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How the mentioned CVE works:
The vulnerability occurs in the Rust compiler for the thumbv6m-none-eabi target (Cortex M0, M0+, M1). When using the portable cmovnz function from the cmov crate, LLVM optimization passes misinterpret the code. The bitnz function returns 0 or 1, which the optimizer treats as a boolean value. Since this intermediate value is not masked by black_box, the subsequent .wrapping_sub(1) becomes a conditional operation. This leads to the emission of non-constant time assembly with conditional branches like bne. The assembly shows a branch that depends on data, creating timing variations. This breaks constant-time guarantees crucial for cryptographic operations, enabling side-channel attacks. The issue is specific to this target due to LLVM’s handling of optimizations for ARM Cortex-M0 series. Attackers could exploit timing differences to leak sensitive information such as keys. The cmov crate warns that the portable version is best-effort, but this compiler behavior introduces a security flaw. Developers relying on constant-time code should avoid the portable cmov or use compiler flags to disable relevant optimizations.
DailyCVE Form:
Platform: thumbv6m-none-eabi
Version: Unspecified
Vulnerability: Non-constant time assembly
Severity: Medium
Date: Not provided
Prediction: Patch date unknown
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Bash command: cargo asm –release –target thumbv6m-none-eabi
Code from src/lib.rs:
![bash]
use cmov::Cmov;
[inline(never)]
pub fn test_ct_cmov(a: &mut u8, b: u8, c: u8) {
a.cmovnz(&b, c);
}
Assembly snippet:
bne .LBB0_2
mvns r3, r3
.LBB0_2:
How Exploit:
Timing analysis attacks
Protection from this CVE:
Use constant-time implementations
Impact:
Potential key leakage
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Sources:
Reported By: github.com
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