-25)
Non-executable memory regions | Yes | 1.8.0 (2016-04-14)
Stack clashing protection | Yes | 1.20.0 (2017-08-31)
Read-only relocations and immediate binding | Yes | 1.21.0 (2017-10-12)
Heap corruption protection | Yes | 1.32.0 (2019-01-17) (via operating system default or specified allocator)
Stack smashing protection | No |
Forward-edge control flow protection | No |
Backward-edge control flow protection (e.g., shadow and safe stack) | No |

1. See https://github.com/rust-lang/rust/tree/master/compiler/rustc_target/src/spec for a list of targets and their default options. ↩

### Position-independent executable

Position-independent executable increases the difficulty of the use of code reuse exploitation techniques, such as return-oriented programming (ROP) and variants, by generating position-independent code for the executable, and instructing the dynamic linker to load it similarly to a shared object at a random load address, thus also benefiting from address-space layout randomization (ASLR). This is also referred to as “full ASLR”.

The Rust compiler supports position-independent executable, and enables it by default since version 0.12.0 (2014-10-09)[10]–[13].

    $ readelf -h target/release/hello-rust | grep Type:
      Type:                              DYN (Shared object file)

Fig. 1. Checking if an executable is a position-independent executable.

An executable with an object type of `ET_DYN` (i.e., shared object) and not `ET_EXEC` (i.e., executable) is a position-independent executable (see Fig. 1).

### Integer overflow checks

Integer overflow checks protects programs from undefined and unintended behavior (which may cause vulnerabilities) by checking for results of signed and unsigned integer computations that cannot be represented in their type, resulting in an overflow or wraparound.

The Rust compiler supports integer overflow checks, and enables it when debug assertions are enabled since version 1.1.0 (2015-06-25)[14]–[20].

    fn main() {
        let u: u8 = 255;
        println!("u: {}", u + 1);
    }

Fig. 2. hello-rust-integer program.

    $ cargo run
       Compiling hello-rust-integer v0.1.0 (/home/rcvalle/hello-rust-integer)
        Finished dev [unoptimized + debuginfo] target(s) in 0.23s
         Running `target/debug/hello-rust-integer`
    thread 'main' panicked at 'attempt to add with overflow', src/main.rs:3:23
    note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace.

Fig. 3. Build and execution of hello-rust-integer with debug assertions enabled.

    $ cargo run --release
       Compiling hello-rust-integer v0.1.0 (/home/rcvalle/hello-rust-integer)
        Finished release [optimized] target(s) in 0.23s
         Running `target/release/hello-rust-integer`
    u: 0

Fig. 4. Build and execution of hello-rust-integer with debug assertions disabled.

Integer overflow checks are enabled when debug assertions are enabled (see Fig. 3), and disabled when debug assertions are disabled (see Fig. 4). To enable integer overflow checks independently, use the option to control integer overflow checks, scoped attributes, or explicit checking methods such as `checked_add`2.

It is recommended that explicit wrapping methods such as `wrapping_add` be used when wrapping semantics are intended, and that explicit checking and wrapping methods always be used when using Unsafe Rust.

2. See https://doc.rust-lang.org/std/primitive.u32.html for more information on the checked, overflowing, saturating, and wrapping methods (using u32 as an example). ↩

### Non-executable memory regions

Non-executable memory regions increase the difficulty of exploitation by limiting the memory regions that can be used to execute arbitrary code. Most modern processors provide support for the operating system to mark memory regions as non executable, but it was previously emulated by software, such as in grsecurity/PaX's PAGEEXEC and SEGMEXEC, on processors that did not provide support for it. This is also known as “No Execute (NX) Bit”, “Execute Disable (XD) Bit”, “Execute Never (XN) Bit”, and others.

The Rust compiler supports non-executable memory regions, and enables it by default since its initial release, version 0.1 (2012-01-20)[21], [22], but has regressed since then[23]–[25], and enforced by default since version 1.8.0 (2016-04-14)[25].

    $ readelf -l target/release/hello-rust | grep -A 1 GNU_STACK
      GNU_STACK      0x0000000000000000 0x0000000000000000 0x0000000000000000
                     0x0000000000000000 0x0000000000000000  RW     0x10

Fig. 5. Checking if non-executable memory regions are enabled for a given binary.

The presence of an element of type `PT_GNU_STACK` in the program header table with the `PF_X` (i.e., executable) flag unset indicates non-executable memory regions3 are enabled for a given binary (see Fig. 5). Conversely, the presence of an element of type `PT_GNU_STACK` in the program header table with the `PF_X` flag set or the absence of an element of type `PT_GNU_STACK` in the program header table indicates non-executable memory regions are not enabled for a given binary.

3. See the Appendix section for more information on why it affects other memory regions besides the stack. ↩

### Stack clashing protection

Stack clashing protection protects the stack from overlapping with another memory region—allowing arbitrary data in both to be overwritten using each other—by reading from the stack pages as the stack grows to cause a page fault when attempting to read from the guard page/region. This is also referred to as “stack probes” or “stack probing”.

