GHSA-3RJW-M598-PQ24
Vulnerability from github – Published: 2026-07-02 17:18 – Updated: 2026-07-02 17:18Summary
The aarch64 implementations of Cmov and CmovEq seem to assume that the high bits when loading a value of size smaller than a register into a register are zero-extended. However, this is not the case and these bits are unspecified. This can result in a left.cmovz(&right, condition) not moving right into left, even if condition == 0.
Details
The Rust reference for inline assembly states that:
If a value is of a smaller size than the register it is allocated in then the upper bits of that register will have an undefined value for inputs [..]. Reference
If the high bits [8..] of the selector loaded into a register in the Cmov implementation or the high bits [16..] of self or other for CmovEq (specifically the implementation for u16 and i16) are set, the inline asm compares will produce a different result than the Rust code expects based on the narrow types.
In other words, the following assert fails, even though condition as u8 is zero:
let condition: u32 = black_box(1 << 8);
let mut left = 1;
let right = 2;
left.cmovz(&right, condition as u8);
assert_eq!(left, right);
Because the ninth bit is set in the original variable, this bit is also set when the truncated condition is loaded into the input register for the cmp, causing the csel to select the wrong value.
The problematic code is located in cmov/src/backends/aarch64.rs here for Cmov and here for CmovEq.
The following function:
#[unsafe(no_mangle)]
pub fn cmovz_wrong_output(left: &mut i32, right: i32, condition: u32) {
left.cmovz(&right, condition as u8);
}
produces the assembly:
cmovz_wrong_output:
.cfi_startproc
ldr w8, [x0]
//APP
cmp w2, #0
csel w8, w1, w8, eq
//NO_APP
str w8, [x0]
ret
which compares the 32-bits of the condition value against 0, instead of the intended 8.
Similarly, the following function using cmoveq
#[unsafe(no_mangle)]
pub fn cmoveq_wrong_output(left: u32, right: u32, input: u8, output: &mut u8) {
(left as u16).cmoveq(&(right as u16), input, output);
}
compiles to:
cmoveq_wrong_output:
.cfi_startproc
ldrb w8, [x3]
and w9, w2, #0xff
//APP
eor w10, w0, w1
cmp w10, #0
csel w8, w9, w8, eq
//NO_APP
strb w8, [x3]
ret
where 32 bits of left and right are compared instead of 16. The same happens for i16.
For CmovEq, it seems the u8/i8 impls are not affected, as they are calling u16::from in the implementation which causes the upper bits to be masked out.
PoC
The following two test cases fail on aarch64-unknown-linux-gnu when compiled with --release (the cmovz one even in debug) emulated with qemu.
> rustc --version
rustc 1.94.0 (4a4ef493e 2026-03-02)
#[cfg(test)]
mod tests {
use core::hint::black_box;
use cmov::{Cmov, CmovEq};
#[test]
fn cmovz_wrong_output() {
// The black box is necessary here, as otherwise the compiler will
// provide a constant 0 to the csel
let condition: u32 = black_box(1 << 8);
let mut left = 1;
let right = 2;
// I added this debug_assert as a sanity check, but funnily it causes
// the wrong cmov behavior in debug as well (as opposed to only in release mode without the debug_assert)
debug_assert_eq!(0, condition as u8);
left.cmovz(&right, condition as u8);
assert_eq!(left, right);
}
#[test]
fn cmoveq_wrong_output() {
let input = 1;
let mut output = 0;
let left: u32 = black_box(1 << 16);
let right: u32 = black_box(1 << 17);
// asserting in release mode here would hide the bug, the debug_assert_eq is
// a sanity check that these values SHOULD be equal
debug_assert_eq!(left as u16, right as u16);
(left as u16).cmoveq(&(right as u16), input, &mut output);
assert_eq!(input, output);
}
}
Impact
Under specific circumstances, this issue can cause Cmov/CmovEq to produce incorrect output on aarch64. However, whether this bug can actually manifest depends on the surrounding code that calls the relevant impls. In the PoC, a narrowing cast is required, which masks out the set bits from Rust's point of view, but which are then used in the inline assembly.
