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.<sup id="fnref:1" role="doc-noteref"><a href="#fn:1"

class="footnote">1</a></sup>

class="footnote">1</a></sup> All examples in this section were built using

nightly builds of the Rust compiler on Debian testing.

The Rust Programming Language currently has no specification. The Rust compiler

(i.e., rustc) is the language reference implementation. All references to “the

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].

assertions are enabled since version 1.0.0 (2015-05-15)[14]–[17], but support

for it was not completed until version 1.1.0 (2015-06-25)[16]. An option to

control integer overflow checks was later stabilized in version 1.17.0

(2017-04-27)[18]–[20].

```compile_fail

fn main() {

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

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

enabled.

```text

Running `target/release/hello-rust-integer`

u: 0

```

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

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.

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](https://pax.grsecurity.net/docs/pageexec.txt)

in grsecurity/PaX’s [PAGEEXEC](https://pax.grsecurity.net/docs/pageexec.txt)

and [SEGMEXEC](https://pax.grsecurity.net/docs/segmexec.txt), 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.

GNU_STACK 0x0000000000000000 0x0000000000000000 0x0000000000000000

0x0000000000000000 0x0000000000000000 RW 0x10

```

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

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

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.

```rust

fn hello() {

println!("Hello, world!");

fn main() {

let v: [u8; 16384] = [1; 16384];

let first = &v[0];

println!("The first element is: {first}");

}

```

Fig. 6. hello-rust-stack-probe-1 program.


Fig. 7. The "unrolled loop" stack probe variant in modified hello-rust.

```rust

fn main() {

let _: [u64; 1024] = [0; 1024];

hello();

let v: [u8; 65536] = [1; 65536];

let first = &v[0];

println!("The first element is: {first}");

}

```

Fig 7. Modified hello-rust.

Fig. 8. hello-rust-stack-probe-2 program.


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

hello-rust.


Fig. 9. The "standard loop" stack probe variant 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.

To check if stack clashing protection is enabled for a given binary, look for

any of the two stack probe variants in the prologue of functions whose stack

size is larger than a page size (see Figs. 6–9).

### Read-only relocations and immediate binding

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).

Figs. 9–10).

<small id="fn:4">4\. And the `DF_1_NOW` flag for some link editors. <a

href="#fnref:4" class="reversefootnote" role="doc-backlink">↩</a></small>

free(): invalid next size (normal)

Aborted

```

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

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

```text

$ cargo run --release

free(): invalid next size (normal)

Aborted

```

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

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.

Heap corruption checks are performed when using the default allocator (i.e.,

the GNU Allocator) (see Figs. 12–13).

<small id="fn:5">5\. Linux's standard C library default allocator is the GNU

Allocator, which is derived from ptmalloc (pthreads malloc) by Wolfram Gloger,

a function. This is also known as “Stack Protector” or “Stack Smashing

Protector (SSP)”.

The Rust compiler supports stack smashing protection on nightly builds[42].

The Rust compiler supports stack smashing protection on nightly builds[40].


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 presence of these cross-references

in Rust-compiled code (e.g., `hello_rust::main`) indicates that the stack

smashing protection is enabled (see Fig. 14).

cross references to `__stack_chk_fail` (see Fig. 14).

### Forward-edge control flow protection

(RAP)](https://grsecurity.net/rap_faq).

The Rust compiler supports forward-edge control flow protection on nightly

builds[40]-[41] <sup id="fnref:6" role="doc-noteref"><a href="#fn:6"

builds[41]-[42] <sup id="fnref:6" role="doc-noteref"><a href="#fn:6"

class="footnote">6</a></sup>.

```text

$ readelf -s -W target/debug/rust-cfi | grep "\.cfi"

12: 0000000000005170 46 FUNC LOCAL DEFAULT 14 _RNvCsjaOHoaNjor6_8rust_cfi7add_one.cfi

15: 00000000000051a0 16 FUNC LOCAL DEFAULT 14 _RNvCsjaOHoaNjor6_8rust_cfi7add_two.cfi

17: 0000000000005270 396 FUNC LOCAL DEFAULT 14 _RNvCsjaOHoaNjor6_8rust_cfi4main.cfi

...