The Rust compiler supports stack clashing protection via stack probing, and enables it by default since version 1.20.0 (2017-08-31)[26]–[29].

Fig. 6. IDA Pro listing cross references to `__rust_probestack` in hello-rust.

    fn hello() {
        println!("Hello, world!");
    }

    fn main() {
        let _: [u64; 1024] = [0; 1024];
        hello();
    }

Fig 7. Modified hello-rust.

Fig. 8. IDA Pro listing cross references to `__rust_probestack` in modified hello-rust.

To check if stack clashing protection is enabled for a given binary, search for cross references to `__rust_probestack`. The `__rust_probestack` is called in the prologue of functions whose stack size is larger than a page size (see Fig. 6), and can be forced for illustration purposes by modifying the hello-rust example as seen in Fig. 7 and Fig. 8.

### Read-only relocations and immediate binding

Read-only relocations protect segments containing relocations and relocation information (i.e., `.init_array`, `.fini_array`, `.dynamic`, and `.got`) from being overwritten by marking these segments read only. This is also referred to as “partial RELRO”.

The Rust compiler supports read-only relocations, and enables it by default since version 1.21.0 (2017-10-12)[30], [31].

    $ readelf -l target/release/hello-rust | grep GNU_RELRO
      GNU_RELRO      0x000000000002ee00 0x000000000002fe00 0x000000000002fe00

Fig. 9. Checking if read-only relocations is enabled for a given binary.

The presence of an element of type `PT_GNU_RELRO` in the program header table indicates read-only relocations are enabled for a given binary (see Fig. 9). Conversely, the absence of an element of type `PT_GNU_RELRO` in the program header table indicates read-only relocations are not enabled for a given binary.

Immediate binding protects additional segments containing relocations (i.e., `.got.plt`) from being overwritten by instructing the dynamic linker to perform all relocations before transferring control to the program during startup, so all segments containing relocations can be marked read only (when combined with read-only relocations). This is also referred to as “full RELRO”.

The Rust compiler supports immediate binding, and enables it by default since version 1.21.0 (2017-10-12)[30], [31].

    $ readelf -d target/release/hello-rust | grep BIND_NOW
     0x000000000000001e (FLAGS)              BIND_NOW

Fig. 10. Checking if immediate binding is enabled for a given binary.

The presence of an element with the `DT_BIND_NOW` tag and the `DF_BIND_NOW` flag4 in the dynamic section indicates immediate binding is enabled for a given binary (see Fig. 10). Conversely, the absence of an element with the `DT_BIND_NOW` tag and the `DF_BIND_NOW` flag in the dynamic section indicates immediate binding is not enabled for a given binary.

The presence of both an element of type `PT_GNU_RELRO` in the program header table and of an element with the `DT_BIND_NOW` tag and the `DF_BIND_NOW` flag in the dynamic section indicates full RELRO is enabled for a given binary (see Fig. 9 and Fig. 10).

4. And the `DF_1_NOW` flag for some link editors. ↩

### Heap corruption protection

Heap corruption protection protects memory allocated dynamically by performing several checks, such as checks for corrupted links between list elements, invalid pointers, invalid sizes, double/multiple “frees” of the same memory allocated, and many corner cases of these. These checks are implementation specific, and vary per allocator.

ARM Memory Tagging Extension (MTE), when available, will provide hardware assistance for a probabilistic mitigation to detect memory safety violations by tagging memory allocations, and automatically checking that the correct tag is used on every memory access.

Rust’s default allocator has historically been jemalloc, and it has long been the cause of issues and the subject of much discussion[32]–[38]. Consequently, it has been removed as the default allocator in favor of the operating system’s standard C library default allocator5 since version 1.32.0 (2019-01-17)[39].

    fn main() {
        let mut x = Box::new([0; 1024]);

        for i in 0..1026 {
            unsafe {
                let elem = x.get_unchecked_mut(i);
                *elem = 0x4141414141414141u64;
            }
        }
    }

Fig. 11. hello-rust-heap program.

    $ cargo run
       Compiling hello-rust-heap v0.1.0 (/home/rcvalle/hello-rust-heap)
        Finished dev [unoptimized + debuginfo] target(s) in 0.25s
         Running `target/debug/hello-rust-heap`
    free(): invalid next size (normal)
    Aborted

Fig. 12. Build and execution of hello-rust-heap with debug assertions enabled.

    $ cargo run --release
       Compiling hello-rust-heap v0.1.0 (/home/rcvalle/hello-rust-heap)
        Finished release [optimized] target(s) in 0.25s
         Running `target/release/hello-rust-heap`
    free(): invalid next size (normal)
    Aborted

Fig. 13. Build and execution of hello-rust-heap with debug assertions disabled.

Heap corruption checks are being performed when using the default allocator (i.e., the GNU Allocator) as seen in Fig. 12 and Fig. 13.

5. Linux's standard C library default allocator is the GNU Allocator, which is derived from ptmalloc (pthreads malloc) by Wolfram Gloger, which in turn is derived from dlmalloc (Doug Lea malloc) by Doug Lea. ↩

### Stack smashing protection

Stack smashing protection protects programs from stack-based buffer overflows by inserting a random guard value between local variables and the saved return instruction pointer, and checking if this value has changed when returning from a function. This is also known as “Stack Protector” or “Stack Smashing Protector (SSP)”.