Additional Finding
PR #1299 fixed a different bug in the aarch64 backend but introduced a small error. The csel! macro expect a cmp expression as its first argument which is never used. The compare is always "cmp {0:w}, 0", even for csel64!, which intends to use cmp {0:x}, 0. Given the bug described above, this oversight actually reduces its impact slightly, as only the bits in position[8..32] can cause issues, even for those impls that use csel64!.
{
"affected": [
{
"package": {
"ecosystem": "crates.io",
"name": "cmov"
},
"ranges": [
{
"events": [
{
"introduced": "0.1.1"
},
{
"fixed": "0.5.4"
}
],
"type": "ECOSYSTEM"
}
]
}
],
"aliases": [
"CVE-2026-50185"
],
"database_specific": {
"cwe_ids": [
"CWE-758"
],
"github_reviewed": true,
"github_reviewed_at": "2026-07-02T17:18:11Z",
"nvd_published_at": null,
"severity": "MODERATE"
},
"details": "### Summary\n\nThe aarch64 implementations of `Cmov` and `CmovEq` seem to assume that the high bits when loading a value of size smaller than a register into a register are zero-extended. However, this is not the case and these bits are unspecified. This can result in a `left.cmovz(\u0026right, condition)` not moving `right` into `left`, even if `condition == 0`. \n\n### Details\n\nThe Rust reference for inline assembly states that:\n\u003e If a value is of a smaller size than the register it is allocated in then the upper bits of that register will have an undefined value for inputs [..]. [Reference](https://doc.rust-lang.org/reference/inline-assembly.html#r-asm.register-operands.smaller-value)\n\nIf the high bits `[8..]` of the selector loaded into a register in the `Cmov` implementation or the high bits `[16..]` of `self` or `other` for CmovEq (specifically the implementation for `u16` and `i16`) are set, the inline asm compares will produce a different result than the Rust code expects based on the narrow types.\n\nIn other words, the following assert fails, even though `condition as u8` is zero:\n```rust\nlet condition: u32 = black_box(1 \u003c\u003c 8);\nlet mut left = 1;\nlet right = 2;\nleft.cmovz(\u0026right, condition as u8);\nassert_eq!(left, right);\n```\nBecause the ninth bit is set in the original variable, this bit is also set when the truncated condition is loaded into the input register for the `cmp`, causing the `csel` to select the wrong value.\n\nThe problematic code is located in `cmov/src/backends/aarch64.rs` [here for Cmov](https://github.com/RustCrypto/utils/blob/dad5e3b9e66d929e86144fe7c8f25371892e35f3/cmov/src/backends/aarch64.rs#L4-L19) and [here for CmovEq](https://github.com/RustCrypto/utils/blob/dad5e3b9e66d929e86144fe7c8f25371892e35f3/cmov/src/backends/aarch64.rs#L60-L72).\n\nThe following function:\n```rust\n#[unsafe(no_mangle)]\npub fn cmovz_wrong_output(left: \u0026mut i32, right: i32, condition: u32) {\n left.cmovz(\u0026right, condition as u8);\n}\n```\nproduces the assembly:\n```asm\ncmovz_wrong_output:\n\t.cfi_startproc\n\tldr w8, [x0]\n\t//APP\n\tcmp w2, #0\n\tcsel w8, w1, w8, eq\n\t//NO_APP\n\tstr w8, [x0]\n\tret\n```\n\nwhich compares the 32-bits of the condition value against 0, instead of the intended 8.\n\nSimilarly, the following function using `cmoveq`\n```rust\n#[unsafe(no_mangle)]\npub fn cmoveq_wrong_output(left: u32, right: u32, input: u8, output: \u0026mut u8) {\n (left as u16).cmoveq(\u0026(right as u16), input, output);\n}\n```\ncompiles to:\n```asm\ncmoveq_wrong_output:\n\t.cfi_startproc\n\tldrb w8, [x3]\n\tand w9, w2, #0xff\n\t//APP\n\teor w10, w0, w1\n\tcmp w10, #0\n\tcsel w8, w9, w8, eq\n\t//NO_APP\n\tstrb w8, [x3]\n\tret\n```\nwhere 32 bits of left and right are compared instead of 16. The same happens for `i16`.