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GHSA-2gqc-6j2q-83qp
No affected components available
Summary
thumbv6m-none-eabi (Cortex M0, M0+ and M1) compiler emits non-constant time assembly when using cmovnz (portable version). I did not found any other target with the same behaviour but I did not go through all targets supported by Rust.
Details
It seems that, during mask computation, an LLVM optimisation pass is detecting that bitnz is returning 0 or 1, that can be interpreted as a boolean. This intermediate value is not masked by a call to black_box and thus the subsequent .wrapping_sub(1) can be interpreted as a conditional bitwise conditional not.
PoC
This is an attempt at having a minimal faulty code. In a library crate with an up-to-date cmov as only dependency, the content of src/lib.rs is:
#![no_std]
use cmov::Cmov;
#[inline(never)]
pub fn test_ct_cmov(a: &mut u8, b: u8, c: u8) {
a.cmovnz(&b, c);
}
The resulting assembly emitted (shown using cargo asm --release --target thumbv6m-none-eabi that uses cargo-show-asm):
.section .text.not_ct::test_ct_cmov,"ax",%progbits
.globl not_ct::test_ct_cmov
.p2align 1
.type not_ct::test_ct_cmov,%function
.code 16
.thumb_func
not_ct::test_ct_cmov:
.fnstart
.cfi_sections .debug_frame
.cfi_startproc
.save {r7, lr}
push {r7, lr}
.cfi_def_cfa_offset 8
.cfi_offset lr, -4
.cfi_offset r7, -8
.setfp r7, sp
add r7, sp, #0
.cfi_def_cfa_register r7
.pad #8
sub sp, #8
movs r3, #0
lsls r2, r2, #24
bne .LBB0_2
mvns r3, r3
.LBB0_2:
ldrb r2, [r0]
str r3, [sp, #4]
str r3, [sp]
mov r3, sp
@APP
@NO_APP
ldr r3, [sp]
bics r1, r3
ands r2, r3
adds r1, r2, r1
strb r1, [r0]
add sp, #8
pop {r7, pc}
The non-constant time assembly is:
bne .LBB0_2
mvns r3, r3
.LBB0_2:
Impact
The exact impact is unclear, especially since cmov clearly warns users that the portable version is best-effort.
The vulnerability can be exploited over the network without needing physical access. It is difficult for an attacker to exploit this vulnerability and may require special conditions. An attacker does not need any special privileges or access rights. No user interaction is needed for the attacker to exploit this vulnerability.
Exploitation attempts have been detected. Elevated vigilance and prompt remediation are advised.
The exploit probability is very low. The vulnerability is unlikely to be exploited in the next 30 days.
We did not find any exploit available. Neither in GitHub repositories nor in the Exploit-Database.
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