The Rust compiler does not support stack smashing protection. However, more comprehensive alternatives to stack smashing protection exist, such as shadow and safe stack (see backward-edge control flow protection).

Fig. 14. IDA Pro listing cross references to `__stack_chk_fail` in hello-rust.

To check if stack smashing protection is enabled for a given binary, search for cross references to `__stack_chk_fail`. The only cross references to `__stack_chk_fail` in hello-rust are from the statically-linked libbacktrace library (see Fig. 14).

### Forward-edge control flow protection

Forward-edge control flow protection protects programs from having its control flow changed/hijacked by performing checks to ensure that destinations of indirect branches are one of their valid destinations in the control flow graph. The comprehensiveness of these checks vary per implementation. This is also known as “forward-edge control flow integrity (CFI)”.

Newer processors provide hardware assistance for forward-edge control flow protection, such as ARM Branch Target Identification (BTI), ARM Pointer Authentication, and Intel Indirect Branch Tracking (IBT) as part of Intel Control-flow Enforcement Technology (CET). However, ARM BTI and Intel IBT -based implementations are less comprehensive than software-based implementations such as LLVM ControlFlowIntegrity (CFI), and the commercially available grsecurity/PaX Reuse Attack Protector (RAP).

The Rust compiler does not support forward-edge control flow protection on Linux6. There is work currently ongoing to add support for the sanitizers[40], which may or may not include support for LLVM CFI.

    $ readelf -s target/release/hello-rust | grep __cfi_init

Fig. 15. Checking if LLVM CFI is enabled for a given binary.

The presence of the `__cfi_init` symbol (and references to `__cfi_check`) indicates that LLVM CFI (i.e., forward-edge control flow protection) is enabled for a given binary. Conversely, the absence of the `__cfi_init` symbol (and references to `__cfi_check`) indicates that LLVM CFI is not enabled for a given binary (see Fig. 15).

6. It supports Control Flow Guard (CFG) on Windows (see https://github.com/rust-lang/rust/issues/68793). ↩

### Backward-edge control flow protection

Shadow stack protects saved return instruction pointers from being overwritten by storing a copy of them on a separate (shadow) stack, and using these copies as authoritative values when returning from functions. This is also known as “ShadowCallStack” and “Return Flow Guard”, and is considered an implementation of backward-edge control flow protection (or “backward-edge CFI”).

Safe stack protects not only the saved return instruction pointers, but also register spills and some local variables from being overwritten by storing unsafe variables, such as large arrays, on a separate (unsafe) stack, and using these unsafe variables on the separate stack instead. This is also known as “SafeStack”, and is also considered an implementation of backward-edge control flow protection.

Both shadow and safe stack are intended to be a more comprehensive alternatives to stack smashing protection as they protect the saved return instruction pointers (and other data in the case of safe stack) from arbitrary writes and non-linear out-of-bounds writes.

Newer processors provide hardware assistance for backward-edge control flow protection, such as ARM Pointer Authentication, and Intel Shadow Stack as part of Intel CET.

The Rust compiler does not support shadow or safe stack. There is work currently ongoing to add support for the sanitizers[40], which may or may not include support for safe stack7.

    $ readelf -s target/release/hello-rust | grep __safestack_init

Fig. 16. Checking if LLVM SafeStack is enabled for a given binary.

The presence of the `__safestack_init` symbol indicates that LLVM SafeStack is enabled for a given binary. Conversely, the absence of the `__safestack_init` symbol indicates that LLVM SafeStack is not enabled for a given binary (see Fig. 16).

7. The shadow stack implementation for the AMD64 architecture and equivalent in LLVM was removed due to performance and security issues. ↩

## Appendix

As of the latest version of the Linux Standard Base (LSB) Core Specification, the `PT_GNU_STACK` program header indicates whether the stack should be executable, and the absence of this header indicates that the stack should be executable. However, the Linux kernel currently sets the `READ_IMPLIES_EXEC` personality upon loading any executable with the `PT_GNU_STACK` program header and the `PF_X` flag set or with the absence of this header, resulting in not only the stack, but also all readable virtual memory mappings being executable.

An attempt to fix this was made in 2012, and another was made in 2020. The former never landed, and the latter partially fixed it, but introduced other issues—the absence of the `PT_GNU_STACK` program header still causes not only the stack, but also all readable virtual memory mappings to be executable in some architectures, such as IA-32 and equivalent (or causes the stack to be non-executable in some architectures, such as AMD64 and equivalent, contradicting the LSB).

The `READ_IMPLIES_EXEC` personality needs to be completely separated from the `PT_GNU_STACK` program header by having a separate option for it (or setarch -X could just be used whenever `READ_IMPLIES_EXEC` is needed), and the absence of the `PT_GNU_STACK` program header needs to have more secure defaults (unrelated to `READ_IMPLIES_EXEC`).

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