Loongson Processors Expose Data Through Leaky Caches

"Our analysis reveals that under certain circumstances, the ‘uncertain’ data originates from the L1 data cache," wrote Lorenz Hetterich, Tristan Hornetz, Fabian Thomas and Michael Schwarz of Germany’s Helmholtz Center for Information Security, summing up a discovery that turns a low-level ISA quirk into a cross-application data leak.

How the researchers found LoongLeak

The team discovered the leakage after fuzzing processors built by China’s Loongson and documented the results on a site called LoongLeakAttack.com. Their analysis began with an instruction in the LoongArch ISA manual that explicitly leaves 32 bits of a memory register in an "uncertain" state. The researchers traced that uncertainty back to the L1 data cache and found that, because that cache is shared between applications, an attacker can extract data belonging to other applications and the operating system.

LoongArch's "uncertain" instruction and cache-set targeting

The paper explains that the uncertain bits can be caused by data originating in the L1 data cache and that the attacker can "prime the CPU’s internal state to target the leakage to a specific cache set." The leakage is architectural rather than incidental: it does not require high-resolution timers or the usual side-channel amplification techniques, and it grants "precise control over cache set and line offset." That combination gives an attacker a direct, low-noise channel into specific cache contents.

Case studies: keys, hashes, and cross-VM leakage

The authors detail practical case studies in a PDF paper that include "recovering full-disk AES keys from the kernel, partial root password hashes from user-space, and bypassing traditional software defenses such as ASLR and stack canaries, all within seconds." They also emphasize that "LoongLeak can be exploited from unprivileged user space, containers, or virtual machines." Critically, the flaw can cross the virtual machine boundary and "leak host data from inside a VM," widening potential impact wherever Loongson silicon is used to host mixed workloads.

Mitigation, patching, and the hardware reality

The researchers assert that software mitigations are not possible. Devices with the vulnerable behavior require hardware-focused responses: either replacement of affected chips or ensuring private data never resides in the L1 cache. One suggested operational mitigation is to disable one thread per core—effectively turning off hyperthreading—to reduce leakage, a step that reduces parallelism on affected systems. Loongson has addressed the issue in an update to its 3A6000 processor model; the vendor-implemented mitigation of evicting cache data reportedly slows performance by just 1.4 percent in the worst case.

What this means for technologists, policymakers, and procurement leaders

• Technologists and security teams: The immediate concern is detection and containment. The researchers found "no specific tools or methods to detect if LoongLeak is being exploited," which forces defenders to consider hardware replacement, strict isolation policies, or disabling logical threads per core where feasible.
• Policymakers and public-sector procurement leaders: China’s government promotes Loongson use as part of a plan to reduce dependence on imported technology and has instructed public-sector buyers to purchase local products. That directive raises the prospect that vulnerable devices could be present in government fleets, creating a hard choice between local sourcing and exposure to an architectural leak.
• Enterprises and device buyers: The practical blast radius may be limited outside China — Loongson chips are "hardly used outside China" — and Lenovo sells Loongson-powered laptops only in China. Other major PC makers told The Register they would adopt Loongson if users demand it or if market participation required it, but the reporting found no non-Chinese company currently shipping those processors.

The LoongLeak findings turn a single ISA note about "uncertain" register bits into a cross-layer security problem: an architectural leakage channel that operates from unprivileged contexts, can jump virtual machine boundaries, and resists software-only fixes. Loongson’s update to the 3A6000 and the modest measured performance hit for an eviction-based mitigation offer a path forward, but the researchers’ admission that they could find "no specific tools or methods to detect if LoongLeak is being exploited" leaves one stark question for administrators and procurement officers alike: if government procurement rules drove broad adoption inside an enterprise or agency, how will defenders identify whether those locally sourced chips are already leaking secrets?

Original reporting: https://www.theregister.com/security/2026/08/13/chinese-loongson_processors_have_leaky_caches_researchers_find/5287137