SpecHammer: Combining Spectre and Rowhammer for New Speculative Attacks

The recent Spectre attacks have revealed how the performance gains from branch prediction come at the cost of weakened security. Spectre Variant 1 (v1) shows how an attacker-controlled variable passed to speculatively executed lines of code can leak secret information to an attacker. Numerous defenses have since been proposed to prevent Spectre attacks, each attempting to block all or some of the Spectre variants. In particular, defenses using taint-tracking are claimed to be the only way to protect against all forms of Spectre v1. However, we show that the defenses proposed thus far can be bypassed by combining Spectre with the well-known Rowhammer vulnerability. By using Rowhammer to modify victim values, we relax the requirement that the attacker needs to share a variable with the victim. Thus, defenses that rely on this requirement, such as taint-tracking, are no longer effective. Furthermore, without this crucial requirement, the number of gadgets that can potentially be used to launch a Spectre attack increases dramatically; those present in Linux kernel version 5.6 increases from about 100 to about 20,000 via Rowhammer bit-flips. Attackers can use these gadgets to steal sensitive information such as stack cookies or canaries, or use new triple gadgets to read any address in memory. We demonstrate two versions of the combined attack on example victims in both user and kernel spaces, showing the attack’s ability to leak sensitive data.