$ readelf -s -W target/release/hello-rust | grep "\.cfi"

5: 0000000000006480 657 FUNC LOCAL DEFAULT 15 _ZN10hello_rust4main17h4e359f1dcd627c83E.cfi

```

Fig. 15. Checking if LLVM CFI is enabled for a given binary[41].

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

The presence of symbols suffixed with ".cfi" or the `__cfi_init` symbol (and

references to `__cfi_check`) indicates that LLVM CFI (i.e., forward-edge

protection, such as ARM Pointer Authentication, and Intel Shadow Stack as part

of Intel CET.

The Rust compiler supports shadow stack for aarch64 only <sup id="fnref:7"

role="doc-noteref"><a href="#fn:7" class="footnote">7</a></sup> on nightly Rust

compilers [43]-[44]. Safe stack is available on nightly Rust compilers

[45]-[46].

The Rust compiler supports shadow stack for the AArch64 architecture<sup

id="fnref:7" role="doc-noteref"><a href="#fn:7" class="footnote">7</a></sup>on

nightly builds[43]-[44], and also supports safe stack on nightly

builds[45]-[46].

```text

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

1177: 00000000000057b0 444 FUNC GLOBAL DEFAULT 9 __safestack_init

678: 0000000000008c80 426 FUNC GLOBAL DEFAULT 15 __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 (see Fig. 16). Conversely, the absence of the

`__safestack_init` symbol indicates that LLVM SafeStack is not enabled for a

given binary.

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).

<small id="fn:7">7\. The shadow stack implementation for the AMD64 architecture

and equivalent in LLVM was removed due to performance and security issues. <a

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

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

25. A. Clark. “Explicitly disable stack execution on linux and bsd #30859.”

GitHub. <https://github.com/rust-lang/rust/pull/30859>.

26. “Replace stack overflow checking with stack probes #16012.” GitHub.

26. Zoxc. “Replace stack overflow checking with stack probes #16012.” GitHub.

<https://github.com/rust-lang/rust/issues/16012>.

27. B. Striegel. “Extend stack probe support to non-tier-1 platforms, and

clarify policy for mitigating LLVM-dependent unsafety #43241.” GitHub.

<https://github.com/rust-lang/rust/issues/43241>.

28. A. Crichton. “rustc: Implement stack probes for x86 #42816.” GitHub.

27. A. Crichton. “rustc: Implement stack probes for x86 #42816.” GitHub.

<https://github.com/rust-lang/rust/pull/42816>.

29. A. Crichton. “Add \_\_rust\_probestack intrinsic #175.” GitHub.

28. A. Crichton. “Add \_\_rust\_probestack intrinsic #175.” GitHub.

<https://github.com/rust-lang/compiler-builtins/pull/175>.

29. S. Guelton, S. Ledru, J. Stone. “Bringing Stack Clash Protection to Clang /

X86 — the Open Source Way.” The LLVM Project Blog.

<https://blog.llvm.org/posts/2021-01-05-stack-clash-protection/>.

30. B. Anderson. “Consider applying -Wl,-z,relro or -Wl,-z,relro,-z,now by

default #29877.” GitHub. <https://github.com/rust-lang/rust/issues/29877>.

39. A. Crichton. “Remove the alloc\_jemalloc crate #55238.” GitHub.

<https://github.com/rust-lang/rust/pull/55238>.

40. R. de C Valle. “Tracking Issue for LLVM Control Flow Integrity (CFI) Support

40. bbjornse. “Add codegen option for using LLVM stack smash protection #84197.”

GitHub. <https://github.com/rust-lang/rust/pull/84197>

41. R. de C. Valle. “Tracking Issue for LLVM Control Flow Integrity (CFI) Support

for Rust #89653.” GitHub. <https://github.com/rust-lang/rust/issues/89653>.

41. “ControlFlowIntegrity.” The Rust Unstable Book.

42. “ControlFlowIntegrity.” The Rust Unstable Book.

[https://doc.rust-lang.org/unstable-book/compiler-flags/sanitizer.html#controlflowintegrity](../unstable-book/compiler-flags/sanitizer.html#controlflowintegrity).

42. bbjornse. “add codegen option for using LLVM stack smash protection #84197.”

GitHub. <https://github.com/rust-lang/rust/pull/84197>

43. ivanloz. “Add support for LLVM ShadowCallStack. #98208.” GitHub.

43. I. Lozano. “Add support for LLVM ShadowCallStack #98208.” GitHub.

<https://github.com/rust-lang/rust/pull/98208>.

44. “ShadowCallStack.” The Rust Unstable Book.