In this paper, we reveal the existence of a new class of prefetcher, the XPT prefetcher, in modern Intel processors which has never been officially detailed. It speculatively issues a load, bypassing last-level cache (LLC) lookups, when it predicts that a load request will result in an LLC miss. We demonstrate that XPT prefetcher is shared among different cores, which enables an attacker to build cross-core side-channel and covertchannel attacks. We propose PREFETCHX, a cross-core attack mechanism, to leak users’ sensitive data and activities. We empirically demonstrate that PREFETCHX can be used to extract private keys of real-world RSA applications. Furthermore, we show that PREFETCHX can enable side-channel attacks that can monitor keystrokes and network traffic patterns of users. Our two cross-core covert-channel attacks also see a low error rate and a 122KiB/s maximum channel capacity. Due to the cache-independent feature of PREFETCHX, current cache-based mitigations are not effective against our attacks. Overall, our work uncovers a significant vulnerability in the XPT prefetcher, which can be exploited to compromise the confidentiality of sensitive information in both cryptography and non-cryptography-related applications among processor cores.

Research into processor-based side-channels has seen both a large number and a large variety of disclosed vulnerabilities that can leak critical, private data to malicious attackers. While most previous works require speculative execution and the use of cache primitives to transmit data, our new approach, called AfterImage, requires neither, capitalizing on vulnerabilities in Intel’s IP-stride prefetcher to both expose and transmit victim data. By training this prefetcher with attacker-known values, and watching for changes to the prefetcher state when execution returns to the attacker, it is now possible to monitor and leak critical data from a large number of common userspace applications and kernel routines without speculation and additional cache accesses. To demonstrate the novel capabilities of AfterImage, we (1) present proof-of-concept attacks that leak data across different isolation levels, (2) present an end-to-end attack that leaks an entire RSA key from a modern, timing-balanced algorithm, and also (3) show how AfterImage can significantly improve the effectiveness of other attacks, such as power side-channel attacks, by using this technique as a high-precision marker.