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Side-channel attacks such as Spectre that utilize speculative execution tosteal application secrets pose a significant threat to modern computingsystems. While program transformations can mitigate some Spectre attacks, moreadvanced attacks can divert control flow speculatively to bypass theseprotective instructions, rendering existing defenses useless.  In this paper, we present Venkman: a system that employs programtransformation to completely thwart Spectre attacks that poison entries in theBranch Target Buffer (BTB) and the Return Stack Buffer (RSB). Venkmantransforms code so that all valid targets of a control-flow transfer have anidentical alignment in the virtual address space; it further transforms allbranches to ensure that all entries added to the BTB and RSB are properlyaligned. By transforming all code this way, Venkman ensures that, in anyprogram wanting Spectre defenses, all control-flow transfers, includingspeculative ones, do not skip over protective instructions Venkman adds to thecode segment to mitigate Spectre attacks. Unlike existing defenses, Venkmandoes not reduce sharing of the BTB and RSB and does not flush these structures,allowing safe sharing and reuse among programs while maintaining strongprotection against Spectre attacks. We built a prototype of Venkman on an IBMPOWER8 machine. Our evaluation on the SPEC benchmarks and selected applicationsshows that Venkman increases execution time to 3.47$\times$ on average andincreases code size to 1.94$\times$ on average when it is used to ensure thatfences are executed to mitigate Spectre attacks. Our evaluation also shows thatSpectre-resistant Software Fault Isolation (SFI) built using Venkman incurs ageometric mean of 2.42$\times$ space overhead and 1.68$\times$ performanceoverhead.

Restricting Control Flow During Speculative Execution with Venkman