\n\nFor CmovEq, it seems the `u8/i8` impls are not affected, as they are calling `u16::from` in [the implementation](https://github.com/RustCrypto/utils/blob/dad5e3b9e66d929e86144fe7c8f25371892e35f3/cmov/src/lib.rs#L119) which causes the upper bits to be masked out.\n\n### PoC\n\nThe following two test cases fail on `aarch64-unknown-linux-gnu` when compiled with `--release` (the cmovz one even in debug) emulated with qemu.\n\n```\n\u003e rustc --version\nrustc 1.94.0 (4a4ef493e 2026-03-02)\n```\n\n```rust\n#[cfg(test)]\nmod tests {\n use core::hint::black_box;\n\n use cmov::{Cmov, CmovEq};\n\n #[test]\n fn cmovz_wrong_output() {\n // The black box is necessary here, as otherwise the compiler will \n // provide a constant 0 to the csel\n let condition: u32 = black_box(1 \u003c\u003c 8);\n let mut left = 1;\n let right = 2;\n // I added this debug_assert as a sanity check, but funnily it causes\n // the wrong cmov behavior in debug as well (as opposed to only in release mode without the debug_assert)\n debug_assert_eq!(0, condition as u8);\n left.cmovz(\u0026right, condition as u8);\n assert_eq!(left, right);\n }\n\n #[test]\n fn cmoveq_wrong_output() {\n let input = 1;\n let mut output = 0;\n let left: u32 = black_box(1 \u003c\u003c 16);\n let right: u32 = black_box(1 \u003c\u003c 17);\n // asserting in release mode here would hide the bug, the debug_assert_eq is\n // a sanity check that these values SHOULD be equal\n debug_assert_eq!(left as u16, right as u16);\n (left as u16).cmoveq(\u0026(right as u16), input, \u0026mut output);\n assert_eq!(input, output);\n }\n}\n```\n\n### Impact\n\nUnder specific circumstances, this issue can cause `Cmov/CmovEq` to produce incorrect output on `aarch64`. However, whether this bug can actually manifest depends on the surrounding code that calls the relevant impls. In the PoC, a narrowing cast is required, which masks out the set bits from Rust\u0027s point of view, but which are then used in the inline assembly. \n\n### Additional Finding\n\nPR [#1299](https://github.com/RustCrypto/utils/pull/1299) fixed a different bug in the aarch64 backend but introduced a small error. The `csel!` macro expect a `cmp` expression as its first argument which is never used. The compare is always `\"cmp {0:w}, 0\",` even for `csel64!`, which intends to use `cmp {0:x}, 0`. Given the bug described above, this oversight actually reduces its impact slightly, as only the bits in position`[8..32]` can cause issues, even for those impls that use `csel64!`.",
"id": "GHSA-3rjw-m598-pq24",
"modified": "2026-07-02T17:18:11Z",
"published": "2026-07-02T17:18:11Z",
"references": [
{
"type": "WEB",
"url": "https://github.com/RustCrypto/utils/security/advisories/GHSA-3rjw-m598-pq24"
},
{
"type": "PACKAGE",
"url": "https://github.com/RustCrypto/utils"
}
],
"schema_version": "1.4.0",
"severity": [
{
"score": "CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:N/VI:L/VA:N/SC:N/SI:N/SA:N/E:P",
"type": "CVSS_V4"
}
],
"summary": "Cmov/CmovEq on aarch64 can produce wrong results if high-bits of registers are set "
}
Sightings
| Author | Source | Type | Date | Other |
|---|
Nomenclature
- Seen: The vulnerability was mentioned, discussed, or observed by the user.
- Confirmed: The vulnerability has been validated from an analyst's perspective.
- Published Proof of Concept: A public proof of concept is available for this vulnerability.
- Exploited: The vulnerability was observed as exploited by the user who reported the sighting.
- Patched: The vulnerability was observed as successfully patched by the user who reported the sighting.
- Not exploited: The vulnerability was not observed as exploited by the user who reported the sighting.
- Not confirmed: The user expressed doubt about the validity of the vulnerability.
- Not patched: The vulnerability was not observed as successfully patched by the user who reported the sighting.